MXPA99010725A

MXPA99010725A - Stimulation of implantable internal ear in multiple channels

Info

Publication number: MXPA99010725A
Application number: MXPA/A/1999/010725A
Authority: MX
Inventors: H Doyle James
Original assignee: James H Doyle Sr
Filing date: 1999-11-19
Publication date: 2001-09-07

Abstract

Se describe un dispositivo para estimular la rama de transmisión auditiva del 80 nervio. Utiliza electrodos diseñados para restringir el campo eléctrico a la región de su grupo de fibras nerviosas respectivo, y producir un campo de gradiente para cada canal, de tal manera que las características de latencia de las fibras nerviosas en un canal dado ocasionarán un disparo (una propagación) una secuencia de las fibras nerviosas durante una porción del impulso de estímulo del canal. En el sistema análogo, los canales de fibras nerviosas se estimulan en secuencia, pero su estímulo se traslapa con su canal anterior por una cantidad igual al período de latencia más corto de dicho canal. Durante el período en que un impulso de estímulo estáocasionando una propagación de las fibras nerviosas, la amplitud del impulso se modula con la información de audio. En adición, durante la propagación de las fibras nerviosas, ya sea un elemento eléctrico, o bien la forma de la sonda, compensar las características de fuerza-duración de las fibras nerviosas individuales. En el sistema digital, no hay propagación alguna, debido a que hay un solo momento de disparo para las fibras nerviosas enun canal. Existe un traslape de canales sobre un número de canales, y la modulación de audio estáen la forma de modulación de frecuencia de la frecuencia de secuencia de canales.

Description

INTERNAL PURPLE STIMULATOR IMPLANTABLE IN MULTIPLE CHANNELS Cross-reference to related requests This application claims priority of the Request for United States Provisional Patent number 60 / 080,268 filed on April 1, 1998.

