Devyani Nanduri

Frequency and amplitude modulation have different effects on the percepts elicited by retinal stimulation.

PURPOSE In an effort to restore functional form vision, epiretinal prostheses that elicit percepts by directly stimulating remaining retinal circuitry were implanted in human subjects with advanced retinitis pigmentosa RP). In this study, manipulating pulse train frequency and amplitude had different effects on the size and brightness of phosphene appearance. METHODS Experiments were performed on a single subject with severe RP (implanted with a 16-channel epiretinal prosthesis in 2004) on nine individual electrodes. Psychophysical techniques were used to measure both the brightness and size of phosphenes when the biphasic pulse train was varied by either modulating the current amplitude (with constant frequency) or the stimulating frequency (with constant current amplitude). RESULTS Increasing stimulation frequency always increased brightness, while having a smaller effect on the size of elicited phosphenes. In contrast, increasing stimulation amplitude generally increased both the size and brightness of phosphenes. These experimental findings can be explained by using a simple computational model based on previous psychophysical work and the expected spatial spread of current from a disc electrode. CONCLUSIONS Given that amplitude and frequency have separable effects on percept size, these findings suggest that frequency modulation improves the encoding of a wide range of brightness levels without a loss of spatial resolution. Future retinal prosthesis designs could benefit from having the flexibility to manipulate pulse train amplitude and frequency independently (clinicaltrials.gov number, NCT00279500).

Retinal prosthesis phosphene shape analysis

A retinal prosthesis system to restore sight for the blind is under development. The system is analogous to cochlear implants, in which photoreceptor input is bypassed and replaced by direct electrical stimulation of the retinal ganglion cells. Currently, six test subjects have been implanted with a 4×4 electrode array and stimulator. We report here psychophysical clinical data examining how stimulation amplitude affects phosphene shape and repeatability on a single electrode. Phosphene shape data was quantified by a set of numerical descriptors calculated from image moments. Comparison of phosphene descriptors for a single electrode across repeated trials and amplitude levels measured the repeatability within an amplitude group. Our experimental findings show that stimulation of the retina creates repeatable percept shapes and that an increase in stimulation amplitude causes a significant change in size and shape of phosphenes.

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

Retinal prostheses, now implanted in over 250 patients worldwide, electrically stimulate surviving cells in order to evoke neuronal responses that are interpreted by the brain as visual percepts (‘phosphenes’). However, instead of seeing focal spots of light, current implant users perceive highly distorted phosphenes that vary in shape both across subjects and electrodes. We characterized these distortions by asking users of the Argus retinal prosthesis system (Second Sight Medical Products) to draw elicited percepts on a touchscreen. We found that phosphene shape could be accurately predicted by simulating the topographic organization of nerve fiber bundles in each subject’s retina. Our model shows that activation of ganglion axons contributes to a rich repertoire of phosphene shapes, successfully replicating percepts ranging from ‘blobs’ to oriented ‘streaks’ and ‘wedges’ depending on electrode location. This work provides a first step towards future devices that incorporate stimulation strategies tailored to each individual patient’s retinal neurophysiology. One Sentence Summary We show that the perceptual experience of retinal implant users can be accurately predicted using a computational model that simulates the topographic organization of each individual patient’s retinal ganglion axon pathways.