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  Electrode multiplexing method for retinal prosthesisUSPTO Application #: 20060241753Title: Electrode multiplexing method for retinal prosthesis Abstract: A method is disclosed for efficient multiplexing of a plurality of electrodes in a nerve stimulator using improved, predetermined, regular, repeatable geometric patterns arranged in a predetermined mosaic to form a desired array. Multiple electrodes within said array are addressed by the nerve stimulator as being a stimulating electrode by an instruction specifying a single identifier, indicating a position within each regular geometric pattern. As such, each electrode within the array, maintaining the specified position within its respective repeatable geometric pattern, becomes a stimulating electrode and is connected to the appropriate electronic circuit for subsequent, potential use in nerve stimulation. (end of abstract) Agent: Gregg Jorgen Suaning - Narara, AU Inventors: Gregg Jorgen Suaning, Nigel Hamilton Lovell USPTO Applicaton #: 20060241753 - Class: 623006630 (USPTO) Related Patent Categories: Prosthesis (i.e., Artificial Body Members), Parts Thereof, Or Aids And Accessories Therefor, Eye Prosthesis (e.g., Lens Or Corneal Implant, Or Artificial Eye, Etc.), Retina The Patent Description & Claims data below is from USPTO Patent Application 20060241753. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention is directed generally to approaches to electronic neuroprostheses. More specifically, the present invention is directed to the use of improved multiplexing of stimulation signals to arrays for retinal prostheses using electrode array configurations and switching logic. BACKGROUND OF THE INVENTION [0002] A single electrode has proven to elicit the perception of a spot of light, a so-called phosphene, in humans with vision impairment. U.S. Provisional Application 60/473,304, filed May 29, 2003 disclosed the implantation of electronic devices wholly or partially at the retina, with an array of electrodes provided to deliver electrical stimulation to remaining intact retinal neurons. An improved arrangement of electrodes was disclosed comprising a stimulation array whereby electrodes are arrayed, in whole or in part, in a staggered pattern allowing for a high density of phosphenes, but wherein the elicitation of discrete phosphene is nonetheless achievable. (See FIG. 1) [0003] In such an array each electrode of said stimulation array is relatively large by comparison to remaining intact retinal neurons, stimulating many neurons when actuated. In such an array, an electrode primarily activates intact retinal neurons that lie on the small, retinal region directly adjacent to the center of said electrode. With increased stimulation, said region of activation increases, a phenomenon approximately modeled by circular regions of increasing radii, concentric to said adjacent region. [0004] For an electronic retinal prosthesis, electrodes effectively render an image by way of phosphenes in the implant recipient's visual field. This is achieved by way of each electrode activating a population of retinal neurons in a discrete region; each population pertaining to the perception of a phosphene. A high density of rendered phosphenes (and therefore a high density of electrodes) is desirable for it allows better visual acuity in the implant recipient. This density, however, is constrained by interference. If any two regions of activation are too close, injected charge will interfere, meaning that the elicitation of discrete phosphenes can not be achieved. For example, an intraocular array of two, small, stimulating electrodes which, when actuated maximally, can activate two large, circular regions of retinal tissue. The two stimulating electrodes need be disparate enough such that the two, said circular regions do not interfere. However, since high density is desirable, the two electrodes should be close enough such that the two circular regions meet tangentially. [0005] In light of the above, the problem as to how to array said stimulating electrodes is analogous to the geometric problem regarding optimally packing equi-sized circles on an unbounded plane. It is a geometric result that the densest packing of equi-sized circles on an unbounded plane is a mosaic exhibiting staggering between successive rows and columns, as illustrated by the four rows and seven columns in FIG. 1. An example of this mosaic is the hexagon. [0006] A stimulating prosthesis of non-trivial complexity must be configured to deliver electrical stimuli. Typically this is achieved by way of switching, via a multiplexing circuit, current or voltage sources to the intended electrodes. Configuring said multiplexing circuit requires instructions to configure, time to convey, and time to act upon said instructions. It is therefore advantageous to reduce either or both the instructions necessary to configure the multiplexing circuit, or the time required for said instructions to be delivered and acted upon. [0007] Utilizing the hexagonal mosaic for electrode layout, or abstracts thereof, a novel multipexing method for configuring and delivering the stimulus or stimuli from said electrode layout is described. SUMMARY OF THE INVENTION [0008] In one embodiment, the present invention proposes that stimulating electrodes be arrayed in geometric correspondence, in whole or in part, with a pre-determined staggered pattern and multiplexed according to instructions to appropriate current or voltage sources wherein said instructions are minimised by way of using the electrode geometry to interpret said instructions. One useful pattern having optimal packing density is a dimensionally staggered hexagonal array. Multiplexing of said hexagonal array is achieved by way of an addressing method based upon the hexagaon layout. [0009] An object of this invention is to provide a method for connecting stimulating electrodes to current or voltage sources in a nerve stimulating prosthesis. [0010] It is a further object of this invention to provide such a method for connecting stimulating electrodes to current or voltage sources utilizing the geometric shape of the hexagon as a fundamental means of addressing said stimulating electrodes. DESCRIPTION OF PRIOR ART [0011] Implantable neural prostheses in mainstream treatment of disease began with the cardiac pacemaker in the 1950s. It is now possible to treat cardiac arrhythmia with pacemakers so that the recipient may lead a near normal life and maintain near normal life expectancy. Later, based upon similar technology, the cochlear implant gave rise to the ability to restore hearing to the deaf and severely hearing impaired. Nenral deafness can be treated with cochlear implants to such an extent that a large proportion of recipient patients