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Scott Greenwald, Alan Horsager, Mark Humayun, Robert Greenberg, Matthew McMahon, Ione Fine; Brightness as a function of current amplitude in human retinal electrical stimulation. Journal of Vision 2009;9(8):1054. doi: 10.1167/9.8.1054.
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Six human subjects with late stage retinitis pigmentosa were implanted with a prototype epiretinal prosthesis consisting of a 4×4 array of sixteen stimulating electrodes with the goal of eventually restoring functional vision. As described previously, this device reliably elicits visual percepts by electrically stimulating intact cells of the neural retina. An important problem for restoring function is to calibrate each electrode to account for differences in the brightness of percepts as a function of stimulation amplitude.
We measured apparent brightness as a function of electrical current amplitude, using both subjective magnitude rating and brightness matching procedures in two chronically implanted subjects. We found that both magnitude rating and brightness matching data can be described using a simple model where apparent brightness increases as a power function of stimulation intensity (B=aCg). For both experiments, good fits for both subjects could be obtained using a fixed value of the exponent g, and only allowing the scaling factor, a, to vary across electrodes.
These results suggest that it should be possible calibrate brightness across an entire array of electrodes by measuring a single parameter for each electrode. For example, single brightness matching measurements made at a relatively high amplitude on each electrode would be sufficient to describe relative perceived brightness across electrodes, and thereby provide a simple method of creating the appearance of equally bright phosphenes across an entire array and across a wide range of brightness levels.
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