BACKGROUND OF THE INVENTION The present invention relates to a system and method for electrical stimulation of the inner ear. More particularly, the present invention relates to an implantable device for the electrical stimulation of the VIII nerve. Still more particularly, the present invention relates to an implantable device for electrical stimulation of the eighth nerve to produce the sensation of hearing. It is well known that the impulses of the brain and nerves are electrical in nature. It is also known that electrical stimuli applied to receptor centers such as nerves cause a reaction dependent on the electrical characteristics of the stimuli. Many devices use these features to compensate for the defective performance of the body's sensory organs. In normal hearing, the ciliary cells are a critical link in the chain of hearing. They serve two functions in association with the brain: (1) they establish a background nervous activity that is perceived as silence ("active silence" as described below); and (2) when sound enters the ear, they generate a potential that varies and modulates this background nervous activity in response to sound. This resulting nervous activity is a constant plus that derived from atmospheric pressure. This derivative or rate of change of pressure carries the information of the sound. Important for the present invention is the recognition that the ratio or frequency or density of the resulting nerve activity can be seen as a sound-modulated carrier. In a deeply deaf patient, the main cause of deafness is loss of function of the ciliary cells. In 30% of the deaf, the loss of nerve fibers from the spiral ganglion to the ciliary cells that do not work is a cause that contributes to deafness. This can be caused due to inactivity of the nerve fibers of the ciliary cells towards the spiral ganglion. Therefore, to restore the hearing of a person with partial or total (profound) hearing loss, a replacement of these functions is required past the point of loss of function, which is a superior link to the brain. In the case of the ear and the associated hearing functions, many devices have been designed to electrically stimulate the auditory nerve of the human body, which is known as the VIII cranial nerve. However, these devices operate on principles derived from an inadequate extrapolation of certain observations made by Beckesy in the thirties. Beckesy's observations refer to the basilar membrane, which extends the entire length of the cochlea. These observations revealed that the basilar membrane vibrates in response to sound vibrations that enter the ear. Beckesy observed, and others confirmed that the sound vibrations caused the membrane to vibrate with a standing wave where the maximum amplitude of the standing wave occurs at a location on the membrane that depends on the frequency of incoming sound vibrations. The activity of the individual ciliary cell in these places was also particularly pronounced in the places of maximum amplitude of the standing wave. The high frequencies result in a maximum amplitude at the entrance of the cochlea. As the frequency decreases, the place of this maximum amplitude moves towards the final end of the cochlea. Although this mechanical action is true and the activity of the individual ciliary cell is emphasized in these places of maximum amplitude, other people have inadequately extrapolated these observations to conclude that the hearing is effected by the response of the individual nerve fibers throughout of the length of the cochlea that were dependent on frequency. In this way, the theory developed, known as the Theory of the Place of hearing, that the nerve fibers in the cochlea conduct different frequencies to the brain depending on their location in the cochlea. It is curious that the absolute length of the cochlear duct, which varies from 5 millimeters in the chicken to 100 millimeters in the whale, does not seem to play a very important role in the frequency range of the cochlea, that is, the whale has a range of frequencies slightly larger than a chicken although the Theory of the Place of the hearing would suggest that, with a cochlea that is twenty times greater, the range of frequencies of the whale should be twenty times greater than that of the chicken. Theory of the Place of the hearing requires that the nerves of the cochlea operate in a different way from the way in which all the other nerves in the body operate. The present invention is based on a listening pattern that is entirely different from that of the Theory of the Place. This invention, in comparison with the Theory of Place, is based on the application of principles of the signal processing to the function of the nerve fibers of the VIII nerve ending in the vestibule and the cochlea, very similar to the operation of the receptors Modern communication systems that use digital signal processing to reduce noise and process information. The nerves that end in the vestibule and / or the cochlea that transfer the sensation of sound are not specific and can be triggered in sequence or as a sustained background nerve activity by a single impulse which, when modulated, produces the sensation of sound of modulation during a given period of time. In accordance with the foregoing, given the principles guiding the present invention, the nerve fibers in the VIII nerve operate in a manner identical to that of the entire body. In particular, the signal sent by the nerves is not specific but the number of nerves that is triggered and the reason for the tripping transports the information to the brain that the brain translates into sound. The number of nerve fibers that are triggered simultaneously or at a rate of repetition so high that they appear simultaneous is a function of the instantaneous sound intensity, the variations of this nerve activity are perceived as sound. The hearing model upon which the present invention is based recognizes that many nerve fibers of the cochlea have other functions than sound conduction. It is recognized that the very regular and orderly spatial arrangement of the sensory elements in the cochlea predisposes it to work based on special principles, however, not according to the Theory of the Place of hearing. It has been observed that the stimulation of many of these fibers does not produce the sensation of sound. The brain uses the cochlea as a mechanism to control the vibrations of sound pressure as a result of sound vibrations and by this serves as a means to control the volume. In this way, some of the outer ciliary cells of the cochlea capture the movement of the basilar membrane, transmitting this information to the brain which in turn sends signals back to many of the ciliary cells in the cochlea to control the rigidity of the basilar membrane and by this control the mechanical impedance at the entrance of the vestibule to the cochlea. This then allows automatic volume control (in the mechanical domain) and possibly a means to control the frequency response to improve intelligibility. The change in the mechanical characteristics of the basilar membrane changes the mechanical transfer of energy to the ciliary cells, thus effecting sensitivity and frequency response. The cochlea can also contribute. to the sound localization process. The audio signals of the voice and music are found to have the majority of their energy concentrated in the low frequency ranges. To achieve an increase in the ratio of signal to noise, pre-accentuation (elevation of the gain of a signal) of the high frequencies and a corresponding deceleration in the detection in the brain should be observed. Consistent with this notion, Beckesy published in 1960 that the vibration patterns of the cochlear partition of cadavers for several frequencies showed a pre-accentuation of the high frequencies in the first distance of 10 millimeters from the abutment. At ± 91, Rhode published a graph of the input-output ratio, in decibels, for the hammer and basilar membrane (Figure 21A). The graph shows an increase of 10 decibels per octave (or 20 decibels per decade) of frequencies between 200 hertz and 8 kilohertz. Also Figure 21B shows that a wide range of frequencies stimulates the ciliary cells in this area. These observations tend to support the concept of pre-accentuation. The observations also suggest that the outer ciliary cells of the cochlea function to provide information to the brain to control volume, dynamic range and have an effect on the frequency response rather than transmitting the sensation of particular frequencies to the brain. Furthermore, it is not generally known that the nervous activity that produces sound consists of the sum of the nervous activity in response to external sound or stimuli that modulate a constant background nervous activity. This constant background nervous activity was described by R. Lorente De No in 1976 as follows, "In the absence of peripheral stimulation, acoustic nuclei are the site of continuous activity maintained by the arrival of nerve impulses initiated simultaneously in the cochlea. It is necessarily accompanied by the circulation of impulses in chains of neurons, since the spontaneous activity in the cochlea and in the acoustic centers is perceived by humans as silence, it must be concluded that the spontaneous activity serves to determine the background states of several subdivisions of the acoustic nuclei, to which the deviations caused by the sound refer, in other words, what we hear is the result of those deviations of the fundamental states or the baseline signal of the acoustic nuclei, which are caused by external sources of sound ". He calls this background state "active silence" to which the perception of sound refers. Although others have observed this activity, nobody has recognized it as a carrier that is the sum of non-specific nervous activity and modulated by external stimuli. The recognition of this principle is an important element of the present invention. This recognition is consistent with the data sample-theorem developed by Hartley of Bell Labs and Nyquist in 1928 when "active silence" is considered as a carrier frequency. It is not necessary that the nervous activity be a sequence of a single nerve fiber activity but that the nervous activity be at a high frequency such that it is beyond the range of the audible sound, cuts or multiples of simultaneous nerve connections can occur. Active silence can be compared with the molecular activity of a gas at a given pressure (silence) and the modulation of this activity by pressure variations due to sound. The mechanical characteristics of the basilar membrane at the entrance of the cochlea (see Figure 21A and 21B) are such that the modulation is maximal for high frequencies and reduced to a ratio of 6 decibels per octave at the lower frequencies. The voice and music audio signals are found to have the most of their energy concentrated in the lower frequency ranges. The accent of the high-frequency components of the audio signal is introduced before the noise is introduced into the nerve activity, to the point where they produce a constant deviation of the background nerve activity as a function of the frequency. This equalization, of the low frequency and high frequency portions of the audio spectrum, allows the signal to completely occupy the bandwidth of the neurons communications link. The spectrum of the noise introduced in the output of the sum of the nerves occupies the total bandwidth. The noise-power spectrum at the summation output is accentuated at the higher frequencies. At the output of the sum of the nerve fibers, the inverse function, the de-emphasis, is introduced into the higher frequency components, which restores the original signal-power distribution. This de-stressing process reduces the high frequency components of the noise as well and thus effectively increases the signal-to-noise ratio. This function of accentuating high frequencies compensates for an inverse function at the far end of the nerve bundle in the brain. It is similar to the accentuation of high frequencies in a frequency modulated transmitter and subsequently the attenuation of high frequencies in the receiver. The result, with the compensation of the basilar membrane of the "receptor" functions of the brain, is an improved signal-to-noise ratio. A secondary feature of the cochlea is that all frequencies in fact stimulate the nerve fibers near the vestibule with pre-accentuation. High frequencies dominate the input and low frequencies dominate the other end. However, the uptake of a frequency is not related to which nerve fibers are stimulated but to the change in the activity of the global nerve fiber when the sum of all the activity of the nerve fibers is observed (see Figure 20). The previous function of the cochlea could be compared with a system of loudspeakers for bass, intermediate range and for high-pitched sounds. When the sounds reach the ear, an individual hears the sum of the activity of each of the speakers. Similarly, the brain receives signals that constitute the sum of the activity and signals sent by the ciliary cells and the associated nerve activity. However, importantly, the nervous activity associated with each stimulated ciliary cell makes a contribution to the nervous activity added completely independent of the contribution made by the nerve activity of other stimulated ciliary cells but is effected by another nervous activity. Thus, frequently, when observed in isolation, nervous activity appears to be frequency selective. However, when viewed closely in light of the recognition of the spontaneous or background activity of the ciliary cells as a carrier frequency for the received sound stimuli, the recognition of the present invention that the modulation of the nerve activity of Bottom line or spontaneous is what you "hear", not the nerve activity associated with individual ciliary cells, it becomes apparent. Although it is true that different frequencies can increase the activity of nerve fibers in different areas of the cochlea, this does not affect the transmission of sound. The added change in the nerve activity of the vestibule and the cochlea is heard as sound, not which nerve fiber is activated at some time or when a given frequency is heard. This concept was first suggested by Rinne in 1865 but he did not have a formal theory to support it. In 1880 Rutherford provided a plausible explanation, the TELEPHONE THEORY. However, at that time little was known about the characteristics of nerve fibers and it would have to be almost 50 years before the DATA SAMPLING THEORY of Hartley and Nyquist. The physiological characteristics of the VIII auditory nerve are equally important to design any system based on the theory of hearing adopted previously. In particular, five characteristics play an important role in designing some of these systems: force-duration, flow production, latency, recovery, and fatigue. The force-duration characteristics of human nerve fibers are plotted in the strength-duration curve shown in Figure IA. The force-duration curve expresses the relation between the shortest duration of an applied current (stimulus) to the nerve fiber and in the shortest time during which the current (stimulus) must flow to reach a threshold for excitation. Expressed differently, the force-duration curve is a graph of the threshold intensity just able to excite an axon and its relation to the duration of the stimulus current. Undoubtedly, nerve fibers will not be excited in response to current densities below a minimum. The force-duration curve is further described in Medical Physiology and Biophysics, Ruch and Fulton, Eighteenth Edition, W. B. Saunders and Co. Ruch and Fulton model this nervous behavior from a single capacitance resistance circuit. The duration of the force combined with a gradient electric field determines the range of impulse length 'for a stimulus impulse to cause "flow production". Flow production is the sequential firing of nerve fibers within a group of nerve fibers or ganglia that have been stimulated by a single impulse stimulus. After stimulation by a single impulse stimulus such as a single square wave through the action of a gradient electric field striking a group of nerve fibers or ganglia, each of the individual nerve fibers within the group will receive a decreasing intensity stimulus as the individual nerve fibers within the group increase in distance from the source electrode. This phenomenon is shown in Figure IB. The trigger ratio of the individual nerve fiber will correspond to that shown in the force-duration curve of Figure 1A. Thus, when the individual nerve fibers within a stimulated nerve group start to fire, those closest to the origin of the gradient field will shoot at a higher ratio while those farther away will shoot at a slower rate. In this way, the nervous group will transmit a series of signals, that is, a production of nervous activity flow, over time. In particular, this "flow production" is characterized by the fact that some members in the group will shoot in succession, these successive shots will be presented at a slower rate than the previous shots. Flow production occurs during the last portion of the single nerve impulse stimulus. The length of the flux production of a group of nerve fibers is limited by the delay of the start of flux production at the beginning of the impulse (not earlier than 0.1 milliseconds according to the force-duration curve of Figure IA) and the time remaining for the end of the stimulus pulse. This behavior is shown in the upper portion of the graph of Figure IA by the line labeled "Nerve Trigger Ratio". The latency period is the delay between the start of the stimulus impulse and the firing of a nerve fiber. With reference to Figure 1A it is shown that the latency time for each nerve fiber is different as defined by the force-duration curve and the gradient field. In practice, it is desirable to cause the start of flow production to be delayed by more than 0.2 milliseconds. As the latency time is shortened by increasing the amplitude of the stimulus, the compensation component necessary to maintain the constant differential latency times during flow production requires increasing the stimulus to an excessive amplitude for a system with a long flow production time. , (as in the channel 4 system). The minimum latency period also determines the overlap time for adjacent channels ie the time in which the adjacent fiber or other nerve fibers must be stimulated to continue the transmission of the total signal once the original nerve fiber channel enter the recovery stage. Moreover, the individual fibers of the VIII nerve are not able to transmit stimuli indefinitely. After receiving and transmitting a stimulus, the nerve must go through a recovery period. If this does not happen, the nerve will be fatigued and stop transmitting. The nerve recovery characteristics limit the repetition rate of the stimulation of the individual channel. The nerves are eventually damaged by prolonged stimuli with an average direct current component. All these stimuli must be done in an alternating current manner. When an electric field strikes the auditory sensory branch of the VIII nerve that includes the brainstem or spiral ganglion, the angle of arrival produces a gradient field through the nerve group that can cause the nerves to fire in sequence. This is illustrated in Figure 1C, which indicates how the strength of an electric field decreases through a group of nerve fibers, between a cathode and an anode. Because of this decreasing force electric field, the nerve fibers are triggered in sequence. Furthermore, due to the combination of the electric field and the force-duration characteristics of the nerve fibers, as the cathode distance increases, the time between successive nerve firings also increases. In contrast, with the arrangement shown in Figure ID, all nerve fibers are subjected to substantially the same electrical potential and thus will shoot substantially simultaneously. If the amplitude of the stimulus is small, so that it does not produce a carrier frequency high enough to be above the range of hearing, the frequency of flux production can be heard. Its frequency is a function of the place of the stimulus on the force-duration curve in relation to the stimulated nerve fibers and the slope of the curve. This varies with the time and amplitude of the stimulus. As mentioned above, the signal sent by the nerves is not specific, but the number of nerves that are triggered and the speed of shots transports information to the brain that the brain translates into sound. For example, as the amplitude increases, the rate of firing of the sequential nerve fibers increases. If the angle of a stimulus is almost perpendicular, a high ratio of sequential shots will be present since the individual nerve fibers of a channel receive almost the same stimulation. If the angle is small, the sequential trigger is at a lower frequency since the difference in stimulation through the stimulated individual nerve fibers will have a greater range. The known devices that have been designed to help the deep deaf by means of electrical stimulation of the VIII nerve but on principles guided by the Theory of the Place, nevertheless, work mainly due to this dependence of angle and stimulus, but with results that are not predictable , repeatable or optimized. In U.S. Patent No. 3,449,768, issued to James Doyle, the system was not designed based on principles of the Theory of the Place of Hearing but was designed to produce a carrier of nerve activity based on multiple channels stimulated in sequence at a sufficiently high ratio to result in a nerve activity carrier suitable for modulation with sound information. That patent describes a device for applying electrical stimuli to the VIII cranial nerve and includes a system of electrodes for placement in the neighborhood of the auditory nerve, means for feeding impulses to a plurality of transmission channels and a modulator that modulates an integral of time- amplitude of each of the impulses. This system was limited due, for example, to the number of channels required and the recovery time allowed for each channel was too short to allow prolonged stimulation without causing nervous fatigue. No consideration was made to the latency characteristics of the nerve fibers (the delay between the start of the stimulus impulse and the firing of a nerve fiber). Moreover, Doyle's previous system failed to allow compensation in the stimulus force to maintain a constant nerve firing ratio and overcome the inherent slowness due to flow production, as described above. Finally, Doyle's previous patent did not recognize or show that the carrier frequency (the frequency or density of background nerve activity) is independent of the reason at which the individual channels are being fired and the number of individual channels. These limitations or failures resulted in a system with low sound fidelity, a signal-to-noise ratio that is lower than what can be achieved otherwise, and a constant tone or hum perceived by the patient.

