A NEUROPROSTHETIC SYSTEM AND METHOD FOR SUBSTITUTING A SENSORY MODALITY OF A MAMMAL BY HIGH-DENSITY ELECTRICAL STIMULATION OF A REGION OF THE CEREBRAL CORTEX

Abstract

A neuroprosthetic system for substituting a sensory modality of a mammal by electrical stimulation of a region of the cerebral cortex corresponding to the neural modality to be substituted. The system includes at least one sensor for generating a sensed data feed by sensing a neural modality to be substituted, an electrode unit including a plurality of three-dimensional arrays of flexible electrode shafts for implantation of a region of the cerebral cortex, a rigid electrode support structure for guiding the flexible electrode shafts into the region of the cerebral cortex and for retraction after implantation, a driving unit for electrically driving the electrode, a recording unit, and a processing unit for analysing sensed data feed for providing stimulation patterns for electrically driving groups of electrical contacts of the electrode unit corresponding to subsets of locations in the region of the cerebral cortex, for substituting the sensory modality.

Claims

1-15. (canceled)

16. A neuroprosthetic system for substituting a sensory modality of a mammal by electrical stimulation of a region of the cerebral cortex of the mammal corresponding to the neural modality to be substituted, comprising: at least one sensor, for use by the mammal, for generating a sensed data feed by sensing a neural modality to be substituted; an electrode unit, comprising a plurality of three-dimensional arrays of flexible, elongated electrode shafts, for intracortical implantation for a dense occupation of the region of the cerebral cortex of the mammal for providing functional coverage of the sensory modality, each shaft comprising multiple electrical contacts for electrical stimulation of subsets of locations in the region of the cerebral cortex; a rigid electrode support structure for simultaneously guiding the flexible electrode shafts of an array into the region of the cerebral cortex of the mammal during intracortical implantation, and for retracting the support structure after implantation of the array of the flexible electrode shafts; a driving unit for electrically driving the electrode unit for stimulating the subsets of locations in the region of the cerebral cortex; a recording unit for obtaining neural recording through the electrode unit in the region of the cerebral cortex; and a processing unit for analysing the sensed data feed for providing stimulation patterns for electrically driving groups of electrical contacts of the electrode unit corresponding to subsets of locations in the region of the cerebral cortex, for substituting the sensory modality.

17. The neuroprosthetic system according to claim 16, wherein: the neuroprosthetic system is operable for substituting visual perception in a visual region of the cerebral cortex of the mammal; the at least one sensor comprises at least one portable imaging unit for capturing images and generating a captured image data feed; the driving unit is operable for evoking phosphenes at locations in the visual region of the cerebral cortex; and the processing unit is operable for providing the stimulation patterns for stimulating groups of electrical contacts of the electrode unit corresponding to subsets of phosphene locations in the visual region of the cerebral cortex for evoking phosphenes for substituting the visual perception comprised of phosphene patterns obtained through semantic segmentation of images of the captured image data feed.

18. The neuroprosthetic system according to claim 16, further comprising a switching device for channelling one or more stimulation signals of the driving unit to subsets of electrical contacts located within the region of the cerebral cortex providing the functional coverage of the sensory modality.

19. The neuroprosthetic system according to claim 16, wherein: each of the multiple three-dimensional arrays comprises a base plate and the flexible electrode shafts extend from one surface of the base plate; the electrical contacts are distributed in a lengthwise direction of the elongated electrode shafts; and each electrical contact connects to an electrical lead supported by the base plate.

20. The neuroprosthetic system according to claim 19, wherein: the base plate has a substantially quadrangular shape and has side dimensions in the range from 4 mm to 30 mm and a thickness in the range from 10 μm to 200 μm; the flexible electrode shafts are ribbon shaped and have a length in the range from 0.5 mm to 40 mm, a width in the range from 5 μm to 100 μm, and a thickness in the range from 1 μm to 30 μm; the electrical contacts are distributed in a lengthwise direction of the electrode shafts with a mutual maximal spacing of 800 μm; and the electrode shafts extend from the base plate with a mutual spacing from 0.1 mm to 2 mm.

