WO2014169241A1

WO2014169241A1 - Headgear for dry electroencephalogram sensors

Info

Publication number: WO2014169241A1
Application number: PCT/US2014/033854
Authority: WO
Inventors: Trevor Austin KERTH; Michael Henry ELCONIN; Yu Mike CHI
Original assignee: Cognionics, Inc.
Priority date: 2013-04-12
Filing date: 2014-04-11
Publication date: 2014-10-16

Abstract

A headgear includes a tensioning assembly for applying pressure to a sensor onto a subject and apparatus forming a spine to conform to a head. The tensioning assembly includes an anchor; and an elastic band having an adjustable length with one end connected to the anchor. The elastic band is adapted for applying pressure to the sensor towards the subject; and the pressure applied by the elastic band to the sensor can be changed by adjusting the length of the elastic band. The apparatus forming a spine to conform to a head includes a plurality of pods; wherein an individual pod contains a joint that permits rotational movement between the pods so that the spine can conform to the surface of different head shapes.

Description

HEADGEAR FOR DRY ELECTROENCEPHALOGRAM SENSORS

DESCRIPTION

The invention relates to headgear and headsets, specifically to apparatuses that enable the rapid and reliable placement of sensors on a subject's head. The focus of this patent is for EEG applications, although the invention is broadly applicable to placing any sensor or transducer on the head of a subject. Conventional recording of EEG signals predominantly involves the use of wet electrodes that utilize a gel for conduction to the subject's scalp. The wet electrodes can be affixed to the skin, sometimes abraded, with a glue or placed in an elastic cap. The high conductivity of the gel allows it to permeate through hair and forms a secure, low-impedance electrical connection between the subject and the recording instrument resulting in a high signal quality. However, the use of electrolytic gels, adhesives and abrasion is often time consuming, irritating and uncomfortable. In response, dry electrodes, which do not require conductive gels, adhesives, or scalp preparation, have been explored as an alternative.

In practice, dry electrodes suffer from numerous usability issues. Although acquiring signals on bare skin (e.g., forehead) is relatively straightforward, most EEG setups also require electrodes across the entire head and over areas with hair. With wet electrodes, any gaps between the electrode and the surface of the scalp is filled by a gel which guarantees a conductive path and buffers against movements and displacements. Dry electrodes, however, have no gel buffer and must be placed directly onto the surface of the scalp. Unlike wet electrodes which can rely on gels to compensate for inadequate contact/pressure, dry electrodes are critically dependent on a harnessing system, called a dry EEG headgear, to secure every electrode to the scalp to achieve acceptable signal quality. The main challenges with dry EEG headgears are conforming to the many variations in human head shape and size while regulating and optimizing each sensor's contact tension for high signal quality without subject discomfort. They must also be easy to apply and remove. The first and simplest approach previous attempted involves adapting the standard elastic EEG cap for dry electrode use, exemplified by Gevins et. al. in US4967038. The elasticity in the cap enables it stretch and conform to different heads. The stretching action generates tension which can also be further increased by using relatively 'tall' electrodes that create more deformation in the cap. However, elastic caps suffer from a numerous problems. Although elastic caps are generally flexible, the generic 'balloon-like' shape does not conform well to all different head shapes (e.g., 'boxy' heads), leading to areas where the cap is overly tight and other areas where the cap is loose. In addition, the closed nature of the cap makes adjusting electrodes difficult (e.g., manipulating ones with bad contact) and the electrodes inside a cap are prone to tipping during application. Finally, the cap has no means for adjusting or optimizing the pressure on individual or small groups of electrodes - all of the electrodes are mechanically coupled together - making tuning contact pressure particularly difficult. With a closed cap, it is common for some electrodes to contact the head with excessive pressure and other not at all.

More sophisticated designs use rigid headgears with mechanical mechanisms to individually place sensors on the head for better performance and reliability. One example is found in US8103328 by Turner et al. where each sensor is mounted on a spring-loaded assembly, with a hinge to a tensioned arm. In a full headgear, an array of arms radiate from a central point at the top of the head. The hinging mechanism in the arm helps orient the sensor to the surface of the head and the arm generates pressure that connects the sensor to the head. This system is effective at providing optimized and individual tension for each of the sensors in the array but is complex and bulky. A simpler variant, exemplified by the Emotiv EPOC product

(http://www.emotiv.com/apps/epoc/299/), replaces the assembly of springs and hinges with a single plastic arm. Deformation of the arm by the head generates tension to hold the sensor on the subject. This greatly simplifies the design but reduces the ability of the headgear to conform to different head shapes. In general, mechanical designs with individual arms for each sensor, or a group of sensors, do not work well unless each sensor and arm contains flexibility and several degrees of freedom to conform to different heads. Most importantly, the complexities of the mechanical systems preclude constructing high-density arrays due to their sheer number of parts.