are able to converse over the telephone, attend mainstream schools and function within society with undetectable disability. Variants of the same technology have been used to restore movement and body function in paralyzed people, and to attenuate tremor in the neurologically diseased. [0012] It may someday be possible to restore useful vision and physical movement to the blind and paralyzed respectively through analogous methods. The division between the two groups described above, those that are commonplace treatments and those that remain in fledgling research, exists as a result of several factors but one of the principal reasons for the substantial successes of the cardiac pacemaker and the cochlear implant is their ability to deliver appropriate electrical stimulation to the appropriate site at the appropriate frequency without exacerbating damage to tissue or the disease condition. For this to occur, an effective means of injecting current and subsequently recovering or safely dissipating this current is required. [0013] The cardiac pacemaker requires only a single stimulation channel (electrode) with the return path for the neurostimulation via the pacemaker capsule itself. For the cochlear implant, substantial benefit can be achieved with as few as 16 stimulation channels and associated electrodes. [0014] In contrast, little visual information can be conveyed with any number of stimulation channels fewer than 64 [Cha1992], with hundreds or thousands of electrodes necessary to approximate the rich sense of vision that sighted people enjoy. To place this in terms of information flow, the optic nerve of a healthy eye carries approximately one million individual fibres, each of which may transmit chemical signals of an on/off (digital) nature at a rate of up to 200 `bits` per second. This equates to approximately 200 Megabits per second per eye. [0015] The order of stimulation frequency for each of the above examples differ markedly. Assuming a single source of stimuli, said source must supply approximately two stimulus pulses per second for the cardiac pacemaker so as to maintain a heart rate of 120 beats per minute, and up to approximately 15,000 pulses per second for the cochlear implant in order to take advantage of the most modern speech processing techniques [Huang1998]. [0016] In the case of a visual prosthesis, the optimal stimulation rate is not yet established and is likely to be the topic of intense debate well into the future. The practical lower limit of the stimulation rate is the so-called critical flicker fusion (CFF) frequency, below which spots of light (phosphenes) conveyed by way of a visual prosthesis appear pulsatile. The CFF is further dependent upon the site of stimulation, e.g. the retina, optic nerve, lateral geniculate nucleus or visual cortex. It is likely that frequency modulation above the CFF will play a substantial role conveying safe and effective phosphene information in image processing and stimulation strategies. In anticipation of such implementation of frequency modulation techniques, the stimulation rate capabilities of any implantable visual prosthesis must be well above the CFF with an upper limit of the signal carrying capacity of individual optic nerve fibres which is of the order of 200 signals per second. In instances where large numbers of electrodes are employed, the stimulation rate is a linear multiple of the electrode quantity. [0017] There exist several locations within the visual pathway that could conceivably be used for neurostimulation. Phosphenes have been elicited in the visual cortex [Brindley1968], the geniculocalcarine tract [Marg1965] and the superior colliculus [Nashold1970]. Optic nerve stimulation has been shown to be a plausible site with successful recording of cortical responses obtained following stimulation [Shandurina1986], and more recently, mapped phosphenes being conveyed in a human subject [Veraart2003]. The correspondence of phosphenes with stimulation of retinal topography has demonstrated the plausibility of retinal stimulation [Humayun1996]. [0018] The aforementioned represent potential intervention points for an artificial vision prosthesis. The direct brain stimulation approaches carry with them an inherent safety risk to the patient that arguably overshadows any potential benefit that could be acquired. Furthermore, it has been recognized since Brindley and Lewin's time that electrode quantities available in present technologies (limited to the order of tens of electrodes) are insufficient to convey useful vision at the visual cortex by virtue of the substantial area dedicated to vision within the brain and the disbursed phosphenes elicitied by electrodes placed in close vicinity to one another in this region. Other areas of the brain do not lend themselves to sufficient degrees of access to justify attempts towards application of visual prostheses. Veraart's work [Veraart2003] has shown the optic nerve to be an intervention site of substantial promise. Of particular difficulty, however, is the lack of corresponding mapping between phosphene percepts and electrode locations. This leads to the assertion that in the presence of viable neurons, the retina itself is the most efficacious site for a neural prosthesis for the restoration of vision and is the target of choice for the present proposed study. The CFF of the intact retina is dependent upon light intensity (Ferry-Porter Law). In an illustrative example of a television, no flicker is observed at a screen refresh rate of 25 Hz (interleaved wherein every second line of pixels is updated with each pass at 50 Hz) at moderate brightness. A different situation applies in electrical stimulation applied to the diseased retina. In these instances, intra-retinal processing is dysfunctional as a result of the disease, and what mechanisms that remain are by-passed by the way in which the bipolar and retinal ganglion cells are preferentially stimulated over other surviving neurons [Greenberg1999][Rattay2003]. The apparent result of this is a reduction in the duration of the perceived phosphenes. [0019] In testing on human subjects, flicker fusion occurs at a two to three times the frequency in electrical stimulation when compared visual flicker fusion of normal eyes [Humayun1999]. [0020] To avoid the perception of flicker, a single stimulation source must be capable of stimulating once every 20 ms. For pulse widths of the order of 1 ms each (2 ms with a charge recovery phase), as few as ten electrodes may be driven in series from a single source. For higher rates of stimulation, this problem is reduced to some extent but stimulation thresholds using constant current waveforms of durations substantially below 0.5 ms have not been reported. Continue reading... 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