21. The neuroprosthetic system according to 19, wherein: the electrode support structure comprises a plurality of mutually coupled elongated rigid insert shafts for guiding the flexible electrode shafts for intracortical insertion in the selected region of the cerebral cortex; the base plate comprises a two-dimensional array of through holes receiving the plurality of elongated rigid insert shafts having a rectangular cross section with a width at least approximately identical to the electrode shaft and a thickness in the range from 10 μm to 50 μm, or a circular cross-section with a diameter of 1 μm to 50 μm wherein the elongated rigid insert shafts are positioned with a mutual spacing in the range of 0.1 mm to 2 mm; each flexible electrode shaft affixes to a respective elongated rigid insert shaft by a brain fluid dissolvable adhesive or by a hole at a top of the flexible electrode shaft that is penetrated by a tip of the insert shaft; and at least one of the elongate rigid insert shafts and the flexible electrode shafts comprises a tapered free end.

22. The neuroprosthetic system according to claim 21, wherein: the flexible electrode shaft comprises a tissue-biocompatible material; the electrical contacts comprise iridium oxide microstructured platinum or titanium nitride; the base plate comprises silicon sheet material; the electrical wires comprise biocompatible electrical conductive material; the elongated rigid insert shafts comprise tungsten or silicon; the dissolvable adhesive comprises polyethylene glycol; and at least one of the elongated rigid insert shafts and the flexible electrode shafts comprises a tapered free end.

23. The neuroprosthetic system according to claim 16, wherein the processing unit is configured to determine correlations between neural signals from the electrodes, and locations in the sensory cortex area, for deducing functional maps of the sensory cortex.

24. The neuroprosthetic system according to claim 16, wherein: the processing unit comprises a transceiver configured for wireless data communication; the neuroprosthetic system comprises a signal transducer arrangement configured for wireless data communication with the processing unit, the signal transducer arrangement configured for controlling the recording unit and the driving unit for driving and recording of the electrical contacts in a calibration mode and an operational mode, wherein the electrical contacts in the operational mode are controlled for signal stimulation and in the calibration mode for signal stimulation and signal reception, and wherein the signal transducer arrangement is configured for controlling the driving unit in the calibration mode for driving the electrical contacts for signal stimulation in a cortex area, and signal reception in one or more other cortex areas for determining potential locations in the region of the cerebral cortex for substituting the sensory modality.

25. The neuroprosthetic system according to claim 24, wherein the signal transducer arrangement is configured for controlling the driving unit in the calibration mode for driving the electrical contacts in one of signal stimulation and signal reception at predetermined regular time intervals, for recalibrating at least one of stimulation signals and potential locations in the region of the cerebral cortex for substituting the neural modality.

26. The neuroprosthetic system according to claim 25, wherein the signal transducer arrangement comprises electrical simulation circuitry operable for simultaneously energizing, during the operational mode, a plurality of electrical contacts corresponding to the subsets of locations for substituting the neural modality in accordance with sensory primitives.

27. The neuroprosthetic system according to claim 24, wherein each of the multiple electrical contacts connects by an electrical lead to a plurality of multiplexers of the transducer arrangement.

28. The neuroprosthetic system according to claim 24, wherein each of the plurality of arrays of flexible electrode shafts comprises at least one multiplexer connected to the multiple electrical contacts of the array and one or more multiplexer leads for electrically driving the electrical contacts by the driving unit.

29. The neuroprosthetic system according to claim 16, wherein the processing unit is configured for analysing the sensed data feed for providing the stimulation patterns based on convolutional neural networks.

30. The neuroprosthetic system according to claim 16, further comprising at least one eye-tracking sensor for sensing perceptual information for augmenting the substitution of the neural modality.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0075] FIG. 1 illustrates, schematically, a neuroprosthetic system according to the present disclosure;

[0076] FIG. 2 illustrates an assembly of a three-dimensional array of electrodes and a support structure according to the present disclosure;

[0077] FIG. 3 illustrates a detailed view of an insert shaft of the support structure for inserting the electrode of the three-dimensional array according to the present disclosure;

[0078] FIG. 4 illustrates the transducer unit and multiple three-dimensional arrays according to the present disclosure;

[0079] FIG. 5 illustrates the transducer unit and the lead connections to the arrays according to the present disclosure;

[0080] FIG. 6 illustrates a side view of a brain with multiple arrays in the visual region of the cerebral cortex according to the present disclosure;

[0081] FIG. 7 illustrates several steps of semantic segmentation of an image captured by the camera of a system according to the present disclosure.

