A model of ganglion axon pathways accounts for percepts elicited by retinal implants

Abstract

12Degenerative retinal diseases such as retinitis pigmentosa and macular degeneration cause 13 irreversible vision loss in more than 10 million people worldwide. Retinal prostheses, now 14 implanted in more than 250 patients worldwide, electrically stimulate surviving cells in order to 15 evoke neuronal responses that are interpreted by the brain as visual percepts ('phosphenes'). 16However, instead of seeing focal spots of light, users of current epiretinal devices perceive highly 17 distorted phosphenes, which vary in shape not just across subjects but also across electrodes, 18 resulting in distorted percepts. We characterized these distortions by asking users of the Argus 19retinal prosthesis system (Second Sight Medical Products, Inc.) to draw percepts elicited by single-20 electrode stimulation on a touchscreen. Based on ophthalmic fundus photographs, we then 21 developed a computational model of the topographic organization of optic nerve fiber bundles in 22 each subject's retina, and used this model to successfully simulate predicted patient percepts. Our 23 model shows that activation of passing axon fibers contributes to the rich repertoire of phosphene 24shapes reported by patients in our psychophysical measurements, successfully replicating visual 25 percepts ranging from 'blobs' to oriented 'streaks' and 'wedges' depending on the retinal location 26 of the stimulating electrode. This model provides a first step towards future devices that 27 incorporate stimulation strategies tailored to each individual patient's retinal neurophysiology. 28 Impact 29 Current retinal implant users report seeing distorted and often elongated shapes rather than small 30 focal spots of light that match the shape of the implant electrodes. Here we show that the perceptual 31 experience of retinal implant users can be accurately predicted using a computational model that 32simulates each individual patient's retinal ganglion axon pathways. This opens up the possibility 33 for future devices that incorporate stimulation strategies tailored to each individual patient's retina. 34Wuyyuru, V., Sahel, J., Stanga, P., et al. (2013). The Argus II epiretinal prosthesis system allows 595 letter and word reading and long-term function in patients with profound vision loss. Br J 596Ophthalmol 97, 632-636. 597

Biophysical model of axonal stimulation in epiretinal visual prostheses

“…Because epiretinal prostheses sit on top of the optic fiber layer, these devices may accidentally stimulate passing axon fibers, which could antidromically activate cell bodies located peripheral to the point of stimulation [3]- [5]. This can cause nontrivial perceptual distortions [6]- [8] that may severely limit the quality of the generated visual experience [9], [10].…”