Abstract:
The invention described herein comprises an electronic retinal prosthesis for the treatment of some forms of blindness in animals (humans inclusive) wherein surviving retinal neurons of an intact, damaged, or diseased retina, may be physiologically activated by way of electrical stimulation delivered from electrodes placed in biologically sustainable positions so as to produce the perception of light in said animal.

Description:
RELATED APPLICATIONS 
   This application claims priority under U.S.C. §119(e) from provisional application No. 60/331,579 entitled “Vision Prosthesis for the Blind, Sustainable Electrode Positioning and Method for Implementing the Same” filed 20 Nov. 2001. 

   FIELD OF THE INVENTION 
   The present invention relates generally to a method and apparatus for treating some forms of blindness, and more particularly to a method and apparatus for stimulating the retina in the eye. 
   BACKGROUND OF THE INVENTION 
   The eyes enable light perception in animals. The animal perceives visual images when an eye senses light and sends visual information to the brain via the optic nerve. Each eye includes the sclera, retina, pupil, lens, and a vitreous chamber filled with vitreous humor. The outer layer of the eye includes an opaque portion, the sclera, that is continuous with a transparent portion, the cornea, for receiving light. The iris regulates the light, entering the eye through the cornea, that passes through to the lens. The lens focuses the light through the vitreous chamber and onto a network of neurons and fibers, known as the neural retina, some cells of which (the retinal ganglion cells) collect together to form the optic nerve. 
   Vision loss and/or blindness results when the neurons in the retina are damaged or destroyed, e.g., by disease. Known prior art efforts for treating this vision loss include electrically stimulating surviving retinal neurons. Passage of electrical current, from one or more stimulating electrode(s), through or across retinal neurons, en-route to a reference electrode or plurality of reference electrodes, results in physiological excitation of said retinal neurons in the form of change in trans-membrane electrical potential of one or more neuronal cells. Said change in trans-membrane electrical potential, if sufficient in magnitude, eventuates, directly or indirectly, the propagation of action potentials along the optic nerve to the vision centers and accessory optic pathways of the brain. Upon receipt of the action potentials by the vision centers and accessory optic pathways, perception of light and other physiological responses occur. 
   The electrical properties of the eyes have been studied and documented for decades. In a 1956 study, “The Passive Electrical Properties of the Frog&#39;s Retina and Sclera for Radial Fields and Currents” (Journal of Physiology, 134:339–352), Giles Brindley revealed a structure he deemed the “R-Membrane”. He described the R-Membrane as, “a retinal structure of high radial resistance and capacity and small thickness.” He went on to suggest that the R-Membrane was associated with the external limiting membrane. Subsequent investigators have determined that the R-Membrane is associated with the tight junctions of the retinal pigment epithelium. The results from these investigations are recorded in “Localization of Electrical Activity in the Cat Retina by an Electrode Marking Method,” by K. T. Brown and K. T. Tasaki (Journal of Physiology, 1961, 158:281–295), and in “On the R-Membrane in the Frog&#39;s Eye: Its Localization and Relation to the Retinal Action Potential,” by T. Tomita, M. Jurakami, and Y. Hashimoto (Journal of General Physiology, 1960, 43:81–94). These studies have documented various electrical properties of the retina. 
   Further, K. T. Brown and T. N. Wiesel disclosed in “Localization of Origins of Electroretinogram Components by Intraretinal Recording in the Intact Cat Eye,” (Journal of Physiology, 1961, 158:257–280) that the vitreous humor approximates an isopotential medium, a fact confirmed by the inventors through experimentation. As disclosed in “Trans-Retinal Electrical Stimulation using a Neuroprosthesis: The Effects of Damage to the R-membrane,” by G. J. Suaning, N. H. Lovell, and Y. A. Kerdraon (24th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, 23 –26 Oct. 2002, Houston, Tex., USA), moving the location of the intravitreal electrode within the vitreous chamber had no discernable effect on the amplitude of the electrically evoked potential (EEP) measured from the visual cortex of the sheep. In the same study, the recorded EEP was of similar amplitude to the visually evoked potential (VEP) derived by strobe-light stimuli. The similarity in these amplitudes is apparently associated with stimulation of large regions of the neural retina. These observations are not surprising and indeed concur with D. R. Crapper and W. K. Noell&#39;s 1963 study, “Retinal Excitation and Inhibition from Direct Electrical Stimulation,” (Journal of Neurophysiology, 26:924–947). According to Crapper and Noell, “The high conductivity of the vitreous humor and a high resistance of uniform distribution at the outer retinal surface probably are responsible for the finding that the trans-retinal pulse stimulated a wide retinal area and that electrical stimulation elicited similar phenomena as diffuse light stimulus.” The above journal articles are hereby incorporated by reference. 
