Characterization of retinal ganglion cell activities evoked by temporally patterned electrical stimulation for the development of stimulus encoding strategies for retinal implants

For successful restoration of visual function by retinal implant, a method for electrical stimulation should be devised so that the evoked activities of retinal ganglion cells (RGCs) should convey sufficient information on visual input. By observing RGC activities under different stimulation constraints, it may be possible to determine optimal pulse parameters, such as pulse rate, intensity, and duration, for faithful transmission of visual information. To test the feasibility of this approach, we analyzed RGC spike trains evoked by temporally patterned stimulation from retinal patches mounted on a planar multielectrode array. Assuming that the intensity of uniform visual input is transformed to amplitudes of pulse trains, we attempted to determine optimal methods for modulating the pulse amplitude so that the information essential for the perception of intensity variation is properly represented in RGC responses. RGC firing rates could be modulated to track the temporal pattern of pulse amplitude variations, which implies that pulse amplitude modulation is a plausible means to enable perception of temporal visual patterns by retinal implants. As expected, specific pulse amplitude modulation parameters were crucial for proper encoding of visual input. RGC firing rates increased monotonically according to the pulse amplitude in a defined pulse amplitude range (20-60 μA). The similarity between the RGC firing rate and the temporal pulse intensity pattern was highest when the pulse amplitude was modulated within this range. The optimal pulse rate range could be similarly determined.