SUMMARY OF THE INVENTION An object of this invention is to improve the systems for stimulating the auditory nerve of the human body. Another object of the present invention is to improve the system described in U.S. Patent No. 3,449,768, issued to James Doyle. Another object of this invention is to produce continuous nerve activity that limits the spontaneous nerve activity present in normal hearing and modulate this nerve activity with an audio signal to provide hearing. Another object of this invention is to provide a new system for stimulating groups or bundles of nerve fibers of the VIII cranial nerve in a manner that causes production of channel flux at a constant rate. Still another object of the present invention is to carry out this constant ratio flow production in a manner that provides the force-duration characteristics of the ribs. Another object of this invention is to modulate this channel flow production with audio information to produce hearing for the profoundly deaf or those with other auditory impairments. The present invention provides a novel system for stimulating groups or bundles of nerve fibers of the eighth cranial nerve so as to cause flux production of channels at a constant rate. With the preferred embodiment of the invention described herein in detail, unlike any prior art device, this constant ratio flow production is carried out in a manner that provides strength-duration characteristics of the ribs. In addition, the system modulates this channel flow production with audio information to produce hearing for the deep deaf or those with other hearing impairments. The present invention further provides a method for stimulating these groups or bundles of nerve fibers. In accordance with the preferred embodiment of this invention, an electrical stimulus is applied so as to cause nerve firing at a constant ratio. The increase in the force of the stimulus provided by the device of the present invention is governed by the strength-duration characteristics or behavior of the nerve fibers or ganglia. By adjusting the gradient field generated by the electrodes placed in proximity of the VIII nerve for the characteristics of the nerves represented by the force-duration curve, - the individual nerve fibers located within the gradient field do not need to trigger simultaneously when they are stimulated, as it is taught by the prior art, but will be triggered in a sequence of nervous activity as time progresses. Preferably, the stimuli do not occur at the ends of the force-duration curve. This is because the high voltage required to obtain a very short latency period can produce undesirable electromechanical reactions. Also, a long latency period results in excessive channel overlap and reduces the time available per channel for flow production. For example, preferably, the latency period is maintained between 0.1 and 4.0 milliseconds; and even more preferably, the latency period can be maintained between 2 and 3 milliseconds. It should be noted that the preferred latency period may vary depending on the specific subject or patient, and, with some individuals it may be appropriate or preferred to operate outside the ranges described above. While this production of nerve activity flow is carried out, the device of the present invention modulates the stimulus impulse to vary the ratio of nerve activity and cause the transmission of signals to the brain by the VIII nerve or the ganglia that are perceived as hearing. It also modulates the combined stimulus impulse and its audio modulation to cause the flow production to remain constant when no sound is present. Therefore it follows that as the amplitude of the stimulus impulse increases, the percentage of modulation due to sound remains constant. In other words, as the amplitude of the stimulus increases, so does the audio modulation component. In embodiments of the present invention in which more than one channel is used, the adjacent channels overlap, due to the characteristics of latency and recovery of the nerve. This is done in a way that transmits a constant stream of summed channels. The device of the present invention therefore provides a constant flow of nerve activity independent of audio modulation that acts as a carrier wave but which will not be perceived by the brain as sound but simply as "active silence" as the term is understood in the technique. In addition to stimulating the nerve fibers so as to create a carrier wave, the present invention further utilizes this carrier wave to cause the sound sensation by modulating it at appropriate times during the duration of the stimulus signal and the response of the nerve fibers. The devices made in accordance with the teachings of the present invention contain a means to generate a background nervous activity, perceived by the brain as silence. These devices use this background nerve activity as a carrier of audio information and modulate this nerve activity with audio information. This modulation causes a variation of the density of background nerve activity which is perceived by the brain as sound. However, this modulation is importantly carried out in such a way that the frequency or density of the background nerve activity is independent of the number of electrical channels used by the device (as long as the number is greater than one) and the reason to which any given channel is being stimulated. In this way, the reason for which any stimulus is applied to any channel, and the duration of any pulse, is secondary in function to the main objects of this invention. Undoubtedly, the added nerve carrier frequency can then be produced at more than one thousand cycles per second, which exceeds the recovery time for a single nerve fiber, and can be independent of the modulation frequency. A method is described for causing a flux of nerve fiber activity resulting in a background state of nerve activity on the audio transmission branch of the VIII nerve to flow to the brain and modulation of this flux (pseudo-carrier) with audio information. A device is described to stimulate the auditory transmission branch of the VIII nerve. It uses electrodes designed to restrict the electric field to the region of its respective nerve fiber group and produce a gradient field for each channel so that the latency characteristics of the nerve fibers in a given channel will cause a sequential trip (flow production) of the nerve fibers during a portion of the channel stimulus pulse. In the analog system the nerve fiber channels are stimulated in sequence but their stimuli overlap the previous channel by an amount equal to the shortest latency period of the channel. During the period when the stimulus pulse is causing nerve fiber flux production, the amplitude of the pulse is modulated with audio information. Also during the production of nerve fiber flux either the electrical means or the shape of the probe or both, through the force of the stimulus, they compensate the force-duration characteristics of the individual nerve fibers so that when it is not present some sound the reason for the flow production is substantially constant resulting in an "active silence", that is, a minimum of sound sensation. In this way, the stimulus impulse is divided into two periods. In the first period, the nerves, to which the impulse is applied, do not fire. All nerve firings occur in the second period of the stimulus impulse. It can be noted that these two periods, typically, are not equal in length; and in fact, with the specific examples described herein in detail, the second period is substantially longer than the first period. The desired flow production - that is, sustained nerve activity and a uniform ratio - can be caused by changing the stimulus impulse during the second period or both the first and the second period. However, the audio modulation of the flow production is only done in the second period of the stimulus pulse. In the digital system, there is no flow production since there is only one trigger time for the nerve fibers in a channel. The channel overlap exists on a number of channels and the audio modulation is in the form of frequency modulation of the frequency of the flow production and is independent of the frequency of the channel sequence. One aspect of the present invention involves producing or enhancing a carrier or background nerve activity that is perceived as silence and modulating the background nerve activity (a pseudoporter) to produce the sensation of sound and restore a degree of hearing when the organ of sound is totally defective. Another aspect of the preferred embodiment of this invention involves a system that transforms sound into a corresponding electrical signal and includes a coding device for converting the analog signal into a train of nerve impulse stimuli applied to at least two groups of nerve channels simultaneously . In addition, the preferred system includes a transmitter coupler, a receiver coupler, and a multi-channel gradient probe to print electrical stimuli to a bundle of nerves and elements to independently adjust each channel stimulus amplitude. Another aspect of the preferred embodiment of the present invention includes an electrode system within the vestibule and / or cochlea to stimulate the individual nerve fibers of the auditory nerve therein, generating the activity of nerve fibers that is transmitted to the brain in u? simple impulse pattern, by which a background nervous activity (pseudoportadora) is modulated by audio information, whereby the modulation of the density of background nerve activity is perceived as sound. See Figure 20. A further aspect of the present invention includes generating a carrier frequency of nerve activity not dependent on the number of stimulus channels and allowing time for "activated nerve fibers to recover sufficiently so that fatigue does not occur in stimulated nerve fibers Another aspect of the present invention involves an element for the stimulation of any number (N) of different fiber groups or portions of the spiral ganglion of the sensory branch of the eighth nerve, set at N spaced time intervals with a portion of each adjacent group overlapped. The time interval between the repetitions of any stimulus group is substantially greater than the natural recovery time of a single nerve fiber or portion of the spiral ganglion after the electrical stimulus. 5 milliseconds are chosen in the preferred modalities to avoid fatigue of the nervous group after an applied stimulus. This represents approximately 5 time constants of the recovery time of a nerve, leaving a residue of approximately 1% of the previous stimulus. An advantage of the present invention is that sufficient recovery time is allowed for the nerve fibers to recover from the previous stimuli so that the nerve fibers are not fatigued. Another advantage is that the present invention provides a continuous flow of nerve fiber activity not directly related to the repetition rate of the channel thereby avoiding the limitations of the system described in U.S. Patent No. 3,449,768 requiring a ratio of Channel dictated by criteria of the Data Sampling Theory. Advantageously, the energy required for the nerve stimulus is reduced due to the proximity of the stimulus electrodes to the nerves in the audio transmission branch of the eighth nerve. Potentials less than 1 volt are sufficient to trigger a nerve fiber without causing electrochemical reactions of the metal / tissue. The use of stimulus pulses of a cycle having an average direct current value of 0 reduces the possibility of electrochemical reactions of the metal / tissue. Hereinafter, the term "bi-phasic" is used to refer to the stimulus pulse having an average direct current value of 0. US Patent No. 5,674,264 mentions that the manufacturers of implant systems of the cochlea have to be careful to control the voltages of the electrodes to keep them in a region where any electrochemical reaction is present at too slow a rate to cause damage. Advantageously, the embodiments of the present invention effectively eliminate these reactions. There are two limits in the use of the force-duration curve. If the latency period is too short, the amplitude of the stimulus will be high, but more important the compensation to keep the flow production constant for an extended period of time will require a very high stimulus. This can put the electrodes at risk of causing electrochemical reactions. As the number of channels increases, this effect is less pronounced. The present invention ensures a constant background nerve activity during a single production of nerve fiber flux. The present invention accomplishes this cancellation of the nonlinear characteristics of nerve fiber flux production, as represented by the time constant of the force-duration curve. This cancellation can be carried out by one or both of the following techniques: (a) modulate the stimulus impulse with a similarly canceling time constant; or (b) cause the gradient field emanating from the electrode to take shape in a manner that cancels the force-duration curve. The embodiments of the present invention provide normal sound sensations to the receiver. For those who have heard in the past extensive training is not required to interpret the sound. The present invention will also produce sound sensation for those whose ciliary cells and nerve cells have been destroyed. Frequently, the cause of deafness or defective hearing is due to the destruction of the ciliary cells in the ear. Under these circumstances, the stimulation of the nerves in the manner described herein will produce the sensation of sound. However, in some circumstances, the nerves that go to the ciliary cells in the ears of deeply deaf patients or patients with defective hearing are also destroyed. In these cases, the present invention will produce sound sensations by electrically stimulating the spiral ganglion in the manner provided herein or by stimulating at a higher level as in the brainstem. In summary, the present invention produces continuous nerve activity that mimics the spontaneous nerve activity present in normal hearing and modulates this nerve activity with an audio signal to provide hearing.

