Visual percepts induced by transcranial magnetic stimulation of the occipital cortex

How do people see? Most would answer ‘with the eyes’. In fact, the eyes are just the first step in processing visual information. The current thesis seeks to build on our knowledge of cortical processes involved with visual perception. Instead of using visual information presented to the eye, we used TMS to the occipital cortex of the brain to induce visual percepts. These percepts, phosphenes and scotomas, have distinct luminance characteristics and therefore should mimic normal visual processing. In Study One, we tested whether both percepts could be generated in the occipital cortex using the same experimental parameters, as they have previously been tested only under different conditions. We investigated their influence on RT, and the effects of luminance intensity and TMS intensity on percept appearance. Each percept was induced under all experimental conditions, was manipulated in a similar manner, and RT were influenced by percept luminance. The results from Study One led to the hypothesis for Study Two, examining the effects of physical versus perceived luminance intensity and contrast on percept appearance. To this end, illusory brightness was used to gauge the effects of perceived luminance on percepts. Results showed that illusory brightness modulated RT to real stimuli and the frequency of artificial percepts when illusory brightness was perceptually different from physical luminance. The findings of Study Two ushered in the design for Study Three, testing if a size contrast illusion exhibiting depth cues could influence percept characteristics. As size contrast illusions are processed in the ventral stream beginning in the occipital cortex and the dorsal stream of the parietal cortex, Study Three incorporated parietally induced phosphenes as well as those arising from occipital stimulation. Here, we found the illusion affected perceived size of and RTs to occipital-induced phosphenes but not parietal-induced phosphenes. Taken together, these findings imply that the visual cortex processes information in a standardized manner, irrespective of whether the information is delivered to the eye or to the occipital cortex of the brain.