GB2295457A

GB2295457A - Monitoring the myotactic activity of a muscle

Info

Publication number: GB2295457A
Application number: GB9423930A
Authority: GB
Inventors: David Kniveton Chadwick
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-11-26
Filing date: 1994-11-26
Publication date: 1996-05-29
Anticipated expiration: 2014-11-26
Also published as: GB2295457B; GB9423930D0

Abstract

A device for monitoring the myotactic activity of a muscle, eg for use as a communications aid for people with muscle paralysis, comprises a sensor 20 producing a signal indicative of the myotactic activity of a muscle, an amplifier 30, and a processing means 11 to analyse the amplified signal to discriminate between at least two different actuations of the muscle and to produce an output signal representative of a particular muscular actuation. As a communication aid, the device also has a message-generating means 120 permitting the consecutive selection of a number of language elements from a menu by means of the output signal thereby to construct a message. The message generating means may be configured to discriminate between long muscular actuations and a short muscular actuations to control the selection of language elements from the menu. The sensor may comprise electrolytically coated electrodes 20a, b, c placed on the face. <IMAGE>

Description

A DEVICE FOR MONITORING THE MYOTACTIC ACTIVITY OF A MUSCLE The present invention relates generally to a device for monitoring the myotactic activity of a muscle, and more especially, a communication aid driven by such a device.

For people with very severe muscle paralysis, such as that experienced by sufferers of cerebral palsy, communication is very difficult. In such cases, voluntary muscle control can be limited to a few facial muscles.

The present invention is founded on the observation that the actuation of a muscle generates a voltage differential across the muscle as illustrated in Figure 1. The voltages generated by muscle actuation are related to the tension in the muscle and are of the order of 100 V peakto-peak in a frequency band between 20Hz and 200Hz. When the voltages across a muscle are measured, signals outside this frequency band are also picked up, but these are more artefacts of sweating, body movement and other bodily processes than artefacts of muscle actuation.

The present invention provides a device for monitoring the myotactic activity of a muscle comprising: a sensor to provide a first signal indicative of the myotactic activity of a muscle; an amplifier to amplify the first signal to produce an amplified second signal; and a processing means to analyse the second signal to discriminate between at least two different actuations of the muscle and operable to produce a third signal representative of a particular muscular actuation.

The present invention also provides a communication aid comprising the above device and a message-generating means, the message-generating means permitting the consecutive selection of a number of language elements by means of the third signal thereby to construct a message.

The processing means can be configured to discriminate between a long muscular actuation and a short muscular actuation.

In one embodiment, the message-generating means is adapted to interpret the long and short muscular actuations as the - (dash) and .(dot) of the Morse code system, whereby the selected language elements comprise single alphanumeric characters.

In another preferred embodiment, the message-generating means includes a menu, and the long and short muscular actuations control the selection of language elements from the menu. The selected language elements may comprise words of predetermined set phrases. Preferably, the menu can be constructed in accordance with the BLISS language system or similar systems.

Preferably, the processing means and the message-generating means are implemented in software on a microprocessor.

The constructed message can be displayed visually or used to drive a voice synthesizer.

The present invention also provides an amplifier means comprising a differential amplifier, a band-pass filter and a boost amplifier cascaded in series.

Exemplary aspects of the invention are hereinafter described with reference to the accompanying drawings, in which: Figure 2 shows an overall block diagram of a communication aid in accordance with the present invention; Figure 3 shows a block diagram of a multi-stage amplifier in accordance with the present invention; Figure 4 shows a detailed circuit diagram of the amplifier in Figure 3 and an isolator; Figure 5 shows an extract of a typical section of the BLISS dictionary; and Figure 6 shows a depiction of the display of a microprocessor system when, in use, constructing a message using the BLISS language system.

Referring to Figure 2, a communication aid, generally denoted 10, comprises a set of three silver electrodes 20a,20b,20c electrolytically coated with black silver chloride for connection to the facial muscles of a subject. Although not shown, each electrode is placed in series with a 10KQ resistor to protect the subject from possible burns.

A first of the electrodes 20a is connected on the centre of a muscle of a facial muscle such as the eyebrow, cheek or jaw, a second of the electrodes 20b serving as a reference electrode, is connected to a site on the face where there is no muscle, say the centre of the forehead, and the third electrode 20c, serving as an earth electrode, is connected just above the hairline. DRACARDTM gel is used between the skin and the electrodes to bring the contact resistance down to around 2us1.

The electrodes 20 are coupled to the input of an amplifier 30. The amplifier is a multi-stage device and is shown in block diagram form in Figure 3. It comprises a differential instrumentation amplifier 32 as its input stage. This amplifier amplifies the difference between the signals on electrode 20a, 20b. This amplifier 32 is a low-pass filter having a gain of 1000 up to a cut-off frequency of about 1 KHz. A differential instrumentation amplifier 32 is used as the input stage because of its high input impedance and its good common mode rejection properties. The second and third stages comprise two band-pass filters 34,36, each having a lower cut-off frequency of 20Hz and an upper cut-off frequency of 200 Hz. The fourth stage of the amplifier is provided by a booster amplifier 38 with a gain of 50 in the 20Hz to 200Hz frequency band.