Brief Description of the Drawings Figure IA is a graph of the force-duration curve and illustrates a gradient electric field imposed in a linear array of nerve fiber terminations and the effect of force-duration characteristics of a nerve in the time when each nerve fiber is triggered in relation to the strength of the stimulus received. Figure 1A also shows the ratio of logarithmic nerve firing or flux production. Figure IB shows a graph illustrating in two dimensions a channel of a gradient probe and the gradient field striking the nerve endings in the vicinity of the probe. Figure 1C and ID illustrate electric fields at two different angles in relation to a group of nerve fibers. Figure 2 is the schematic of the clock signal generator, modulator and four phases for a 16-channel system. Figure 3 is the schematic of the ramp generator for a 16-channel system. Figure 4 is' the 16-channel multiplexer scheme. Figure 5 is a diagram showing the timing relationship between the 16 channels, the electric waveforms that include force-duration compensation, and the continuous force activity on the auditory branch of the eighth nerve. Figure 6A is a diagram showing the increase in the force of the stimulus to compensate for the force-duration characteristics of a medium to produce a constant ratio of nerve activity. Figure 6B shows the audio modulation imposed on the last portion of the stimulus pulse. Figure 7 is a configuration of a 16 channel multichannel gradient probe.

Figure 8 is a block diagram of a 4-channel system. Figure 9 is a 4-channel clock, sequencer and modulator scheme. Figure 10 is the schematic of one of four compensation circuits of the force-duration curve. Figure 11 is a block diagram of the interconnections between Figure 8 and Figure 9. Figure 12 is the door schedule of the latency period. Figure 13 is the 4-channel output attenuator scheme and one of the four digital-to-analog converters used to perform channel gain adjustment. Figure 14 is a graph of the wave and timing forms of the 4-channel system including the modulation of the last portion of the stimulus waveforms. Figure 15A is a drawing of the 4-channel probe using electrodes perpendicular to the probe and an enlarged ground-simple. Figure 15B is a drawing of the 4-channel probe showing its suitability to the shape of the scale eardrums and their location relative to the spiral ganglion. Figure 16 is a block diagram of the digital system.

Figure 17 is a diagram of the external power source, the microphone, the frequency modulator and the radio frequency coupler of the digital system. Figure 18 is a diagram of the internal unit of the digital system. Figure 19 is a diagram of the waveforms of the channels of the digital system. Figure 20 is a diagram showing the changes in activity of the VIII nerve as a result of the displacement of the acoustic basilar membrane. (Honrubia V, Strelioff D, Stiko S, Ann Otol Rhinol Laryngol 85: 697-701, 1976). Figure 21A graphs the output input ratio, in decibels, for the hammer and the basilar membrane. (Rhode WS: Ann Otol Rhinol Laryngol 86: 610-6126, 1974). Figure 21B shows the vibration patterns of the cochlear partition of the corpse specimen for various frequencies. (Beckesy: Experiments in Hearing, New York, McGraw-Hill, 1960). Figure 22 shows a block diagram of the system. This includes an implant that uses a microprocessor, the external unit for the patient, a test computer and a brief sample of the computer screens. Figure 22A represents the ability to adjust the amplitude of each stimulus channel independently, both amplitude and gain, and to select the channel frequency. Figure 22B represents the adjustment capacity of the force-duration compensation of each channel, to select an initial time constant for all channels and the ability to trim the force-duration compensation in one base per channel. Figure 22C provides the ability for the optimal channels of the probe to be connected to the stimulator, to adjust the stimulus level for each channel, to control the soft start ratio, and to input a stimulator or identification number. for a permanent record in the patient's file.

Detailed Description of the Preferred Modality The nerve fibers in the VIII nerve that transmit audio information to the brain are divided into N separate sections. Each section consists of a number of nerve fibers or portions of the spiral ganglion. Each section is stimulated independently by an electrical impulse, which is divided into two periods of time. During the first period, the amplitude of the stimulus of each section is maintained at a constant so that the activated nerve fibers have a latency period substantially equal in time to the other sections. During the second period, the pulse amplitude will vary so as to cause some of the remaining nerve fibers in that section to be activated at a constant rate at the end of the stimulus pulse. This compensation can also be presented during the first period as it remains fixed and does not contain any audio component. During the second period the audio modulation is superimposed by varying the nerve-trigger ratio. If the compensation of the pulse amplitude includes the first period, it is still important to recognize that the audio modulation overlaps only in the second period, that is, when the production of flux in the nerve fiber group is occurring. The sections are stimulated cyclically. The stimulation section N + l begins at the moment when the second period begins at the end of the stimulus impulse of the N-th section. During the second period of each channel stimulus pulse, the audio information modulates the stimulus impulse. Since the second period of consecutive channels is presented without any time gap between them, the flow of the audio modulation information is continuous. The electrical probe used to stimulate a section of the nerve fibers is configured so that the amplitude of stimulation is different for different nerve fibers in that section. This causes the latency periods to be different for different nerve fibers, causing the nerve fibers to fire sequentially during the second period.