   Currently, some prior art methods stimulate the retina by attaching electrodes to the retina. For example, U.S. Pat. No. 5,935,155 to Humayun et al., herein referred to as the Humayun patent, discloses a vision prosthesis that includes an electrode array secured to the retina and stimulating electronics connected to the electrode array. An external camera, and the corresponding electronics, captures and processes images, encodes the resulting data, and transmits the data to the stimulating electronics. The stimulating electronics interpret the received data and activates one or more electrodes in the electrode array to stimulate the retina. 
   U.S. Pat. No. 6,427,087 to Chow et al., herein referred to as the Chow patent, discloses a vision prosthesis that includes stimulating electrodes and a ground electrode surgically implanted in the eye. The stimulating electrodes contact one side of the retina while the ground electrode passes through the retina and is positioned on the opposite side of the retina from the stimulating electrodes. Generally, the stimulating electrodes consist of photodiodes that detect light entering the eye and produce a stimulating electrical signal corresponding to the light. Because the polarity of the ground electrode in the Chow patent is fixed, the Chow invention delivers only monopolar stimulation (no charge balance). 
   The Humayun and Chow patents are herein incorporated by reference. While active electrodes attached to the retina, as in the Humayun and Chow patents, will stimulate the retina and produce light perception, studies indicate that electrodes in direct physical contact with the retina may cause irreversible damage to neurons and tissue. 
   According to a 1998 thesis by R. J. Greenberg at Johns Hopkins University entitled “Analysis of Electrical Stimulation of the Vertebrate Retina—Work Towards a Retinal Prosthesis,” placement of a retrobulbar electrode in mono-polar stimulation (stimulation behind the eye that occurs with respect to a distant electrode) may be inappropriate owing to the current spread associated with the R-Membrane. While this is apparently the case in a healthy eye, the inventors suggest that in a diseased eye, the passive electrical properties may be rather different or can be made appropriate (through modification with laser, chemical treatment or other appropriate means) to allow for radial current flow thus making trans-retinal stimulation a viable option. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a method and apparatus for producing light perception in an eye comprising a vitreous chamber, a sclera, and a retina. In an exemplary embodiment, a visual prosthesis system includes first and second electrodes spaced from the retina, stimulating electronics connected to the first and second electrodes, and image processing electronics to capture and process images and transmit a corresponding data signal to the stimulating electronics. 
   In exemplary applications, a physician implants the first electrode, spaced from the retina, at one of first and second locations. Further, a physician implants the second electrode, spaced from the retina, at one of the first and second locations. The physician may also connect the first and second electrodes to the stimulating electronics. The stimulating electronics activates the first and second electrodes to stimulate at least a portion of the retina. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an embodiment of a visual prosthesis according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a visual prosthesis system, generally indicated by numeral  10 , according to the present invention. Visual prosthesis system  10  includes various components, both internal and external to the eye  70  and the patient. Eye  70  includes retina  72 , sclera  74 , vitreous chamber  76 , and optic nerve  78 . For simplicity, other areas of the eye such as the cornea, lens, iris, and conjunctiva are not referenced in  FIG. 1 . 
   Visual prosthesis system  10  includes imaging electronics  20 , data link  40 , stimulating electronics  42 , stimulating electrodes  50 , and reference electrodes  60 . The imaging electronics  20  include a camera or other image capture device  22  connected to processing electronics  26  via a data communications line  24 . Processing electronics  26  include image processor  28 , memory  30 , and encoder  32 . 
   In exemplary embodiments, image capture device  22  captures an image from the environment and transfers the image to the image processor  28  within processing electronics  26 . Image processor  28  processes the image in any one of numerous ways such as panning, magnification, edge detection, regional averaging, wavelet decomposition, etc., to produce a pixelized abbreviation of the original image. Each pixel of the abbreviated image corresponds to a single or multiple stimulation electrodes  50 . Memory  30  stores the processed image. 
   Upon completion of the formation and storage into memory  30  of the abbreviated image, each of the image&#39;s pixels to be conveyed is encoded in encoder  32  according to a pre-determined data transfer protocol for subsequent broadcast across data link  40 . Some pixels from the abbreviated image will not be conveyed due to their low level of brightness or other attribute that may render their conveyance inefficient or unnecessary. Data link  40  may be a direct, physical connection of one or more wires conveying both power and data or it may be in the form of inductive or optical coupling, as shown in  FIG. 1 , so as to avoid the need for wires. The present invention is applicable, but not limited to, the split intraocular-extraocular units as described above. Another embodiment might be a single intraocular device encompassing an image sensor, processor, and stimulating electronics. 
