Image-dependence of the detectability of optogenetic stimulation in macaque inferotemporal cortex

Artificial activation of neurons in early visual areas induces perception of simple visual flashes.1,2 Accord-ingly, stimulation in high-level visual cortices is expected to induce perception of complex features.3,4 How-ever, results from studies in human patients challenge this expectation. Stimulation rarely induces any detectable visual event, and never a complex one, in human subjects with closed eyes.2 Stimulation of the face-selective cortex in a human patient led to remarkable hallucinations only while the subject was looking at faces.5 In contrast, stimulations of color-and face-selective sites evoke notable hallucinations indepen-dent of the object being viewed.6 These anecdotal observations suggest that stimulation of high-level visual cortex can evoke perception of complex visual features, but these effects depend on the availability and con-tent of visual input. In this study, we introduce a novel psychophysical task to systematically investigate char-acteristics of the perceptual events evoked by optogenetic stimulation of macaque inferior temporal (IT) cortex. We trained macaque monkeys to detect and report optogenetic impulses delivered to their IT cortices7-9 while holding fixation on object images. In a series of experiments, we show that detection of cortical stimulation is highly dependent on the choice of images presented to the eyes and it is most difficult when fixating on a blank screen. These findings suggest that optogenetic stimulation of high-level visual cor-tex results in easily detectable distortions of the concurrent contents of vision.