DETAILED DESCRIPTION

[0082] FIG. 1 schematically illustrates a neuroprosthetic system 100 for substituting a sensory modality of a mammal, in this case a patient, by electrical stimulation of a region of the cerebral cortex of the patent which corresponds to the neural modality that is to substituted.

[0083] Although the examples illustrated in the figures and described in the description below are directed to a visual neuroprosthetic system in which visual perception is substituted in the visual region of the cerebral cortex, the present invention is not particularly limited solely to the visual sensory modality. The skilled person will appreciate that the present invention is also applicable for other sensory modalities such as the auditory or somatosensory modality.

[0084] Merely as in illustration, the system 100 as demonstrated in FIG. 1, contains several separate physical units, which are contained in separate housings. The skilled person will appreciate that some units however, may also be combined into a single housing.

[0085] The system 100 at least consists of a sensor 150. The sensor may comprise a digital camera or cameras, photodetectors or photosensors, for operating in the visual and/or InfraRed, IR, spectrum, and mounted at glasses or the like. In the example shown in FIG. 1 the sensor is an image sensor which is mounted on glasses 140 of the patient. The sensor may however also be a separate camera that is attachable to the body or can be worn by patient in any other way. The camera may also be a separate hand held camera unit or incorporated in a portable device such as a mobile phone or portable computer device like a tablet.

[0086] The camera 150 is pointed in a direction that corresponds to the field of view 160 of the patient. As such, the camera is able to record or capture a data stream, in this example a data stream consisting of a plurality of images. The data stream contains data or images that cover at least most of the field of view 160. The term field of view refers to the restriction of what is actually visible by the camera 150. Since the human eye may have a larger field of view than most camera devices, the system may also comprise an eye-tracking unit, which is not shown in FIG. 1. The eye-tracking unit is able to detect the position of the eye of the patient, and thus where the eye is directed to. In this way, the camera device may be focused to his direction and generate a data stream that corresponds to the focus area in the field of view of the patient.

[0087] The system 100 also consists of an electrode unit 130. The electrode unit 130, in practice, consists of multiple three-dimensional arrays 130a, 130b, 130c. The system 100 consists preferably of approximately 20 of these arrays 130, for example 10 implanted into the left hemisphere and 10 in the right hemisphere.

[0088] Each three-dimensional array 130a, 130b, 130c consists of multiple flexible, elongated electrode shafts. These electrode shafts have a shape and are made of a material that makes them suitable for intracortical implantation in the brain 180 of the patient. To make them suitable, these electrode shafts may be manufactured of a tissue-biocompatible material such as a polyimide or SU8. The skilled person will however appreciate that other biocompatible insulating materials in combination with electrical conductive materials may also be suitable.

[0089] The electrode shafts are distributed in such a three-dimensional manner over the array that they are able to make electrical contact with a large three-dimensional region of the cerebral cortex, i.e. with most of, or approximately all of the visual region of the cerebral cortex of the patient. This means, that not only in two directions the spacing dl in FIG. 2 between the individual electrode shafts is small, e.g. up 0.4 to 1 mm, but that the electrodes shafts are also longer then electrodes of known electrode arrays, with a length up to 20 or even 40 mm. Preferably, the length of the electrode shafts are in the range of at least 2 to 5 mm till 20 or 40 mm. More preferably, the length is in the range of 5 till 30 mm, even more preferably in the range of 7 till 25 mm, and most preferably in the range of 15 till 20 mm. This way, the electrodes are able to extend beyond a single, or cover even plural folded gyrus in the cerebral cortex.

[0090] Each electrode shaft consists of multiple electrical contacts. These contacts may be controlled individually such that some of the contacts of a single electrode shaft are electrically driven or activated by applying a stimulation signal, and others are not operated. Each contact may also be operated for either signal stimulation or signal recording. In an operational mode, the contacts are operated for signal stimulation only. In a configuration mode, the contacts may however also be controlled to operate in a signal recording modus such that neural activity or neural response to the stimulation signal may be recorded accordingly. In an example, each contact may be arranged such that is can operate both in the stimulation and recording modus, and in an alternative example, contacts may be dedicated for either signal stimulation or signal recording.

