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D. Nanduri, J. D. Weiland, A. Horsager, M. S. Humayun, R. J. Greenberg, I. Fine; Selective Adaptation Using Electrical Stimulation in Humans. Invest. Ophthalmol. Vis. Sci. 2007;48(13):651.
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© ARVO (1962-2015); The Authors (2016-present)
Since 2002, 6 human patients with severe retinitis pigmentosa have been implanted with 4x4 epiretinal electrode arrays (Humayun, 1999). Our goal was to examine how perceptual detection thresholds resulting from electrical stimulation are affected by an adapting/preconditioning pulse train.
The study protocol was approved by the University of Southern California IRB. 7 electrodes were tested, across 2 subjects. Adapting stimuli were 45 Hz biphasic electrical pulse trains of 1s duration. Adaptor pulse widths were either 0.075 or 3ms. Adaptor amplitude was set at 2x threshold for that pulse width. Test stimuli consisted of 15 Hz biphasic pulse trains of 1s duration. Test stimuli could have pulse widths of 0.075, 0.45, 3 or 7.05ms. On each trial there was a 0.5s delay between adaptor and test pulse trains. Perceptual threshold (the current amplitude needed to detect the stimulus on 79% of trials, corrected for false alarms) were measured before and after adaptation, using a 1up-1down technique with 18% interleaved catch trials.
Data were analyzed separately for each test pulse width using 2 factor ANOVAS (electrode x adapt condition, p<0.05). For the 0.075 test pulse we found significant adaptation for both 0.075 and 3ms adaptor pulse trains (0.075=66 % increase in perceptual threshold, 3ms=55%), and there was no significant difference in the magnitude of the adaptation effect between the two adaptor pulse widths. For the 0.45ms test pulse, adaptation was only significant for the 3ms adaptor (~43% threshold increase), with the effect of the 0.075ms adaptor falling below significance. For test pulses of 3 or 7ms we only saw adaptation effects (~21% threshold increase) for the 3ms adaptor. In summary, adapting trains of long pulses increase perceptual thresholds for short, medium and long test pulses, but adapting trains of short pulses only increase perceptual thresholds for shorter stimulus pulses.
Long pulses should adapt cells with both slow and fast current integration, whereas short pulses should selectively adapt cells with fast current integration. Our finding that short-pulse adaptation does not affect the threshold for long test pulses is consistent either with threshold for these longer pulses being mediated by ganglion cells with relatively long chronaxies, or with electrical adaptation having little effect on the ability of slow integrating bipolar cells to indirectly stimulate ganglion cells.
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