Figure IA shows a graph illustrating a gradient electric field 12 imposed on a linear array of nerve fiber terminations. The graph illustrates a force-duration curve 14 in (relative) volts per milliseconds. The force-duration curve is a graph of the intensity of the threshold just able to excite an axon and its relation to the duration of the stimulus current. The force-duration curve expresses the relation between the lower force of an applied current and the shortest time during which the current can flow in order to reach a threshold for excitation. There is a minimum current density below which excitation does not occur. The force-duration curve does not show the effects of stimuli below the threshold after excitability. Current flow below the threshold can advantageously be used in a preferred embodiment of the present invention as a means of overlapping adjacent channels or to increase the background state of nerve activity. Figure IA further shows at 16 a trigger sequence of a group of nerve fibers when a stimulus pulse is applied which has a potential gradient that causes the amplitude of the stimulus of each nerve fiber to be different than its adjacent nerve fibers. Figure IB shows a two-dimensional graph illustrating a channel of a gradient probe and the gradient field. Figure IB shows the location of the active and ground electrodes 20 and 22 for a single stimulus channel. The active electrode and each of its associated ground electrodes produces a gradient field between the electrodes and for a small distance above the electrodes. The gradient probe is placed so that the nerves to be stimulated are in this gradient field either between the electrodes or immediately adjacent to the electrodes. Figure IB also shows the gradient field 24 in terms of a voltage. The illustrated range for the voltage is exemplary and does not represent the actual voltages used. Those skilled in the art will appreciate that nerve fibers can be stimulated with as little as 100 minivolts. 16-CHANNEL STIMULATOR CIRCUIT A 16-channel system that uses a repetition rate of 200 hertz produces a channel-switching frequency of 3,200 hertz. It requires an average flow production of 7.5 nerve fibers per channel to achieve a 24-kilohertz neuron-carrier frequency. Modulation of the amplitude of the impulse stimulus (to provide the sound sensation) is transformed into frequency modulation of the neuron-carrier frequency. The precision in tracking the force-duration curve is not required since only a small portion of the strength-duration curve is used. With this number of channels, the flow production time per channel is only 0.25 milliseconds and the variation of the slope during this time is small. Figure 2 is the schematic of the clock, modulator and signal generator. Its function is to appropriate the modulation of a portion of biphasic pulses. Starting at the left of Figure 2 are terminals 30a and 30b of the power source of +6 volts and -6 volts. Through both the positive and negative terminals are filter capacitors Cl and C2 to provide stable, change-free voltages due to variations in current requirements. The Schmitt trigger IC1 (1/6 of 74C14) together with Rl and C3 form an oscillator, which provides the system clock. The values of Rl and C3 determine the clock frequency. The output of the Schmitt trigger controls a 4-bit binary counter (74C93) to IC2. This counter divides the clock signal by 16. The two least significant digits of the counter output IC2 control the least significant inputs of a 4028 BCD to Decimal Decoder IC3. Both Most significant inputs are connected to ground making the 4028 in effect a 2-digit binary decoder. The four outputs that represent logical values from 0 to 3 (4 separate states) control the 4001 Quad 2-Input ÑOR Gates IC4. The 4 outputs of the ÑOR doors are biphasic signals that swing between the power supply rails of +6 volts and -6 volts and are phase shifted by 90 degrees (see Figure 2). These outputs control the 220K R 2 resistors, 3, 4 and 5. These outputs will be added to the modulation signal. In the lower right part of Figure 2, the modulation input goes through a 10K resistor R14 to the input to the middle of a dual 4-channel analog multiplexer 4052 IC5. The channel selection inputs of IC5 are in parallel with the 4028 IC3 inputs. The outputs of IC5 are modulation signals delayed by 90 degrees from the outputs of IC4. -The outputs of IC5 control the resistors of 220 K, R 6, 7, 8, and 9 that are added with the outputs of R 2, 3, 4, and 5. Timing causes that the modulation is not only imposed in the components of the last half of the biphasic pulses. R14 is a resistor in series with the modulation signal. It provides an input resistance of 10K to IC5 from the modulation source. The outputs of the 4052 also go through resistors R 10, 11, 12, and 13 to electrical ground. Its function including R14 is to keep the impedance substantially constant at the input to the resistors R 6, 7, 8, and 9 so that the value of the summed outputs remains substantially independent of which channel IC5 is selected. There are three outputs of this circuit. In the lower left is the 4-digit ADDRESS BAR, which is controlled by the outputs of IC2. On the right is the SIGNAL BAR that contains the biphasic 4-phase commuted analog stimulus signals that include their modulation, and the ramp control signals in the bottom center. In the lower right of the center are the outputs of the RAMP CONTROL. Figure 3 is the schematic of the ramp generator. To the left of the Figure are the three control signals coming from the ramp control of Figure 2. Line A leads a control line of a bilateral switch 4066, IC7 and through an IC6 inverter to a second control line of IC7. The phase A and phase B signals coming from Figure 2 charge capacitors C4 and C5 through resistors R32 and R15. When the IC7 switches are open, a voltage ramp is displayed. IC8 and IC9 are operational amplifiers each having a positive gain of 11 due to negative feedback through resistor networks R16 and R17 and R18 and R19. Resistors R20 and R21 introduce positive feedback causing the ramp on C4 or C5 to produce an upwardly curved ramp similar to the curve of the force-duration curve at the output of the amplifiers. R22 and R23 add these ramp output signals from IC8 and IC9 to the SIGNAL BAR with signals A and B of Figure 3. ICIO and IC11 are connected operational amplifiers with a gain of -1. Resistors R24 and R25 and R26 and R27 determine this. Resistors R28 and R29 reduce the drag of the direct current keeping the two inputs of each amplifier at the same impedance. R30 and R31 add the outputs of ICIO and IC11 to lines C and D of the SIGNAL BAR. Resistors R22 and R23 add the outputs of IC8 and IC9 to lines A and B of the SIGNAL BAR. Figure 4 is the 16-channel multiplexer. The lines of the STEERING BAR of Figure 2 are connected to the entrance of the address bar in the lower left part of the Figure. The STEERING BAR leads the IC12 address input lines, a 74C154 of 4 lines to a 16-line decoder. The output of decoder IC12 has 16 lines (from 0 to 15) which are turned on one at a time in sequence in a cyclic manner. The first 4 lines from 0 to 3 go to the input of a NAND gate with 4 inputs IC13A (1/2 of a NAND gate with 4 dual inputs 4012 IC13). The output of the door on NAND IC13A is high through the first 4 positions of the 16-line encoder. In a similar way the lines from 1 to 4 of IC12 go to the inputs of the NAND IC13B. Your departure will be delayed by an IC13A account and thus through the eight 4012 ICs, from IC13 to IC20. Note that this is done in a cyclic manner so that the output of the IC20B gate initiates a clock pulse before the output of IC13A. These outputs that last 4 clock pulses and overlap by 3 clock pulses of their adjacent channel control the analog 2-channel analog multiplexer ICs 21A, B, C to IC26A. The multiplexer when switched on selects the biphasic signal A to connect it to its output when it is turned on and the signal to ground when it is turned off. In a similar manner IC21B switches signal B, IC21C switches signal C, IC22A switches signal D, IC24B switches signal A and so on through IC26A and then again up to IC21A. When the switches are not connected to one of the signal lines A, B, C, D, they are grounded to avoid interference and prevent current leakage. The outputs of the multiplexer switches are low impedance and provide a voltage output. Resistors R33 through R48 change the voltage output to a current to drive the nerve probe. Its resistance value is substantially higher than the resistance of the electrodes of the nerve probe thereby ensuring that the nerve conduction is relatively independent of the path of resistance of the probe to the ribs. The fixed adjustment of the output current can be done by varying the voltage of the power supply which has a small effect on the clock frequency or by placing variable drift resistors through each signal line A, B, C, and D to the ground signal which will reduce the voltage change of these points and therefore reduce the current conduction to the nerve. Each variable resistor effects only 1/4 of the 16 outputs, therefore the 4 variable resistors are coupled. Figure 5 shows the waveforms of the 16 channels. As shown in the Figure, each channel is delayed by 90 degrees of a full impulse cycle. The amplitude of the stimulus pulse is set so that the production of nerve flux (or tripping) starts at the beginning of slope compensation. Note that slope compensation can occur at the start of the stimulus pulse (not shown in Figure 5) or the second portion of the stimulus pulse (as shown in Figure 5). However, modulation does not occur until the production of nerve flux is in place. Figure 6A shows the force-duration curve 40 with the stimulus pulse 42 crossing the force-duration curve and the compensation added to the stimulus pulse to generate a constant firing ratio of the nerve fibers. Figure 6B shows the audio modulation 44 superimposed on the last portion of the stimulus pulse. Both figures show the stimulus in a positive direction. This is only for simplicity and does not necessarily indicate the polarity of the stimulus impulse. Figure 7 is a drawing of a 16-channel probe. This is connected to the channel outputs of Figure 4. Figure 15A shows a 4-channel probe. Its design is typical of 4 channels of a 16-channel probe. 4-CHANNEL STIMULATOR CIRCUIT The 4-channel analog system is a configuration with the preferred minimum number of channels. Therefore, it requires a preferred maximum number of flow producing nerve fibers that are stimulated per channel and a preferred maximum of compensation for the force-duration characteristics of the nerves. To achieve a carrier-neuron frequency or 24-kilohertz flow production, suppose a repetition ratio of 200 pulses per second per 4 channels results in a total-of 800 impulses of stimuli per channel per second. If 30 nerve fibers are activated in a uniform sequence per channel, this will result in a neuron-carrier frequency of 24_ kilohertz. The modulation of the amplitude of the impulse (to provide audio sensations) then in effect becomes a frequency modulation of the neuron-carrier frequency (more or less than 30 nerve fibers are triggered by impulse). Figure 8 is a block diagram of a 4-channel system. On the left is the microphone driving an audio amplifier and the limiting / automatic gain control. The output of the audio amplifier drives the external transmitter. The internal receiver drives the clock modulator, stimulus generator, and the force-duration ramp generator. The output of this module is fed through an attenuator to the 4-channel multiplexer. The output of the multiplexer then leads to the probe. Figure 9 is the circuit of the audio preamplifier, clock, and sequencer. In the lower left of the Figure is the clock oscillator 60. IC27 is a section of a 4093 used as a Schmitt trigger. R48 and R49 provide feedback to the entrance. C6 together with the sum of R48 and R49 determine the clock frequency. R49 provides a means of adjusting the clock frequency. The clock output leads the input of a 4-bit counter 74C393 IC28 and also of the force-duration ramp (SD) generator 62. The output of IC28 drives a 4-line to 16-line decoder, IC29. In the lower right part of the Figure is the wiring of the Schmitt Nand triggers of 2 Quad 4093 wiring inputs to build set-reset retention circuits. In the upper left of the Figure are six 4093, IC30 to IC35 wired as shown to provide set-reset retention circuits. The first holding circuit IC30A is fixed at position 0 of output IC29. The second holding circuit IC30B is fixed in position 4, the third holding circuit IC31A in position 8 and the fourth holding circuit IC31B in position 12. In a similar manner the first holding circuit is reset to position 5. , the second holding circuit is reset to position 9, the third holding circuit is reset to position 13 and the fourth holding circuit IC31B is reset to position 1. The output of these four holding circuits is fed through of the resistors R50, R52, R54 and R56 to be added with the audio modulation. In the lower center of Figure 9 is the microphone input 64 which conducts a closed-loop amplifier IC36 with a gain of approximately 80. The ratio of R58, the impedance of the microphone and R59 determine the gain. The alternating current C7 is coupled to the next stage through the volume control R60. The value of C * 7 can be selected to provide pre-accentuation. IC37 adds an additional gain of 3 determined by the values of R61 and R62. The output of amplifier IC37 is the audio signal. It is powered by 4 single-pole, single-pole CMOS switches IC39B, IC39C, IC40A and IC40B. The holding circuits IC32A, IC32B, IC33A and IC33B control these switches. The arm of the switches is connected through the resistors R51, R53, R55, and R57 to be added to the outputs of the channel stimulus. The IC32A retention circuits, IC32B, IC33A and IC33B control the timing of the switches. IC32A is set to IC29 in position 1 and reset to position 5, IC32B fixed to position 5 and reset to position 9. IC33A is set to position 9 and reset to position 13. IC33B is set to position 13 and reset to position 1. The summed outputs for channels 1 to 4 are fed into CMOS switches IC38A, IC138B, IC138C and IC139A. The output of these switches selects either the summed outputs of channels 1 to 4 or a reference level to ground. In the lower left part of Figure 9 there are 4 outputs 66 that go to the compensation circuit SD. These come from IC41 and IC42. IC43 a 74C08, and AND gate, drives its establishment and reset times as follows. RESET Cl is set to position 0 and 6 (the low output sets the holding circuit) and is reset to positions 5 and 9. Similarly, RESET- C.2 sets to positions 5 and 9 and resets to positions 8 and 14. RESET C.3 establishes in positions 8 and 14 and is reset in positions 1 and 13. RESET C.4 is set in positions 1 and 13 and reset in positions 0 and 6. Figure 10 is the schematic of a channel of the four channels of the force-duration compensation circuit. In the lower left part the clock inputs' of the reset signals of Figure 9 enter. The clock signal enters the clock input 74C393, IC45 and the clock input of 74C174, IC46. The IC45 is kept at zero by the reset / clear signal. When the reset signal stops, the counter starts counting. IC46 avoids counting timing errors by capturing the counter value after some undulations have been set. The output of the IC46 register drives two digital-to-analog converters composed of 4053 CMOS switches IC47, IC48, IC49, and IC50. These switches drive two binary ladder networks LR1 and LR2. In the upper left of Figure 10 the input signal of channel 1 of Figure 9 is fed into a damped amplifier IC53 which conducts the first stair switches IC47 and IC48. The output of LR1 is fed into a damped amplifier IC51, which conducts the reference of the second converter from digital to analogue switches IC49 and IC50. In this way a square-law curve is generated to reflect the force-duration curve. This curve that is generated by the channel 1 signal also has the audio modulation generated in Figure 9. A second damped amplifier IC54 feeds the channel 1 signal through the 200K R63 resistor to the output of the second digital converter in analog. The outputs are added and fed into the damped IC52 amplifier. This circuit for the compensation of the force-duration curve is repeated for each of the four channels as shown in Figure 11. In the central lower part of Figure 11 is a block called LATENCY LOGIC 70. When this is The active momentum of each channel is shortened to the latency time, the time before the first nerve fiber is activated in a channel. It only allows one channel to be activated at a time. This is to allow adjustment of each channel stimulus amplitude that has its own latency time and is also used to establish strength-duration characteristics for each nerve group during the test. Figure 12 is the logic diagram of the LATENCY PERIOD DOOR. The signals in Figure 9 on the lower right side lead IC53 to 74C08 and AND gate. The 100K resistors R64, R65, R66 and R67 are connected to ground at one input of each AND gate. When SW1 is in position 1 the IC53 input that has been kept low by R64 becomes high allowing the IC44 signal to pass through the IC53 output. In this way each channel can be selected by SWl. When SWl is in the OFF position, a high signal is given to IC54 which causes the outputs of IC54 a 74C32 to turn on. The output of IC54 drives the IC55 analog switches, and IC56 to select ground and the respective signal channel output or allow all channels to feed through. Figure 13 is the circuit that adjusts the amplitude of each channel. IC57 is a Quad Schmitt double entry door that in combination with R69 and C7 generates a clock. The clock is' enabled when either the UP switch SW2 or the DOWN switch SW3 is activated. When the switch is activated below, a low signal leads to counters from top to bottom IC58 and IC59 to inputs down. In this way, the counter will count up or down when it is ordered. Only 6 bits of the possible 8 bits of the up / down counter are used. DI diodes up to D8 are used to prevent the one-turn counter. When the counter reaches full scale it will not return to 0, and also when the counter drops to 0 it will not return to full scale. The output of the counters IC58 and IC59 drive four digital-to-analog converters configured for the force-duration compensation circuit. This circuit in effect replaces a four-gauge potentiometer. The inputs of the four digital-to-analog converters are the output of four signal channels of the latency period gate Figure 12. Figure 14 shows the waveforms for each of the 4 channels together with their modulation component. This Figure shows an overlap of 0.2 or 0.3 milliseconds between two channels and the superposition of the modulation and compensation of the strength-duration characteristics of the nerve fibers in all but the first 0.2 or 0.3 milliseconds of each channel. Each channel stimulus pulse is biphasic to avoid introducing a direct current component into the system. The modulation is only on the nervous firing portion of the impulse. Since the modulation has an average value of 0 over time, it is not necessary to modulate the portion of the stimulus pulse that follows the modulation. The portion of the stimulation pulse that follows the modulation is an inversion and results in an average direct current value of 0. It is not shown in this Figure but the background state of nerve activity is shown in Figure 5 as typical when no sound is present. This background state when modulated results in the perception of sound. Figure 15A shows a four-channel probe 80. The conduction area of the electrodes is vertical producing a gradient field between the electrodes and providing a maximum surface area for each electrode. In this way the contact surface area is not determined by the proximity of the electrodes and a precise gradient field is produced. The distance between the ground electrodes can be as small as 20 microns and over 200 microns. The ciliary cells are typically 10 micras separated. A separation of 20 micras allows only one row of ciliary cells between the active electrode and its associated earth. However, if the probe is tilted so that the ciliary cells are staggered between the active and ground electrodes, flow production will occur. Although the ciliary cells are nonfunctional in the deep deaf, the location of the ciliary cells is an indication of the location of the ends of the nerve fibers that transmit sound sensations to the brain. Behind the electrodes to ground and active is an insulation layer. This is to avoid producing an electric field in the back. Behind the insulator is an additional ground plate to increase the area to ground to a maximum. The maximum ground area is desired since it reduces the contact impedance of the earth with the conductive fluid and provides a gradient field that minimizes the perturbations of the channel. Also, not shown in this Figure, insulating material may be placed at the lateral ends of each channel to prevent current flow away from the lateral ends. Figure 15B shows a four-channel probe located near the spiral ganglion. As in Figure 15A, the electrodes are mounted perpendicular to the gradient field. The diameter of the probe at its largest point is approximately 2 millimeters, and the diameter of each electrode at its largest point is approximately 1 millimeter. The distance between the electrodes to ground is between 0.5 mm and 2 mm. The electrodes of the probe are mounted in a flexible isolation structure allowing the probe to take the shape of the scale eardrum or scale vestibule. The length of the area of the probe that houses the electrodes is between 2 millimeters and 8 millimeters. In systems with more than 4 channels, the separation between the electrodes to ground would be smaller.