Yet another general approach to constructing a dry EEG headgear is a rigid 'helmet-like' shell as a super-structure that is nominally larger than the subject's head, as exemplified by Delic et. al. in US 2007/0238945 Al. Each sensor is mounted on a spring-loaded or compressible extension directed towards the surface of the head. The compressibility of the mounts enable the sensor array to conform to the variations in human head size and shape. In practice, hard shells have fit problems due to high variation in human head size and shape. Although the

compressibility of the sensor mounts helps compensate for variations in head size, it is still difficult to guarantee that all the sensors hit the head at the correct angle due to variations in head shape. As a result, sensors in the array may contact the surface of the head in a non- perpendicular orientation, resulting in discomfort and poor signal quality.

In light of the limitations with prior art dry EEG headgear, the invention maximizes conformance to variations in head size and shape and provides a system to regulate and control sensor contact pressure for optimal comfort and signal quality. The entire system is specifically designed to accommodate a high density array (32+ channels) in a compact form-factor. A first aspect of the invention is a compact, adjustable tensioning assembly that is capable of applying regulated pressure to an array of sensors on the head. A second aspect of the invention is a mechanical apparatus that provides structure and stability to the headgear while maintaining sufficient flexibility to conform to many variations in human head shape. These two aspects are combined to provide a complete headgear that is easy-to-use and can reliably secure a high-density dry electrode array.

The tensioning assembly takes advantage of the fact that the head is a curved surface. The tensioning assembly operates by stretching an elastic band across the upper perimeter of the head with sensors placed beneath. When the elastic band is tightened, a force is generated along the band in a direction perpendicular to the curved surface of the head to apply approximately uniform pressure to each of the sensors under the elastic band. Since the elastic band is adjustable, the amount of pressure generated can be precisely set and fine-tuned by the user for optimal signal quality and comfort.

A plurality of mechanical apparatus called a spine are used to support a full array of sensors and tensioning assemblies. An individual spine has the advantage of defining a structure for the headgear to improve ease of handling. In an exemplary embodiment, an individual spine includes multiple sub-divisions called pods, each of which contains a reel for changing the length of the elastic bands. The pods are joined together by hinges that run longitudinally down the center of the head from the forehead to the back. Each pod serves as an attachment for one or more elastic bands that run transversely across each side the head.

Individual elastic bands terminate together on anchors near the sides of the head, usually above the ears. The anchors serve to hold the entire headgear in place on the subject's head and may also attach to, a chinstrap or other head straps for improved stability. Fig. 1 is a side view of a first exemplary embodiment of the invention showing the tensioning assembly and spine in a complete headgear on a subject's head.

Fig. 2 is a cutaway view of a pod and two tensioning assemblies taken in a transverse perspective to the view of Fig. 1.

Fig. 3 is a detailed view of a tensioning assembly in the first embodiment.

Fig. 4 is a detailed view of a pod in the first embodiment.

Fig. 5 is a detailed view of two connected pods in the spine in the first embodiment. Fig. 6 is a detailed view of two connected pods in the spine where the tendon is pulled in the first embodiment.

Fig. 7 is a cutaway view of a pod and tensioning assembly in the second embodiment.