[0091] In order for the electrical contacts to process the signal for neural stimulation or neural recording, the system 100 consists of a driving unit and a recording unit. These units may be contained in separate housings, at separate locations in or on the patient, but are preferably contained in a transducer device which may be close to or integrated with the shaft or positioned directly under the scalp on the skull, for example in or near the layer of loose connective tissue of the scalp.

[0092] In order for flexible electrode shafts to successfully penetrate the surface tissue of the cerebral cortex, the system 100 also consists of a rigid electrode support structure. To this end, support structure consists of an array of rigid or stiff insert shafts. The flexible elongated electrode shafts are attached or fixed to the insert shafts of the support structure upon intracortical implantation in the cerebral cortex. Once inserted, the insert shafts disengage such that the insert shafts can be retracted, whereas the electrode shafts remain in place in the cerebral cortex. Attaching and disengaging can be achieved in different ways. For example, the inserts and respective electrodes may be temporarily attached by a brain fluid dissolvable adhesive such as PEG. This example may well work with flat ribbon shaped electrodes. In an alternative, non-flat shape of the ribbon, e.g. with a round or circular cross sectional shape, the tip of the electrode may be provided with an engaging element such as a hole that is to be penetrated by the tip of the insert shaft.

[0093] The electronics of the driving unit, to electrically drive the electrode unit for stimulation of the subset of locations, as well as the recording unit, to obtain neural recording through the electrode unit in the region of the cerebral cortex, can be packaged inside an implantable transducer casing 120 and shielded from the tissue by the material of the casing, which is preferably made of titanium. In case the electronics or electronic circuitry may reside on the electrode unit or more particular, on the three-dimensional arrays, they are embedded in a tissue compatible packaging material such as LCP or polyimide.

[0094] The implant 120 also consists of a switching unit. The switching unit consists of electronic circuitry which channels stimulation signals from the driving unit to the actual electrical contacts located in the cerebral cortex. What that means is, that the driving unit may be equipped with circuitry to generate a plural stimulation signal, i.e. a stimulation pulse of a particular waveform such as a sine wave form, a square wave form, a rectangular wave form, a triangular wave form, a sawtooth wave form, a pulse wave form or any combination of wave forms. The generated signals may not only differ in shape but also in amplitude and can be channelled to different individual or subsets of electrical contacts. This way some groups or subsets of contacts may not be activated at all, others with a first stimulation signal, yet another group with a second, different stimulation signal, and so on. To this end, the switching unit is arranged to achieve electrical channelling of the different signal generators and the subsets of electrical contacts.

[0095] To control the driving unit, the recording unit, and thereby the electrodes of the electrode unit, the system is also provided with a processing unit 170. The processing unit is preferably a handheld device, since that will increase independence and quality of life of the patient. The processing unit or device could be a general-purpose hand-held device, such as the mobile phone as shown by way of example in FIG. 1, which is programmed to operate as processing unit 170 for the neuroprosthetic system 200 according to the invention. The processing unit or device is however more preferably a dedicated device but with dimensions that are preferably similar to those of a mobile phone.

[0096] The processing unit 170 may communicate with the transducer arrangement or transducer device 120 by use of a wireless communication unit 110. This wireless communication unit 110 not only provides wireless technology to control high-channel-count high-density cortical implantable arrays 130a-130c, but also provides a wireless power transfer interface that can send the signals of the recording unit. This can be achieved for example through capacitive, or magnetodynamic, but preferably through inductive coupling between the wireless unit 110 and the transducer implant 120.

[0097] By having both wireless power and data communication between the wireless unit 110 and the transducer implant 120, the risk of infection that is associated with skin-penetrating cables and connectors is reduced.

[0098] With the processing unit 170, the system 100 is able to receive and analyse the data stream sensed by the sensor. In the example shown in FIG. 1, these are images of the data stream captured by the camera 150. These images have to be processed in such a way that the correct electrical contacts are powered or provided with a stimulation signal to evoke a phosphene, i.e. a phenomenon characterized by the experience of seeing light or a specific light dot, at a phosphene location which corresponds to the actual position of the electrode shaft or more precise, the electrical contact of that particular electrode shaft. The subset of phosphene locations should correspond to one or more objects that are captured by the camera and thus in the scene in the field of view 160 of the patient.