THE DIGITAL SYSTEM The limit of this system leads to a system where only one nerve fiber is stimulated per channel and the system becomes purely digital. Again to reach the carrier frequency of 24 kilo-hertz in the digital system, where the trigger repetition rate of an individual nerve fiber is 200 pulses per second, 120 channels are required. (120 X 200 = 24,000). In this digital system, instead of tracking the force-duration curve for each channel, as is done in analog systems, a high-amplitude stimulus impulse is used to place the firing of the nerve fiber in the inclined portion of its force-duration curve. This minimizes the differences in the activation time between the channels. The electric field is restricted to only 1 or a small constant number of nerve fibers, which seems to trigger simultaneously, producing the neuro-carrier frequency of 24 kilohertz. In this digital system, the modulation is carried out modulating the frequency of the carrier frequency. No modulation of the amplitude of the impulse stimulus is required. From this it becomes apparent that the system which is in the transition region between the amplitude modulation as well as the analog and digital frequency can be used to advantage. Figure 16 is a block diagram of the digital system. On the left of the Figure is the external unit 102. This consists of a microphone, an audio amplifier, automatic gain limiter / control, oscillator, frequency modulator, cycle antenna and a power source. At the center of the Figure is the internal unit 104, a cycle that receives the antenna, diodes to provide both positive and negative voltage, voltage regulators and a 128-position counter / decoder and individual latch circuits for each channel. "To the right of the Figure is the probe 106 that is placed near the nerve fibers that conduct the sound to the brain Figure 17 is a version of the external unit A_ on the left of the Figure is a capacitor microphone Ml connected to the input of the IC62 Schmitt with feedback through the resistors R72 and R73 that form a frequency modulated oscillator.R73 is variable to adjust the center frequency.The output of the oscillator is divided by 2 in IC63, a counter 74C93. IC63 is damped by IC64 to avoid capacitive load on meter output The output of IC64 damper drives IC65 a MOSPOWER HALF-BRIDGE CONDUCTOR The output of IC65 is an impedance switch that switches from one power rail to the other. it is fed through C9 to the closed antenna Ll.The capacitor C9 resonates with the inductance of the closed antenna forming a tuning circuit.The capacitor C8 is through the connections of IC65 power. The overlap is such that there is a minimum area within the current path as the resonance currents through the cycle can be high and any area outside the closed antenna will reduce efficiency. Figure 18 is the internal digital system unit. In the upper left part of the Figure is the closed antenna L2 and its CIO tuning capacitor. The output of the closed antenna goes through diodes D8 and D9 to produce positive and negative voltages. Capacitors Cll and C12 filter the direct current voltages. The voltage regulators VR1 and VR2 regulate the voltages. C13 and C14 provide stability to the outputs of the voltage regulators. R74 is connected to the output of the closed antenna to provide a clock to the implanted system. The diodes DIO and Dll limit the voltage swing at the input of IC66 a Schmitt trigger. IC64 provides the clock signal to the IC67 7-bit counter decoder. The 128 positions of the IC67 counter (from 0 to 127) lead to the set-reset retention circuits IC68 to IC95 formed by the CMOS 74C00 gates. As shown in the lower left of Figure 18, these retention circuits are set at a given number N, with the first retention circuit restored N + 4 and the second retention circuit restored at N + 8. The pairs of subsequent retention circuits are changed one count down as shown in the drawing. The outputs of the first pair of latching circuits goes to a 4053 IC196A. When switch IC196A is OFF, the switch output is grounded or potential 0. When the switch is ON, the output of the first latch circuit is connected to the output. This will occur for a complete cycle of the first retention circuit. In this way the average potential of a given channel will be 0. The output of these switches have their voltage changed to a current through the resistors the same as R73 on channel 1 and also through capacitors such as C15 to ensure additionally no long-term direct current component. Figure 19 shows the waveform of the individual channels of the digital system. The circles in the waveform indicate the time when the nerves are triggered. The delay between the channels is 40 microseconds. The amplitude of each stimulus pulse is such that the excitation time of a nerve fiber will occur in the inclined portion of the force-duration duration after 120 microseconds and the stimulus pulse will last beyond the excitation of its fiber. nervous, in this drawing 160 microseconds. The repetition ratio of each channel is 5.12 milliseconds or approximately 5 time constants, which allows the recovery of the nerve to approximately 1% of its initial state before the stimulus. In all analog and digital systems, a carrier of approximately 25 kilohertz is used only as an example. Higher frequencies would require a higher stimulus amplitude which would cause a higher flux production ratio and provide a higher frequency response of the modulation. It is also recognized that with any carrier system the modulation bandwidth must be restricted to less than 1/2 the carrier frequency.

