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J.F. Rizzo, R.J. Jensen, J. Loewenstein, J. Wyatt; Unexpectedly Small Percepts Evoked by Epi-Retinal Electrical Stimulation in Blind Humans . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4207.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To consider mechanisms that yield unexpectedly small percepts following epi-retinal stimulation in blind humans despite use of relatively large stimulating electrodes. Methods: Comparison of animal in-vitro and human in-vivo experiments of electrical stimulation of the epi-retinal surface. Platinum-iridium (25 and 125 µm diameter) electrodes were used to stimulate rabbit retinas. For human experiments, the size of perceptual responses elicited by 400 µm diameter platinum-iridium electrodes was considered. A distant return electrode was used for all experiments. Results: Rabbit experiments using the 25 µm electrode and 8 µA, 100 µsec pulses drove an inhomogeneous cluster of ganglion cells, with 50% probability of ganglion cells directly under the electrode, and a 33% probability that cells 50 µm away from the electrode, would be stimulated. Therefore, there was a minimum of a 33% probability of driving ganglion cells within a 100 µm diameter area around the electrode, which corresponds to 0.5° of visual angle. In human experiments, the most commonly reported singe percept following slightly supra-threshold stimulation was round and roughly the size of a pea (as if viewed at arm’s length), which corresponds to roughly 0.3° of visual angle, a finding which is surprising given the much larger electrodes used for these experiments. Rabbit retinal stimulation with 125 µm electrodes has revealed "direct" and delayed "indirect" responses; the latter were eliminated by drugs that blocked synaptic input to ganglion cells (reported last year). Newer, preliminary findings suggest a center-surround organization of ganglion cell responses to electrical stimulation (i.e. the center responses being the "direct" responses). The center responses were very sensitive to the location of the stimulating electrode, whereas the surround responses did not degrade despite moving the electrode over a much wider area (i.e. 100s of microns). Conclusions: The 400 µm diameter electrodes used with blind humans would be expected to produce relatively large percepts, which typically did not occur. The spatial and temporal disparities in the center vs. surround responses of ganglion cells following electrical stimulation might "filter" and therefore narrow the appearance of the percept. Other, unknown mechanisms likely contribute to the surprisingly small percepts so often reported by blind subjects.
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