[0099] In order to make such a translation between actual captured image data and the subset of phosphene location, which together form a phosphene pattern or patterns, the processing unit 170 employs semantic segmentation.

[0100] With semantic segmentation, the image is processed such that each pixel is assigned to a particular object class. The object is in a way labelled and contains a set of pixels that enclose or outline the object.

[0101] Once the processing unit 170 processed the image data through semantic segmentation, an outline, contour, label, shape or other primitive object is determined which can then be used to as phosphene or stimulation pattern to apply stimulation signals to the appropriate subset of phosphene location. Once these stimulation signals are applied, the invoked phosphenes at these phosphene locations will substitute the visual perception of the patient such that he or she will recognize the actual object as a car or whatever object is in the field of view 160 of the patient

[0102] In FIG. 2 an assembly 130 is shown of a three-dimensional array 133 of flexible, elongated electrodes or electrode shafts with a rigid electrode support structure 132 for guiding the flexible elongated electrodes 135 upon intracortical implantation in a region of the cerebral cortex of a patient.

[0103] The support structure 132 enables simultaneously guiding of the flexible electrode shafts 135 of the array 133 upon implementation and once implemented the insert shafts 134 may detach from the electrodes 135 and the support structure 132 can be retracted. All insert shafts 134, which preferably are manufactured from tungsten, are mutually connected and connected to a metal rod which makes simultaneous, concurrent insertion and retraction possible.

[0104] As clearly indicated in FIG. 2, both the number and the distribution of the insert shafts 134 of the support structure correspond to a number and distribution of through holes in the base plate 131 of the array 133, such that each individual insert shaft 134 is guided by a single respective though hole, thereby ensuring that all electrode shafts or electrode ribbons 135 arrive at the correct position and at the correct depth in the visual region of the cerebral cortex. This could preferably be primary cerebral region V1, but also mid-ranged or high regions such as V2, V3 or V4.

[0105] All individual electrodes 135 preferably have the same length of approximately 0.2-4 cm. The spacing between the individual electrodes is preferably approximately between 0.1 and 2 mm.

[0106] Each electrode shaft 135 contains multiple electrical contacts 136 which are distributed preferably in the longitudinal direction of the shaft. The contact 136 may however also have a lateral distribution pattern or could be distributed in both a lateral and longitudinal direction of the shaft.

[0107] Each electrical contact 136 is electrically wired through an electrical lead. The plurality of leads 138 are electrically connected to the transducer unit 120 as shown in FIG. 1.

[0108] As illustrated in the two detailed views in FIG. 2 there are several ways in which the electrode shafts 135 can engage with the insert shafts 134 of the support structure 132. In the example shown in the left detailed view the rigid insert shafts 134 have a rectangular cross section with a width that is identical or least approximately identical to the electrode shaft 135. The thickness in that particular example is in the range of 10-50 μm. In this example, it is preferred to use a dissolvable material such as PEG.

[0109] In the example shown in the right detailed view the rigid insert shafts 134 may or may not have a circular cross-section with a diameter of approximately 10-50 μm. In this example, the insert shafts 134 have a tapered or smaller diameter tip or free end to engage with a hole in the tip or free end of the electrode shaft 135.

[0110] Although the right detailed view might suggest that the electrical contacts are located on the insert shafts 134, they are actually located on the electrode shaft 135. The electrode shafts 134 can be sleeve shaped to receive the insert shafts 134, but these are more preferably flat opaque ribbon shaped electrodes with a through hole or other receiving element to receive the tip of the insert shaft for ease of surgical insertion into the brain.

[0111] In this example, there is no need for a dissolvable material such as PEG. Once the electrode shafts 135 are in position, the rigid insert shafts can easily be retracted, leaving the electrodes 135 in place.

[0112] FIG. 3 shows a more detailed view of the base plate 131 of the electrode array 133 with a two-dimensional distribution pattern of the through holes 137. FIG. 3 only shows one single electrode and insert shaft. The electrode unit and support structure will however contain a similar number of electrodes and insert shafts that correspond to the amount of through holes 137.