Fig. 8 is a cutaway view of a pod and tensioning assembly in the third embodiment. The invention is best described by first considering the embodiment shown in Fig. 1, which shows a complete headgear 1 on a subject's head. The headgear 1 is composed of several main pieces including the tensioning assembly 2a-i and the spine 4. As previously explained, the spine is built from a series of interconnected pods 6a-i. Each of the pods 6a-i connects back to a tensioning assembly 2a-i and contains internal mechanisms, described later, for pressure adjustment. The tensioning assemblies 2a-i are connected, on the other end, to a common anchor 10. The tensioning assemblies 2a-i and anchor 10 are also mirrored on the other side of the head but are not shown in the side view of Fig. 1 for clarity and simplicity. The anchor 10 along with the pods 2a-i, form the mechanical 'super structure' of the headgear and hold the tensioning assemblies 2a-i, on to the subject's head. The array of sensors (not shown in this view) are placed beneath each of the tensioning assemblies 2a-i. In the embodiment shown in Fig. 1, a chinstrap 12 is additionally connected to the anchor points and looped beneath the subjects head to maximally secure the headgear 1 to the user's head. In other embodiments, the chinstrap can be removed if the anchors apply sufficient 'clamping' force on the headgear 1 to prevent it from moving off the subject's head.

The headgear 1 accomplishes several objectives to construct a reliable and easy to use headgear. First, the spine 4 and anchor 10 provide a defined structure for the headgear making it easy to handle compared to a traditional elastic cap that has no structure. At the same time, the flexibility in the spine 4 and adjustability in the tensioning assemblies 2a-i enable to headgear to conform to the contours of the subject's individual head size and shape. Conformability of the headgear 1 starts with the spine 4. Because the spine 4, is built from a series of hinged pods 6a-i, it can bend onto the outline of a variety of head shapes. Tightening the tensioning assemblies 2a- i, pulls them, along with the sensors underneath, towards the subject's head.

Splitting the headgear into separate tensioning assemblies 2a-i, that span across the head in bands offers several advantages compared to prior art. Unlike cap based systems which have minimal, if any adjustment points, each of the tensioning assemblies 2a-i, can be independently adjusted, improving fit and optimizing sensor contact quality. At the same time, the invention is simpler and more compact than prior mechanical headgear apparatuses because the tensioning assemblies 2a-i, can efficiently cover an entire line of sensors, enabling higher density headgears.

Fig. 2 is a cutaway in the transverse perspective of a single pod 6 and it's tensioning assemblies in the headgear 1. A pod 6 is shown connected to two tensioning assemblies 2 and 3. The assembly 2 represents the same tensioning assemblies as shown in Fig. 1. Because the transverse view shows both sides of the headgear 1, a mirror tensioning assembly 3, hidden from the view in Fig. 1, is also shown. An array of sensors 24a-h are connected to the tensioning assemblies for contacting the subject's head. Depending on the desired number of sensors, the length of the tensioning assembly 2, 3 and the number of sensors placed may be varied in different embodiments.

A detailed view of a tensioning assembly 2 and the junction area between a tensioning assembly 2 and its pod 6 is shown in Fig. 3. The outer layer of the tensioning assembly 2 is made of a sleeve 21 which protects the two inner layers: an elastic band 20 and a sensor band 22. The sleeve 21 contains openings 23 through which the sensors 24 extend from the sensor band 22.

The pod 6 contains a slot 60 at which the pod 6 is attached to the sleeve 21. There is also a slot on the opposite side of the pod 6 to which the tensioning assembly 3 is attached to the pod 6, as shown in Fig. 4. The slot 60 also allows for the elastic band 20 to extend inside the pod and to be wound onto a reel 26.

The elastic band 20 and the sensor band 22 in this embodiment are not physically attached together, thereby permitting the elastic band 20 and the sensor band 22 to slide against each other. This prevents adjustments and movements in the elastic band 20 from displacing the sensor band 22. In the embodiment shown, a sensor 24 is attached to the sensor band 22 which is a thin flexible printed circuit board that also contains wiring for transmitting signals from or to the sensor 24. The elastic band 20 is attached, at the other end, to the anchor 10, in the manner shown in Fig. 1.

The tension and pressure applied to the sensor 24, is adjusted by varying the length of the elastic band 20. In the embodiment shown, a reel 26 is used to store unused lengths of the elastic band 20 inside the pod 6. Other embodiments may alternatively adjust the length of elastic band 20 at the anchor end. The advantage of the reel 26 mechanism, used in the embodiment shown, is that it can adjust both the left and right-side tensioning assemblies 2,3 symmetrically, taking advantage of the fact a subject's heads is mostly symmetrical in the transverse axis, thereby simplifying the adjustment process and preventing the headgear from become skewed to one side due to asymmetric adjustments.