   Stimulating electronics  42 , located either internal to or external from the body, receives and interprets the data transmitted by processing electronics  26  via data link  40 . The data contained within the predetermined protocol may include, but is not limited to: information specifying which stimulating electrodes  50  are to be activated with a stimulation pulse, which reference electrodes  60  are to be used as the return path for the stimulation pulse, and the amplitude and pulse width of the stimulation pulse. In the event that a biphasic stimulation pulse is to be delivered, amplitude and pulse width data for each phase of the biphasic stimulation may be directly or indirectly specified. Further, the data may include repetition information that defines the number of times one or more stimulating electrodes  50  are activated with the same stimulating pulse. This repetition may be achieved by way of repeating identical data transmissions or by sending additional data specifying the quantity and frequency of stimulation. The former method is the preferred method as it provides an additional safety mechanism by precluding the possibility of multiple stimulation errors being brought about by a single transmission of corrupt data. 
   Electronics  42  contains a single or plurality of current sources or sinks that are either multiplexed or directly connected to stimulating or reference electrodes  50 ,  60  by wires (not shown). The stimulating electronics  42  may execute a variety of functions that are designed to enhance the perception of light in the damaged eye  70 . For example, techniques involving steering of current or blocking of particular neuronal regions  72  may be applied to further modulate perception. In addition, variations in stimulus parameters may yield further modulation of perceptions of light. Further, application of blocking currents (anodic blocks, refractory stimulation and the like) may facilitate “steering” particular stimuli to particular and more limited regions of retina  72 , and thus further specify perception of light. 
   The present invention also provides for placing a plurality of stimulating electronics  42  at suitable locations in or near the eye  70 . The plurality of stimulating electronics  42  allows for increased quantities of stimulation sites, increased frequency of stimulation, or a combination of both. 
   Stimulating electrodes  50  and reference electrodes  60 , connected to stimulating electronics  42 , may be of various sizes and shapes and are positioned at various locations, spaced from the retina  72 , on and within the eye  70 . Spacing the electrodes  50 ,  60  from retina  72  avoids unacceptable damage or trauma to the eye  70  and/or orbit (not shown). In exemplary embodiments, electrodes  50 ,  60  may be in close proximity to the sclerotic coat or sclera  74 , a hard, dense and unyielding fibrous membrane that maintains the form of the globe of the eye  70 . For example, stimulating electrodes  50  and/or reference electrodes  60  may be wrapped in an envelope of perforated silicone (not shown) and separated from the sclera  74  by approximately 1 mm. Alternatively, electrodes  50 ,  60  may engage the sclera  74  by attaching to the surface of sclera  74  or by penetrating sclera  74 . In this embodiment, the quantity of electrodes  50 ,  60  is limited only by the surface area of the sclera  74  and the size of each stimulating electrode  50 . Other locations for positioning electrodes  50 ,  60  include, but are not limited to, the vitreous, aqueous, lenticular, iridal, or corneal media, beneath the conjunctiva and in contact with the sclera  74 , upon the cornea or beneath the eyelid, and remote to the eye  70  and orbit, e.g., embedded within the temporal bone. 
   For purposes of illustration,  FIG. 1  illustrates stimulating electrodes  50  positioned on the surface of sclera  74 , reference electrodes  60  positioned within the vitreous chamber  76 , and an optional reference electrode plate  60   a  positioned on the surface of the sclera  74 . It should be noted that the role of the stimulating and reference electrodes  50 ,  60  may be reversed through electronics, thereby introducing a method of charge balance, which is important for tissue survival. Further, this ability to reverse the role of the electrodes  50 ,  60  introduces a new configuration wherein electrodes  50 ,  60  on both sides of the retina  72  may function as either stimulating or reference electrodes  50 ,  60 ; neither specifically acts as a ground electrode. Making use of the plurality of locations for both stimulating and reference electrodes  50 ,  60  in conjunction with reversal of the role of the stimulating and reference electrodes  50 ,  60 , shall facilitate substantial quantities of combinations of current paths and, as a consequence, substantial sites wherein perception of light may be conveyed. The external and internal positioning, with respect to the eye  70 , of electrodes  50 ,  60  makes it feasible to employ a plurality of stimulating electrodes  50 . This increases the quantity of electrodes  50  stimulated in parallel, increases the frequency of stimulation (and, as a result, the rate at which the image may be updated), increases the charge injected to a given region, and simultaneously blocks particular neurons or region of neurons with one stimulating electrode  50  by way of application of anodic blockage or other appropriate means of obtaining a refractory state of said particular neurons, while stimulating with another stimulating electrode  50  so as to further modulate perception of light. The positioning allows for a plurality of stimulating electronics  42 , as mentioned above, that will allow for all of the above to occur. As a consequence, a plurality of electrodes  50 ,  60  would also be present. 