[0113] In FIG. 4 the electrode array 133 also contains additional circuitry 139 In one example of the invention all electrical contacts of each electrode shaft of each array are electrically wired to the transducer 120. In the example shown in FIG. 4 the array however comprises a multiplexer to multiplex and de-multiplex the signals of the large number of electrical wires 138 to one or more likely a lower number of wires. Since per insert, the number of wires could be as high as 50,000 the use of several multiplexers is desirable. The multiplexer may however be located either on the array 133, as shown in FIG. 4, or may also be located in the transducer 120.

[0114] In FIG. 5 each array 130a-130d comprises a multiplexer 139. Although it might be preferable to have the multiplexers as near as possible to the huge number of wires, thus on or near the array, this would reduce the number and length of the leads or wires but could also produce too much heat that may affect the brain. In that case, it is preferred to have all wires run to the transducer and have them connected to multiplexers within the transducer casing 120.

[0115] Although the driving unit and the recording unit could also reside in each array 130, they are preferably located within the transducer unit 120. In the example shown in FIG. 4 however at least some electronic circuitry 139 is located on the array itself. In this example the electronic circuitry 139 may be arranged to multiplex the plurality of signals that are obtained in the recording modus through the large number of electrodes, but is preferably also arranged as switching unit to perform the channelling of the signal generators. Thus, to connect groups of electrical contacts with one of the signal generators.

[0116] FIG. 6 shows a side view of the brain 180 of the patient. In the example shown in FIG. 6, several arrays 130 are implanted the visual region of the cerebral cortex. Most of the arrays 130a, 130b etc. are implanted in area V1. However, at least one of the arrays 130c is however implanted to achieve at least partly or fully coverage of area V4, this in order to perform the recording and stimulation in such a higher visual cortex region.

[0117] As indicated above, this visual region is only an example in which the arrays according to the present disclosure can be implanted. Other sensory modality regions of the cerebral cortex may be implanted with similar arrays according to the present disclosure. These regions, such as the auditory regions or auditory cortex may for example be implanted with similar arrays which however have electrode shafts with different dimensions adapted to that particular region.

[0118] In view of the above, the skilled person will appreciate that the dimensions such as length of the electrode shafts as indicated relate in general to the visual cortex. Although these dimensions may very well also be suitable for other regions such as the auditory, tactile, somatosensory or olfactory functionality. Some or all of these regions may however require different dimensions.

[0119] FIG. 7 shows several steps a-h of the translation of the images of the data stream feed into a phosphene pattern by making use of semantic segmentation. In step a, the camera images are electrically imparted to the retina or cortex using an image transformation algorithm in a portable device. In step b, an example is shown of an outdoor scene that is within the field of view of the patient shown in figure a. Imagine that the patient approaches the car. In step c, the locations of 64 phosphenes are shown that the patient can perceive when retinal electrodes are stimulated. Step d, shows the potential locations of 1,000 phosphenes that can be achieved by the stimulation of cortical area V1. In step e, a limitation of known prosthetic devices is shown which are not able to distinguish between relevant and irrelevant contours (such as foreground and background information in the image). Semantic segmentation algorithms however delineate the pixels in the image that belong to the car. The result of semantic segmentation as shown in step f, can be used as template to determine the stimulation patterns as shown in step g. If semantic segmentation is used to determine the pattern of activated phosphenes (subset of those in step c), perception of the approximate location and shape of the car is expected, resulting in a large improvement of vision through the prosthetic device. In step g, a cortical prosthesis according to an aspect of the invention can give rise to an even higher resolution percept.

[0120] Other variations to the disclosed examples can be understood and effected by those skilled in the art in practicing the claimed disclosure, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not construed as limiting scope thereof. Similar reference signs denote similar or equivalent functionality.

[0121] The present disclosure is not limited to the examples as disclosed above, and can be modified and enhanced by those skilled in the art beyond the scope of the present disclosure as disclosed in the appended claims without having to apply inventive skills and for use in any data communication, data exchange and data processing environment, for example for use of the neuroprosthetic system for substituting auditory perception.