To position the sensor 24 on the surface of the subject's head, the reel 26 is rotated which retracts the elastic band 20. Initially, only excess slack in the elastic band 20 is taken in until the sensor 24 reaches the surface of the subject's head. At this point, further retracting the elastic band 20 into the reel 26 begins to stretch and deform the elastic band 20, generating a downward pressure on the sensor band 22 and holding the sensor 24 onto the surface of the head. This system permits precise adjustment of the downward pressure on the sensor 24. Although only one sensor 24 is shown in Fig. 3, multiple sensors can be placed beneath the sensor band 22, as depicted in Fig. 2, using the same basic arrangement.

Fig. 4 shows the inside of a pod 6 and the mechanisms to adjust the elastic band 20. For simplicity the elastic band 20 is not shown in this illustration to better illustrate the features inside the pod 6. Each pod 6 contains a dial 62 which turns a worm drive 28. The worm drive 28 engages with the teeth on the reels 26a-b and causes them to rotate. Two reels 26a-b are shown in Fig. 4 since each pod 6 is normally attached to both tensioning assemblies 2, 3, as shown in Fig. 2. Once the elastic band 20 is wound onto a reel 26, its length can be controlled by rotating the dial 62.

Splitting the headgear into multiple pod segments 6a-i allows for the headgear to fit across many different head sizes and shapes. Fig. 5 shows a detailed view of two pods 6a-b and how they fit together to construct the spine 4 of the headgear 1. Each of the pods 6a-i contains identical parts. In this section, all pods will be generically refered to as 6 unless otherwise noted. Each pod 6 contains a slot 60 at which the pod 6 is attached to tensioning assemblies 2,3, as shown in Fig. 3. The elastic band 20 runs into the pod 6 through the slot 60 and onto the reel 26. A dial 62 rotates the reel 26 that controls the extended length of the elastic band 20, as explained previously. Pods are connected at a joint formed by a slot 64 and pin 66 mechanism. The slot 64 of one pod fits around the pin 66 of another pod. Other embodiments of the invention have utilized a simple rotating hinge, but the slot 64 and pin 66 mechanism significantly increases the degree of freedom of movement for the spine 4 by allowing for both rotational and lateral movements at the joint, which helps the headgear 1 conform to irregular head shapes.

Because the spine 4 is highly flexible, a mechanism is ideally needed to bring the spine into a fixed and Open' position (minimum concavity) for ease of handling, especially when putting the headgear 1 on to a subject. The spine 4 contains a tendon 70, normally a thin string or cord that runs inside and through all of the pods 6a-i. In the embodiment shown, the tendon is attached to the spine 4 at one end (not shown) and remains loose at the other. This allows the tendon 70 to be pulled at the loose end to cause the series of pods 6a-i to lock up and tighten up.

To better define the shape and position for the open mode of the spine 4, each pod contains a stopper 68. Fig. 6 shows two pods locked against each other by the stopper 68 as the tendon 70 is pulled with the stopper 68 preventing further movement.

The first embodiment described above focuses on constructing a system for high sensor densities. For applications that do not require as many electrodes, alternative and simpler embodiments of the invention may be advantageous.

Fig. 7 shows a second embodiment of the invention, specifically the tensioning assembly.

Similar to Fig. 2, the transverse view is shown but with only one side drawn for simplicity of illustration. In this version of the tensioning assembly, a single sensor 70 (or a small array of sensors) is mounted underneath a pod 76, which is located between the top and the side of the head. Elastic bands 72a, 72b connect the pod 76 to a top anchor 78a at the crown of the head and a side anchor 78b at the side of the head. This embodiment relies upon the same principle of generating pressure, tangential to the curved surface of the head by controlling the length of elastic bands. The pod 76 contains a knob, which extends or retracts the elastic bands 72a and 72b by rotating two reels 74a, 74b. Use of two reels 74a, 74b enables symmetric control of the elastic that maintains the relative position of the sensor 70 irrespective of the absolute length of the elastic bands 72a, 72b. Compared to the first embodiment, placement of the pod 76 above a fewer number of sensors offers more precise control over each sensor at the cost of lower sensor density.

To fully illustrate the fundamental concepts of the invention, it is useful to refer to Fig. 8, which shows a third, minimal embodiment. In this embodiment, the sensor 80 is attached to a pod 86. An elastic band 82 connects one side of the pod 86 to a side anchor 88b. Pulling on a small tab 84 controls the length of the elastic band 82 and, hence, the pressure on the sensor 80, using the same principles of tension generation as the first and second embodiments. A second band 83, which may or may not be elastic, connects the pod 86 to the top anchor 88a to complete the structure. It is also possible to swap the locations of the second band 83 and the elastic band 82 without affecting the function of the tensioning assembly.