PROCEDURE The procedure for implanting the device in a human ear is as follows: The placement of the gradient probe is critical and requires precise placement for optimal performance. With an adult patient who has heard in the past the probe is placed while the patient is alert. Using a local anesthetic the probe moves in its place having activated it and the patient indicates when he hears an intelligible sound. Then for the exact placement each channel is stimulated only to establish that each channel requires approximately the same intensity of stimulus to produce the sensation of sound and to verify the force-duration characteristics. For the second time the patient is asked to confirm that their hearing is normal and then the probe is permanently fixed in its place. During this procedure a template that is mounted on the patient's head is used to hold the probe so that the movement of the head will not affect the location of the probe in relation to the nerve fibers. With patients who have never heard in the past the probe is located as above except that the last step is to confirm that a minimum of sound is heard when no external sound is present and the sounds are comfortable when they are generated externally. For children who are unable to provide direct help, another means of establishing the presence of sound sensations can be used through the measurement of nervous activity at a higher level or brain activity. One method is through the measurement of nerve activity in the cochlea. The method of the present invention directly stimulates nerve fibers of the audio transmission portion of the VIII nerve with electrical signals representing the audio sounds captured in sequence, thereby imparting the sensation of hearing a deaf patient. The method comprises implanting a receiver with a closed antenna and with connections to an electrode probe, composed of an array of electrodes formed to produce multiple gradient fields in the patient, in the audio portion of the VIII nerve and generate a representative electrical signal of audible sounds captured. An electrical signal is divided into multiplexed time channels by which each multiplexed channel is connected to a corresponding gradient probe channel and contains the audible representation of the entire audio spectrum and elements to limit the audio spectrum. Each channel is processed to produce a biphasic stimulation signal that overlaps with its adjacent channel biphasic stimulation signal (over time) but does not overlap its component that is representative of the audible sounds. The method further comprises aligning a transmitting / receiving external closed antenna with the closed antenna of the receiving / transmitting implant so that there is a distance at least equal to the thickness of the patient's skin separating the external antenna from the internal antenna thereby providing a means to transmit both energy and representations of audible sounds to the implant recipient, and locate the gradient probe in the vicinity of the nerve fibers that transmit sound sensations to the brain. In addition, the method comprises compensating movements in the patient's head during the placement of the gradient probe; and fix it permanently the gradient probe. Although embodiments of the invention have been shown, it will be understood that changes and modifications to the methods and devices shown herein may be made by those skilled in the art and therefore the appended claims are intended. cover all these changes and modifications that fall within the true spirit and scope of the present invention. In the circuits described above, well-known standard components are shown. This is to provide specific examples of how detailed functions can be performed. Does not suggest programmable door arrangements, microprocessors or other components excluded. In fact devices such as the 8XL51FX COMMERCIAL / EXPRESS LOW VOLTAGE CHAMPS SINGLE-CHIP 8-BIT MICROCONTROLLER from Intel are an excellent choice for machining the functions of the invention. When operating at a clock frequency of 3.5 Megahertz it attracts less than 6 mA. Figure 22 is a block diagram of the system that includes the use of all these devices. In the upper left part of the Figure is an outline of the external module 120 to be used by the patient. It is designed to be worn behind the ear in a manner similar to some conventional hearing aids. It houses a high-energy rechargeable battery as an energy source. The unit, when not in use, can be mounted on a charger shown below the unit. The external module has a volume control, a null background tone adjustment, a power switch and a microphone located in the unit behind a small hole. The spiral of the antenna is connected to the unit through a small cable and mounted adjacent to the receiving spiral of the implant. To the right of the package of the external module is a block diagram 122 of the circuits. The microphone conducts the automatic gain control circuit. The volume control sets the input to an analog to digital converter. Above the AGC block are the top and bottom controls which lead an up / down counter. This is a fine cut of the stimulus level. This value in addition to the output of the digital-to-analog converter is formatted to feed the modulator, which over imposes the data on the oscillator output and is fed to the antenna. Located below the external module and the charger is a drawing of the computer module of the doctor's office 124 consisting of a portable computer with a microphone, a transmitter / receiver and software to perform the following functions: 1) Establish a communication link bidirectional between the external computer and the implant computer, 2) adjust the channel selection ratio (or channel frequency), the stimulus amplitude for each channel both independently and in a coupled fashion. 3) Adjust the compensation for the force-duration curve. This setting is in the form of a computer table. It establishes a constant curve in time that can be cut at all points of the curve to nullify the carrier's variations in nerve activity. This curve interacts with the stimulus amplitude settings since its values are in terms of the percentage of the stimulus amplitude. The time axis of the table is independent of the channel selection ratio. t 4) The adjustment of both the audio level (thick) and the soft start speed. The soft-start / stop function causes the background carrier to gradually turn on or off with a minimum of discomfort for the user. 5) Adjust the number of channels that will be used. Four are a minimum and eight are a maximum. It also selects which channels of the probes are to be used. First the doctor explores each channel and establishes its functionality in terms of its sensitivity, and strength-duration characteristics. In the case of the choice of 4 channels to be used, then the doctor selects which channels of the probe will be used. All unused channels are grounded at the common return electrodes. In the case of the 6-channel system, the best 6 channels of the probe are used. During this selection process the switch of the electrode array remains at the electrodes of the selected channel and all other channels are connected to ground. Figure 22C shows the selection of 4 channels of an 8-channel probe. Figures 22A, 22B and 22C are on-screen displays for the following functions of computer screens. In the center of Figure 21 is the block diagram 130 of the implant unit. The closed receiver / transmitter antenna is at the left end of the block diagram. It feeds the receiver that develops a voltage that is then regulated and provides protection to component failure, up and down energy of the integrated microprocessor, high voltage for non-volatile write memory and a soft start and soft off of the probe stimulus. A second output of the receiver is the serial data output. This feeds the data decoder and then the microprocessor. Both the command signals and the audio data enter the microprocessor. The programmed function values of the microprocessor are fixed in a non-volatile memory. Eight converters from digital to analog (these can be either of the analog type or can be produced by pulse width modulation as the sübumbral stimulus acts in an additive manner and has an effect of increasing the excitability of the nerve cell membranes) to drive eight current sources of which four to eight can be used. The outputs of these current sources produce both negative and positive currents to establish a direct current potential of zero average. These current source channels are then selected by the array of electrode switches to be fed through capacitors, producing the possibility of a direct current component residue, to the electrodes of the probe. As those skilled in the art will understand, the present invention can be incorporated in many different specific ways, and the specific details described herein are only examples of how to practice the invention. For example, many alternate circuit and block diagrams are equivalent to the example diagrams shown in the drawings, and can be used to realize this invention. Also, the necessary electrical circuits can be manufactured as integrated circuits or they can be integrated into a microprocessor, or a combination of integrated circuits and microprocessor can be used. In fact, a microprocessor, because it can be programmable because of its small size, can be particularly advantageous as a device for use in the present invention. Although it is apparent that the invention described herein is well calculated to cover the previously established objectives, it will be appreciated that numerous modifications and modalities can be designed by the claims covering all these modifications and modalities that fall within the true spirit and scope of the invention. present invention.