This final stage is required to bring the overall gain of the amplifier 30 to a level at which the signals provided by the electrodes 20 can be readily processed. The electrode 20c is connected to the signal ground of the amplifier 30. A detailed circuit diagram of the amplifier is shown in Figure 4. As a point of practical circuit construction, it will be noted that the OP-amp used in the third stage is not a 741, the industry standard, but an LTC1050CN8 because of its negligible DC offset.

Figure 4 also shows the next major stage in the system - an optical isolator 40. This was required to protect the amplifier 30 from noise generated by the mains. In order to provide signal level compatibility between the amplifier 30 and the optical isolator 40, it was necessary to include a level shifter 39 at the output of the amplifier 30. The output of the isolator 40 is then sampled at high frequency by an analogue - to digital converter (ADC) (not shown) to produce digital data which is then fed to the input port of a microprocessor 100.

The software on the microprocessor 100 includes a module, referred to hereinafter as the processing means 110 which analyses the data from the ADC and discriminates between long and short muscle acutations. It achieves this by the following method.

For a predetermined period, 0.25s has been found to be sufficient, each item of sampled data is tested to see whether it is positive or negative. If it is positive it is added to a first variable X and if it is negative its modulus is added to a second variable Y. At the end of the predetermined period, a variable Z is calculated thus: Z = (x-y) / (number of samples taken during the predetermined period).

If Z > a first constant, say 1.1, then the muscle actuation is deemed to be a long muscle actuation.

If Z < the first constant and Z > a second constant, say 0.55, then the output is deemed to be a short muscle actuation, otherwise there is deemed to have been no muscle actuation. At the beginning of every predetermined period, the variable x and y are, of course, reset to zero.

Discrimination between various types of muscle actuation not only on the basis of the duration of the actuation but according to other criteria also falls within the ambit of the present invention. Accordingly, more sophisticated discrimination algorithms including neutral networks and fast Fourier transforms can be employed.

The software on the microprocessor 100 also includes a module, hereinafter referred to as the message-generating means 120. The function of this module is to take the output of processing means 110, and use this output to determine the selection of a number of language elements, whereby a message can be constructed.

In one embodiment, the message-generating means is adapted to interpret the long and short muscular actuations as the - (dash) and .(dot) of the Morse code system. In this way, consecutive long and short muscle actuations can build up a string of text.

In accordance with a preferred embodiment of the invention, the BLISS language system is employed. The BLISS language system is a system which facilitates communication by handicapped people. It comprises approximately 500 symbols corresponding to words or commonly used phrases. The dictionary of BLISS is in the form of an eight page booklet, the pages of which are colour coded. Each page is divided into eight sections, each of which is colour coded in the same way. Finally, each section is divided into eight squares which carry the BLISS symbol and its meaning shown in English such as "hello". A typical section of the BLISS dictionary is shown in Figure 5.

The system is utilised by the present invention as follows.

A screen including three menus as shown in Figure 6 is displayed.

The cursor starts at the top of the PAGE menu. In order to select a word or phrase, a series of a short muscle actuations causes the cursor to descend this menu until the cursor is opposite the required page colour.

A long muscle actuation selects the page colour and moves the cursor to the top of the SECTION menu. As before, a series of short pulses causes the cursor to descend this menu until the cursor is opposite the required section colour. A long muscle actuation selects the section colour and moves the cursor to the top of the SQUARE. A similar process selects the appropriate option from the PAGE menu. At this point, the selected BLISS phrase appears at X. The above process is repeated until a string of phrases covers the screen.

It has been found that employing the BLISS system an A4 page of text can be communicated in 5 to 25 minutes using only one controlling muscle.

In order to improve the controllability of the cursor, it is preferred to include a software module to compensate for signal transients between muscle actuations. This module is notionally positioned between the ADC and the processing means 110 and function as follows. Once data from the ADC indicates that a signal is being generated by the electrodes, a 0. ls delay is initiated. Once this delay has elapsed, the data then incoming from the ADC is passed directly to the processing means 110, which then processes the data as previously described. Once the data from the ADC indicates that the signal is no longer being generated by the electrodes, a further 0.ls delay is initiated. This module is particularly useful in testing when switches with debounce are used to simulate muscle actuation.

Claims

1. A device for monitoring the myotactic activity of a muscle comprising: a sensor to provide a first signal indicative of the myotactic activity of a muscle; an amplifier to amplify the first signal to produce an amplified second signal; and a processing means to analyse the second signal to discriminate between at least two different actuations of the muscle and operable to produce a third signal representative of a particular muscular actuation.

2. A communication aid comprising the device of Claim 1 and a message-generating means, the message-generating means permitting the consecutive selection of a number of language elements by means of the third signal thereby to construct a message.

3. A communication aid as in Claim 2, wherein processing means is configured to discriminate between a long muscular actuation and a short muscular actuation.

4. A communication aid as in Claim 3, where the message-generating means is adapted to interpret the long and short muscular actuations as the - (dash) and . (dot) of the Morse code system, thereby to construct a message.

5. A communication aid as in Claim 4, wherein the messagegenerating means includes a menu, the long and short muscular actuations controlling the selection of language elements from the menu.

6. A communication aid as in Claim 5, wherein the menu is constructed in accordance with the BLISS language system.

7. A device constructed, arranged and adapted to operate substantially as hereindescribed in relation to the accompanying drawings.

8. A communication aid constructed, arranged and adapted to operate substantially as hereindescribed in relation to the accompanying drawings.