Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session WW7: The Physics and Bioengineering of Artificial Sight |
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Sponsoring Units: DBP Chair: Robert Greenberg, Second Light LLC Room: LACC 408B |
Thursday, March 24, 2005 5:30PM - 6:06PM |
WW7.00001: Electrodes in Human Eyes: An Update of Blind Patient Psychophysical Testing for Artificial Sight Invited Speaker: Experiments were conducted to measure the accuracy with which subjects implanted with an Intraocular Retinal Prosthesis (IRP) can perform visual tasks. The Food and Drug Administration and the Institutional Review Board of the University of Southern California approved the protocol. Test subjects who met the protocol requirements were implanted with an epiretinal prosthesis consisting of 16 electrodes in a 4x4 distribution. The tests were divided in two categories: computer controlled tests and video camera tests. Results from computer controlled tests include: sequential activation (4 alternative forced choice (AFC)) 70{\%} correct (p$<$ .001); orientation of lines of electrodes (2 AFC) 78{\%} correct (p$<$ .01). The patients also could recognize the direction of movements of a white bar in 59{\%} (p$<$ .01) in camera controlled tests. Comparing one vs. multi-pixel resolution, subjects required less time to provide a correct answer when multiple pixels were used (counting objects 27s, p$<$ .0001; L orientation 80s, p$<$ .0003) and a trend towards better performance when using multiple pixels (the number of pixels varied between subjects). In summary, test subjects with no better than light perception vision can perform simple visual tasks using the Model-1 IRP device. An increased number of pixels and electrodes in the Model 2 IRP device may produce greater functionality. \newline \newline Work done in collaboration with Douglas Yanai, MD, James D. Weiland, PhD, Manju Mahadevappa, PhD, Eugene de Juan, Jr., MD. [Preview Abstract] |
Thursday, March 24, 2005 6:06PM - 6:42PM |
WW7.00002: Microsystems Technology for Retinal Implants Invited Speaker: The retinal prosthesis is targeted to treat age-related macular degeneration, retinitis pigmentosa, and other outer retinal degenerations. Simulations of artificial vision have predicted that 600-1000 individual pixels will be needed if a retinal prosthesis is to restore function such as reading large print and face recognition. An implantable device with this many electrode contacts will require microsystems technology as part of its design. An implantable retinal prosthesis will consist of several subsystems including an electrode array and hermetic packaging. Microsystems and microtechnology approaches are being investigated as possible solutions for these design problems. Flexible polydimethylsiloxane (PDMS) substrate electrode arrays and silicon micromachined electrode arrays are under development. Inactive PDMS electrodes have been implanted in 3 dogs to assess mechanical biocompatibility. 3 dogs were followed for 6 months. The implanted was securely fastened to the retina with a single retinal tack. No post-operative complications were evident. The array remained within 100 microns of the retinal surface. Histological evaluation showed a well preserved retina underneath the electrode array. A silicon device with electrodes suspended on micromachined springs has been implanted in 4 dogs (2 acute implants, 2 chronic implants). The device, though large, could be inserted into the eye and positioned on the retina. Histological analysis of the retina from the spring electrode implants showed that spring mounted posts penetrated the retina, thus the device will be redesigned to reduce the strength of the springs. These initial implants will provide information for the designers to make the next generation silicon device. We conclude that microsystems technology has the potential to make possible a retinal prosthesis with 1000 individual contacts in close proximity to the retina. [Preview Abstract] |
Thursday, March 24, 2005 6:42PM - 7:18PM |
WW7.00003: Spike timing control in retinal prosthetic Invited Speaker: Frank Werblin To restore meaningful vision to blind patients requires a retinal prosthetic device that can generate precise spiking patterns in retinal ganglion cells. We sought to develop a stimulus protocol that could reliably elicit one ganglion cell spike for every stimulation pulse over a broad frequency range. Small tipped platinum-iridium epiretinal electrodes were used to deliver biphasic cathodal electrical stimulus pulses at frequencies ranging from 10 to 125 Hz. We measured spiking responses with on-cell patch clamp from ganglion cells in the flat mount rabbit retina, identified by light response and morphology. Single electrical 30 pA cathodal pulses of 1 msec duration elicited both by direct electrical activation