This third, minimal, embodiment, which exemplifies the invention in the simplest sense, need not involve any complex mechanics. In the most basic form, the invention simply attaches an elastic band to an anchor. Adjustments in the length of the elastic band controls the amount of tension applied on to a sensor placed underneath the elastic band, towards the user's head.

In addition to the spine arrangement shown in the first embodiment (Fig. 1), it is also possible to use different geometries to connect the pods, elastic bands and sensors, especially for lower channel count systems. As an example, the embodiments shown in Fig. 7 and Fig. 8 can form a complete headgear apparatus by sharing a common top anchor with an array of elastic bands extending radially, each connected to a set of pods, sensors and side anchors. A radial arrangement is particularly conducive to a standard 10-20 EEG sensor layout.

Further combinations are also possible. The simplest geometry would comprise of a single pod, elastic band and sensor.

It is worth noting that although the discussion revolves around a headgear for supporting EEG electrodes, the invention can be generally adapted to any transducer system that requires an array contacting the subject's head. As an example, optical emitters and sensors can be installed in the headgear in a near infrared spectroscopy system. The applications also do not necessarily need to be limited to sensing - another application could involve using the headgear for placing electrodes to deliver current into a subject (e.g., transcranial stimulation).

Although the focus of the application has been on the sensing of EEG biopotential signals, the headgear of the present invention is broadly applicable to a variety of other embodiments that involve the placement of an array of transducers on the surface of a person's head. As an example, one embodiment involves placing an array of temperature sensors on the head. Another embodiment involves placing an array of infrared emitters and detectors to measure blood oxygen saturation or near infrared spectroscopy on the head. In yet another embodiment, an array of electrical transducers is placed on the head, using the present invention, to deliver electrical current for transcranial stimulation, rather than or in addition to sensing bioelectric potentials.

Before usage, all of the tensioning assemblies are adjusted to minimum tension and the spine is loosened by manipulation of the tendon. The loosened headgear is placed on the head of the subject and the tendon is relaxed to allow the spine to follow the outline of the subject's head. The headgear is then secured to the head by closing the chinstrap. The user then adjusts each of the tensioning assemblies via the dial on each pod to apply pressure on the sensors until sufficient contact and signal quality is achieved for an acceptable EEG recording. When the user is finished with the headgear, the system can be removed by opening the chin strap and loosening each of the tensioning assemblies.

Claims

1. A tensioning assembly for applying pressure to a sensor onto a subject comprising: an anchor,

an elastic band having an adjustable length with one end connected to the anchor, wherein the elastic band is adapted for applying pressure to said sensor towards said subject; and

wherein said pressure applied by the elastic band to said sensor can be changed by adjusting the length of the elastic band.

2. The system of claim 1, wherein said elastic band is placed over a plurality of sensors for applying said pressure.

3. The system of claim 1, wherein the length of the elastic band can be adjusted by the rotation of a reel.

4. The system of claim 3, wherein a plurality of said elastic bands is attached to a said reel that is adapted for adjusting said plurality of elastic bands simultaneously.

5. An apparatus forming a spine to conform to a head comprising:

a plurality of pods,

wherein an individual pod contains a joint that permits rotational movement between the pods so that the spine can conform to the surface of different head shapes.

6. The apparatus of claim 5, wherein the pod contains a reel that is adapted for adjusting the length of an elastic band.

7. The apparatus of claim 6, wherein the reel is connected to a dial that is disposed on the outside of the pod for rotating the reel.

8. The apparatus of claim 5 wherein the joint of the pods further comprises a slot for allowing both lateral and rotational movements between the pods.

9. The apparatus of claim 5, wherein a tendon is placed inside the spine for controlling the shape of the spine when the tendon is pulled.

10. A headgear, comprising:

a tensioning assembly for applying pressure to a sensor onto a subject; and

apparatus forming a spine to conform to a head,

wherein the tensioning assembly comprises:

an anchor,

1. The headgear of claim 10, wherein the apparatus for forming a spine to conform to a head comprises:

a plurality of pods,