Claims

1. A device for applying electrical stimuli to any branch of the VIII nerve comprising: a generator of stimuli for applying electrical stimuli to nerve fibers of the eighth nerve during at least two time channels to produce or increase a flow of nervous activity at a generally constant rate , independent of audio modulation, that is able to act as a carrier wave and that is perceived by the brain as active silence.

2. A device according to the claim 1, wherein: the stimulus generator applies electric fields to at least two groups of nerve fibers to cause the fibers in each group to produce flow in a generally uniform ratio.

3. A device according to the claim 2, where the stimulus generator applies a time that varies the electric field of each group of fibers that compensates the strength-duration characteristics of the fibers in the group in order to cause the fibers in the group to produce flow in the usually uniform reason. A device according to claim 2, wherein the stimulus generator applies a space that varies the electric field to each group of fibers that compensates for the strength-duration characteristics of the fibers in the group in order to cause the fibers in the group produce flow at the generally uniform ratio. 5. A device according to the claim 2, wherein the stimulus generator includes: a plurality of electric field generators, each electric field generator positioned adjacent to the respective group of nerve fibers to apply electric fields to the adjacent group of nerve fibers; and a signal generator for applying signals to electric field generators to cause electric field generators to apply electric fields to the nerve fibers to produce or increase the flow of nerve activity at a generally constant rate. 6. A device according to the claim 1, wherein the stimulus generator includes elements to modulate the carrier wave to produce the sound sensation. 7. A device according to the claim 6, where: each stimulus is applied during one of the time channels to an associated group of nerve fibers; each time channel includes a latency period and a flow period; when one of the stimuli is applied to the associated group of nerve fibers during one of the time channels, the fibers in the associated group of fibers flow at a generally uniform rate during the flow period of the time channel to produce or increase the wave carrier, and the means for modulating the carrier wave modulates that an electrical stimulus during the flow period of the time channel. 8. A device according to claim 2, wherein: each stimulus is applied during one of the time channels to one of the fiber groups; each time channel includes a latency period; When one of the stimuli is applied to one of the fiber groups during one of the time channels, that stimulus includes compensation for the strength-duration characteristics of the fibers in that group to cause the fibers of the group to produce flux at a time. Generally uniform ratio, and compensation for force-duration characteristics occurs after the latency period of that time channel. 9. A device according to claim 1, wherein the stimulus generator applies electric fields to a multitude of sets of nerve fibers to cause the fibers of each set to trigger substantially simultaneously. 10. A device according to claim 1 wherein the stimulus generator applies stimuli to a multitude of nerve fiber groups to produce the flow of nerve activity. 11. A device according to the claim 10, wherein: the stimulus generator applies electric fields to the nerve fiber groups to cause the fibers in each group to produce flow at a generally uniform rate. 12. A device according to the claim 11, wherein: the stimulus generator applies electric fields to each of the groups of nerve fibers for a respective time channel and in a defined sequence, and where adjacent channels over time overlap in time to compensate for the latency period in nerve fiber firing. 13. A device according to the claim 12, wherein pairs of adjacent channels over time overlap during a substantially constant time course. 1

4. A device according to claim 12, wherein: during a defined cycle time, the stimulus generator applies electric fields to several fiber groups; and at this defined cycle time, the stimulus generator does not apply electric fields to each fiber group for the remainder of the time period to provide the nerve fibers of each group with a recovery time. 1

5. A device according to claim 12, wherein: during a defined cycle time, the stimulus generator applies electric fields to a given number of fiber groups; and in this defined cycle time, the stimulus generator applies electric fields to all the given fiber groups during a generally equal time course. 1

6. A device for applying electrical stimuli to any branch of the eighth nerve, comprising: a generator of stimuli for applying electrical stimuli to a multitude of nerve fiber assemblies of the eighth nerve during a multitude of time channels, to cause the fibers from each set they shoot substantially simultaneously and to produce a flow of nervous activity at a generally constant rate, independent of audio modulation, which is able to act as a carrier wave and which is perceived by the brain as an active silence . 1

7. A device according to claim 16, wherein the amplitude of the electrical stimulus is such that each set of fibers fires within 120 microseconds after one of the stimuli is applied to the set of fibers. 1

8. A device according to claim 16, wherein: the stimulus generator applies electric fields to each of the sets of nerve fibers during a respective time channel in a defined sequence, and wherein the adjacent channels in time they overlap in time to compensate for a latency period in nerve fiber firing. 1

9. A device for applying electrical stimuli to a branch of the VIII cranial nerve, which comprises in combination: elements for the stimulation of a number (N) of different groups of nerve fibers of the VIII cranial nerve, these groups of nerve fibers being fastened in N separate intervals; and elements for repeatedly applying electric fields to fiber groups to produce or increase a constant flow of nerve activity, independent of audio modulation, which is capable of acting as a carrier wave and which is perceived by the brain as active silence; and where the carrier wave, when modulated, results in the perception of sound; and wherein, for each fiber group, a range is provided between applications of the electric fields to the fiber group, and this range is not less than the natural recovery time of the nerve fibers in the group. 20. A device according to claim 20, wherein: the element for applying the electric fields applies a time that varies the electric field of each group of fibers that compensates the force-duration characteristics of the fibers in the group with the In order to cause the fibers in the group to produce flow at a generally uniform rate. 21. A device according to claim 20, wherein: the element for applying the electric fields applies electric fields to pairs of fiber groups during a period of time overlap to compensate for a latency period in the firing of nerve fibers. 22. A method for applying electrical stimuli to a branch of the VIII cranial nerve, comprising: using a stimulus generator to apply electrical stimuli to the nerve fibers of the VIII cranial nerve during at least 2 channels of time to produce or increase a flow of stimuli. nervous activity at a generally constant ratio, independent of audio modulation, that is capable of acting as a carrier wave and that is perceived by the brain as active silence. 23. A method according to claim 22, wherein: the step of using the stimulus generator includes the step of applying electric fields to at least two groups of nerve fibers to cause the fibers of each group to produce flow to a ratio generally uniform. 24. A method according to claim 23, wherein the step of using the stimulus generator also includes the step of applying an electric field that varies the time of each fiber group that compensates the strength-duration characteristics of the fibers in the group in order to cause the fibers in the group to produce flow at the generally uniform ratio. 25. A method according to claim 23, wherein the step of using the stimulus generator includes the steps of: placing a respective electric field generator adjacent to each of the nerve fiber groups to apply electric fields to the adjacent group of nerve fibers, - and use a signal generator to apply signals to electric field generators to cause electric field generators to apply electric fields to nerve fibers to produce the generally constant flow of nerve activity. 26. A method according to claim 22, further comprising the step of modulating the carrier wave to produce the sound sensation. 27. A method according to claim 22, wherein the step of using the stimulus generator includes the step of applying stimuli to a multitude of fiber channels to produce the flow of nerve activity. A method according to claim 27, wherein the step of using the stimulus generator includes the steps of: applying electric fields to each of the nerve fiber groups for a respective time channel and in a defined sequence; and overlapping the adjacent channels in time to compensate for the latency period in nerve fiber firing. 29. A method according to claim 28, wherein pairs of adjacent channels over time overlap for a substantially constant time duration. 30. A method according to claim 28, wherein the step of using the stimulus generator further includes the steps of: during a defined cycle time, applying electric fields to several of the fiber groups; and in this defined cycle time, provide each fiber group with a rest period, where the electric fields are not applied to the fiber group, to provide the nerve fibers of each group with a recovery time. 31. A method according to claim 28, wherein: during a defined cycle time, the stimulus generator applies electric fields to a given number of fiber groups; and in this defined cycle time, the stimulus generator applies electric fields to all the given fiber groups for a generally equal duration of time. 32. A method for directly stimulating the nerve fibers of the VIII cranial nerve with electrical signals representing audio sounds picked up in sequence to impart the sensation of hearing the deaf patient, the method comprising the steps of: implanting in the patient a receiver with a antenna and with connections to an electrode probe, composed of an array of electrodes formed to produce multiple gradient fields in the area of the cochlear nerve; generating an electrical signal representative of audible sounds captured, - dividing the electrical signal into multiplexed time channels, wherein each multiplexed channel is connected to a corresponding gradient probe channel and contains the audible representation of the entire audio spectrum and means to limit the audio spectrum; process each channel to produce a biphasic stimulation signal that overlaps a biphasic stimulation signal of adjacent time channel but does not overlap its component that is representative of the audible sounds, - aligning an external transmitting / receiving antenna with the implanted antenna so that there is a distance at least equal to the thickness of the patient's skin separating the external antenna from the internal antenna, wherein the external antenna provides a means of transmitting both energy and audible sound representations to the implanted receiver; locate the gradient probe in proximity to the nerve fibers that transmit sound sensations to the brain; and permanently fix the gradient probe in place in the patient.