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Alan Horsager, Ione Fine; Evidence for synchrony using direct electrical stimulation of the human retina. Journal of Vision 2007;7(9):114. doi: 10.1167/7.9.114.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The visual representation of objects and contours may involve binding related features through synchronous neural activity. While task and stimulus dependent synchronous activity on a fine temporal scale has been recorded in early visual cortex, it remains uncertain whether this activity actually has a perceptual role. Since 2002, 6 human subjects with severe retinitis pigmentosa have been implanted with 4×4 epiretinal electrode arrays (Humayun, 1999). In these subjects, visual percepts are mediated by directly stimulating the inner retina, rather than via temporally sluggish photoreceptors. This allowed us to test whether synchronous retinal activity was perceptually distinct from asynchronous activity at rates higher than the critical flicker fusion limit.
Methods: 500 ms biphasic pulse trains were presented on four neighboring electrodes with 800 µm center-to-center separation. Stimulation was suprathreshold at 80 Hz or above. Subjects performed a two-alternative same/different task in which each interval contained either synchronous or asynchronous stimulation. During synchronous stimulation all pulses all four electrodes were stimulated simultaneously. During asynchronous stimulation, the pulses on each of the four electrodes were temporally interleaved, but each individual electrode continued to be stimulated at a rate of 80Hz or above. Data was collected on 10 4-electrode sets across 2 human subjects.
Results and Conclusions: Subjects had no difficulty (85–100% accuracy) differentiating synchronous from asynchronous stimulation, even at frequencies above the critical flicker fusion limit. Synchronous stimulation generally appeared brighter. While it is possible that these results are due to retinal interactions, it is more plausible that the synchronous and asynchronous activity patterns elicited within the retina were transmitted to early visual cortex and produced distinguishable percepts. Our data therefore support the notion that synchronous activity enhances brightness or salience and plays an important role in perception.
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