Sequence of visual cortex stimulation affects phosphene brightness in blind subjects

Background: Visual cortical prostheses (VCP) could potentially benefit a majority of the blind population. Feasibility testing of these VCP opens new avenues to characterize stimulation of visual cortex in blind subjects. Objective/hypothesis: To determine if sequential stimulation of visual cortex produces a perception bias in phosphene brightness. Methods: We stimulated three blind subjects implanted with the Orion array with sequences of two and three electrodes and asked them to determine the brighter phosphene, using interval forced-choice paradigms. We selected a set of reference electrodes as the constant stimuli across sequences and compared across three different amplitude levels keeping all other stimulation parameters fixed across electrodes. Results: For two subjects, we measured a significant increase in the probability of perceiving a lower-level amplitude just as bright or brighter than a higher-level amplitude when stimulated later in the sequence (p < 0.001, Wilcoxon rank sum test). The probability of reference electrodes selected as brighter was also higher during the second phase, across most amplitude comparisons. For the third subject, there were measurable but not significant changes, where the first stimuli were perceived as brighter. The effects were consistent within subjects in the three-electrode sequences, where the probability of the reference electrode selected as brighter was correlated to when it was presented in the sequence. Conclusions: We showed evidence of temporal interactions in non-overlapping sequences of electrodes, where the direction of the effect was subject specific but consistent across a variety of electrode locations and current amplitude levels.