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S.D. Elfar, N.P. Cottaris, R. Iezzi, G.W. Abrams; How the Retinal Network Reacts to Epiretinal Stimulation to Form the Prosthetic Visual Input to the Cortex: Computational Modeling of a Normal Retina . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5275.
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© ARVO (1962-2015); The Authors (2016-present)
We examined how the normal, intact retinal network may interact with electrical epiretinal stimulation in shaping the spike trains of ON and OFF ganglion cells, and thus the synaptic input to first–stage cortical neurons. This information is crucial for interpreting results from experiments in which the efficacy of epiretinal stimulation is assessed in normally–sighted animals.
We developed a biophysical model of the retinal network that includes the basic retinal cell types and their interconnections, and first–stage visual cortex neurons (simple cells). Neurons were modeled as leaky integrators with added membrane and synaptic conductances. Epiretinal stimulation via a planar disk electrode was modeled. It was assumed that only cell bodies are stimulated by the extracellular potential gradient. Retinal output was assessed at the level of simple cells which receive convergent but opponent input from ON and OFF ganglion cells.
Our simulations of electrical stimulation of a fully–functional retina show that epiretinal stimulation alone causes an indiscriminate excitation of ON and OFF ganglion cells, resulting in a patchy input to the cortex with isolated regions of excitation amid regions of no net excitation. The retinal network progressively biases the excitation of ON relative to OFF ganglion cells, shaping the synaptic input to the cortex. These interactions gradually interpolate and focus the initially patchy synaptic input. Compared to results from simulations of a retina in an early phase of a degenerative disease, the normal retina exhibits spatiotemporally tighter, but otherwise similar, dynamics in response to electrical stimulation.
Neurophysiological experiments using sighted animals may be a valid approach to assess the effects of epiretinal stimulation in initial stages of retinal degeneration as long as the differences described above are taken into consideration. Currently, we are testing the model's predictions in vivo by recording the cortical activation patterns in response to electrical epiretinal stimulation.
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