   The following discusses the basic operation of an exemplary embodiment of the present invention. In general, imaging electronics  26  process and encode an image captured by image capturing device  22 , and transmit the processed image via data link  40  to stimulating electronics  42 , as described above. Electronics  42  analyzes the received data so as to ensure compliance with the pre-determined data transfer protocol and acts upon the data so as to pre-configure the stimulating electronics  42  for the delivery of the specified stimulation event or set of stimulation events to one or more specified stimulating and reference electrodes  50 ,  60 . Subsequent to the appropriate pre-configuration, the actual stimulus is delivered to the specified stimulating electrodes  50 . Activating a stimulating pulse that travels from one or more stimulating electrodes  50  positioned upon the sclera  74  to one or more reference electrodes  60  positioned in the vitreous chamber  76  shall pass stimulating current through or across retinal neurons  72 . When the stimulation is of sufficient magnitude (current amplitude, pulse duration, injected charge, etc.), perception of light in the region of the stimulated retinal neurons  72  is observed by the individual wearing the invention. 
   In a non-limiting illustrative example of operation concerning the perception of a point of light with the visual field, stimulating electrode  50  upon sclera  74  is energized by electronics  42  with a cathodic, constant current pulse of programmable amplitude and duration following pre-configuration as described above. Reference electrode  60  may be any one or more of reference electrodes  60  inside or outside of the eye  70  dependent upon the specific nature of visual perception to be delivered to the eye as determined by image processor  28  and encoder  32  based upon the image obtained by image capture device  22 . In this instant, the current passes across the scleral tissue  74  and through neuronal layers of the retina  72 , causing activation of some of the retinal neurons  72  as it passes. Activation of neurons within the retina  72  leads to perception of light in the vicinity and possibly the surrounding regions of the current path. Following the completion of the first current pulse, a charge recovery pulse may be delivered by the electronics  42 . This pulse is of programmable current amplitude and duration and is anodic in polarity with respect to stimulating electrodes  50 . 
   A second non-limiting example illustrates how peripheral vision may be conveyed when gross movement detection is of higher importance than detailed vision. In this instance, reference electrode  60  comprises a reference electrode plate  60   a . Reference electrode plate  60   a  is an external plate, juxtaposed to the periphery of retina  72  and positioned upon sclera  74 . Stimulating electrode  50  may be any one or more of electrodes  50 ,  60  inside or outside of the eye  70 , dependent upon the specific nature of the visual perception to be delivered as determined by image processor  28  and encoder  32 . With respect to the stimulating electrode  50 , cathodic current pulse passes across the tissue of the sclera  74  and through neuronal layers of the retina  72 , causing activation of a relatively large region of retinal neurons  72  as it passes through as a consequence of the reference electrode plate  60   a , being used as reference electrode. Activation of said neurons  72  leads to perception of light or movement in the vicinity and possibly the surrounding regions of the current path. Following the completion of the first current pulse, a charge recovery pulse may be delivered by the electronics  42 . This pulse is of programmable current amplitude and duration and is anodic in polarity with respect to stimulating electrode  50 . As a result, reference electrode plate  60   a  facilitates the physiological activation of large numbers of neurons in the retina  72  at the periphery of the visual field through a single event or a relatively low plurality of events of electrical stimulation thus facilitating the conveyance of gross movement detection at the periphery of the visual field and stimulation of the accessory visual pathways. 
   In summary, the present invention serves to place electrodes  50 ,  60  for stimulating the retina  72  in locations whereby biological reactions to the presence of the electrodes  50 ,  60  are relatively atraumatic to the remaining functions of the eye  70  and orbit not affected by disease. This positioning does not further exacerbate (to significant degree) the effects of disease and facilitates electrical stimulation of neurons  72  not damaged or destroyed by disease. At the same time, the electrode locations facilitate the ability to reliably regionalize electronic stimuli to produce light perception in localized regions of the visual field, and to the accessory optic pathways. In stimulating the accessory optic pathways, mechanisms other than those leading to conscious light perception may notably be elicited (e.g., neurohumoral secretion, ocular reflexes, etc.). In addition to the conveyance of rudimentary light perception through the delivery of stimulus, delivery of successive stimuli to a plurality of electrodes  50 ,  60  is intended to provide rudimentary patterned vision for use in the conveyance of visual cues, navigation aides, patterns, characters, rudimentary images, and other appropriate visual stimuli. 
   The foregoing description and drawings describe and illustrate the present invention in detail. However, the foregoing only describes some embodiments of a visual prosthesis system. Accordingly, the present invention may be carried out in specific ways other than those set forth herein without departing from the essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.