of ganglion cells and synaptic excitation and inhibition. Direct activation elicited a single spike that followed the onset of the cathodic pulse by about 100 $\mu $sec; presynaptic activation typically elicited multiple spikes which began after 10 msec and could persist for more than 50 ms depending on pulse amplitude levels. Limiting the pulse duration to 100 $\mu $sec eliminated all presynaptic activity: only ganglion cells were driven. Each pulse elicited a single pike for stimulation frequencies tested from 10 to125 Hz. Our ability to elicit one spike per pulse provides many important advantages: This protocol can be used to generate temporal patterns of activity in ganglion cells with precision. We can now mimic normal light evoked responses for either transient or sustained cells, and we can modulate spike frequency to simulate changes in intensity, contrast, motion and other essential cues in the visual environment. [Preview Abstract] |
Thursday, March 24, 2005 7:18PM - 7:54PM |
WW7.00004: Design of a High-resolution Optoelectronic Retinal Prosthesis Invited Speaker: It has been demonstrated that electrical stimulation of the retina can produce visual percepts in blind patients suffering from macular degeneration and retinitis pigmentosa. So far retinal implants have had just a few electrodes, whereas at least several thousand pixels would be required for any functional restoration of sight. We will discuss physical limitations on the number of stimulating electrodes and on delivery of information and power to the retinal implant. Using a model of extracellular stimulation we derive the threshold values of current and voltage as a function of electrode size and distance to the target cell. Electrolysis, tissue heating, and cross-talk between neighboring electrodes depend critically on separation between electrodes and cells, thus strongly limiting the pixels size and spacing. Minimal pixel density required for 20/80 visual acuity (2500 pixels/mm2, pixel size 20 um) cannot be achieved unless the target neurons are within 7 um of the electrodes. At a separation of 50 um, the density drops to 44 pixels/mm2, and at 100 um it is further reduced to 10 pixels/mm2. We will present designs of subretinal implants that provide close proximity of electrodes to cells using migration of retinal cells to target areas. Two basic implant geometries will be described: perforated membranes and protruding electrode arrays. In addition, we will discuss delivery of information to the implant that allows for natural eye scanning of the scene, rather than scanning with a head-mounted camera. It operates similarly to ``virtual reality'' imaging devices where an image from a video camera is projected by a goggle-mounted collimated infrared LED-LCD display onto the retina, activating an array of powered photodiodes in the retinal implant. Optical delivery of visual information to the implant allows for flexible control of the image processing algorithms and stimulation parameters. In summary, we will describe solutions to some of the major problems facing the realization of a functional retinal implant: high pixel density, proximity of electrodes to target cells, natural eye scanning capability, and real-time image processing adjustable to retinal architecture. [Preview Abstract] |
Thursday, March 24, 2005 7:54PM - 8:30PM |
WW7.00005: Molecular Photovoltaics and Artificial Sight Invited Speaker: The goal of this project is insertion of purified Photosystem I (PSI) reaction centers or other photoactive agents into retinal cells where they will restore photoreceptor function to people who suffer from age-related macular degeneration (AMD) or retinitis pigmentosa (RP), diseases that are the leading causes of blindness world-wide. Although the neural ``wiring'' from eye to brain is intact, these patients lack photoreceptor activity. It is the ultimate goal of this project to restore photoreceptor activity to these patients using PSI as the optical trigger. In principle, the approach should work. PSI is a robust integral membrane molecular photovoltaic device. Depending on orientation, it can depolarize or hyperpolarize the cell membrane with sufficient voltage to trigger an action potential. The first objective of this work, reported here, is to impart photoreceptor activity to mammalian cells using the previously determined molecular photovoltaic properties of isolated Photosystem I reaction centers. Incubation of WERI-Rb-1 retinoblastoma cells with functional PSI reaction centers that were isolated from spinach leaves and reconstituted into proteoliposomes resulted in a light-induced PSI-dependent increase in intracellular Ca$^{2+}$. The increase, due to Ca$^{2+}$ uptake, was dependent on the presence of extracellular Ca$^{2+}$ ions. [Preview Abstract] |
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