Abstract
Stimulation of visually sensitive regions of ventral temporal cortex in humans alters visual perception, yet the precise nature of these effects remains unclear. Clarifying the perceptual nature of these perturbations is essential for bridging the causal gap between neuronal activity and vision as a behavior, and for the development of effective visual prosthetics for patients with severe visual impairments. To test whether stimulation of inferior temporal (IT) cortex causes additive (“hallucinatory”) versus distortive effects, we carried out optogenetic stimulation in macaque monkeys performing a stimulation-detection task while viewing images of objects that varied in visibility. Visibility was degraded by reducing contrast, saturation, and spatial frequency of object images to gray in five steps. Hypothetically, if stimulation causes an additive effect, varying visibility of the visual input should not affect detectability of stimulation. If anything, stimulation should be easier to detect when the visual input is less visible since there would be no underlying image to parse from the hallucinatory percept. If on the other hand cortical stimulation has a distortive effect, it should be easier to detect when the visual input is more visible, as the effect would necessarily be a function of the visual input. Two macaque monkeys were implanted with Opto-Arrays over a region of their IT cortex transduced with the depolarizing opsin C1V1. In each trial, following fixation an image was displayed for 1s. In half of the trials, randomly selected, a 200ms illumination impulse was delivered halfway through image presentation, and the animal was rewarded for correctly identifying whether the trial did or did not contain cortical stimulation. Visibility of the visual input significantly affected stimulation detectability (ANOVA, p < 0.003 for both animals). Consistent with the distortion model, the animals were more accurate at detecting stimulation when the visual input was more visible.