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A. Horsager, J. D. Weiland, R. J. Greenberg, M. S. Humayun, I. Fine; Spatiotemporal Integration of Perceptual Brightness in Retinal Prosthesis Patients. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3011.
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The goal of microelectronic retinal prostheses is to establish functional vision using spatiotemporal patterns of stimulation across 2-dimensional arrays of electrodes. A key question is whether each electrode behaves independently - creating isolated, discrete, punctate phosphenes. Here, we evaluate the question of electrode independence by measuring perceptual brightness for pulse trains presented across pairs of electrodes.
Using a two-alternative forced-choice task, retinal prosthesis patients were asked to identify which of two intervals contained the brighter stimulus. One interval contained 2 time-synched 50 Hz suprathreshold pulse trains presented on a pair of electrodes. The other interval contained the same pulse trains (i.e., same pulse width, frequency, etc.) on the same electrode pair, but the pulse trains were temporal phase shifted across the two electrodes. The phase shift varied between 0.075-9 ms. The pulse amplitudes of these phase-shifted stimuli were varied across trials to find the point of equal brightness to the time-synched stimulus. The effect of the phase shift on perceptual brightness was measured for electrode pairs separated, center to center, by 800 micrometers.
For nearly half of the electrode pairs that we tested apparent brightness was dominated by the percept generated by the brightest electrode. For the other half of the electrodes there appeared to be brightness summation across the electrode pair. For these electrodes the amount of summation decreased as a function of the phase shift, and by ~2ms brightness matches once again appeared to be dominated by the brightest electrode.
We see evidence for spatiotemporal interactions between electrodes. However phase-shifting pulses across electrode pairs appears to minimize these interactions. Further research, combining psychophysical and electrophysiological techniques, will be required to determine to what extent these interactions are mediated by electric fields, retinal circuitry, and/or cortical integration.
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