Optogenetic stimulation in macaque V4 cortex induces robust detectable visual events

Understanding the nature of the perceptual events evoked by neural perturbations is essential for bridging the causal gap between neuronal activity and vision as a behavior. Here we assess behavioral detectability of optogenetic stimulation in monkey V4 cortex. Two macaque monkeys were chronically implanted with LED arrays over a region of V4 cortex transduced with the depolarizing opsin C1V1. The animals were trained to detect stimulation while fixating at different images. In each trial an image was displayed on the screen for 1s. In half of trials, randomly selected, a 200ms optical impulse was delivered halfway through image presentation, and the animal was rewarded for correctly identifying whether the trial contained cortical stimulation. Both animals learned to perform the task significantly above chance within 11 and 7 sessions respectively (Chi-sq, p-values < 0.01) and improved their performance to 90% and 83% after 27 and 13 more training days (Chi-sq, p-values < 0.001). After the training phase, 20 novel images were used to test whether stimulation detection depends on the choice of onscreen image. The choice of image had a significant effect on stimulation detection (permutation test, p-values < 0.001). Further, the effect varied as a function of cortical location. Taken together, the results suggest the effect of stimulation is visual in nature and stimulation of different subregions evoke different perceptual events. Next we asked whether the stimulation-evoked events are additive in nature, by varying the visibility of the onscreen images. In contrast to inferotemporal cortex, reducing the visibility of the onscreen images did not systematically reduce stimulation detection. These results suggest the events evoked by stimulation in V4 are additive. The findings show for the first time that optogenetic stimulation of V4 cortex induces robust detectable visual events, opening the door to systematic causal studies of V4 with optogenetic methods.