Size-contrast illusions are processed in the ventral stream of the occipitotemporal cortex and the dorsal stream of the parietal cortex. Greater interactions between these two regions are associated with stronger illusions. The present study tests if visual percepts (phosphenes) induced by TMS of the occipital and parietal cortices are also susceptible to size-contrast illusions. Using the Ponzo illusion, wherein a two-dimensional image exhibits an illusory third dimension of depth, we predicted that the size of phosphenes and their reaction times would be modulated by perceived distance, similar to results found with normal external stimuli. A novel image illustrating a business park with buildings depicted as being 'near' or 'far' from the observer was used to elicit depth cues. In Experiment 1, single-pulse TMS was administered to the left primary visual cortex and left intraparietal sulcus to induce phosphenes. Participants (N=10) were to respond as fast as possible by pressing a button when they experienced a phosphene, and to report its size and location on the monitor. Previous work has shown that participants are faster to respond to an object whose perceived size is larger. We verified in a second experiment, with the same participants, that a size illusion was present for physical stimuli (i.e. no TMS phosphenes) on the same background using an adjustment task. In Experiment 1, we found the illusion affected perceived size of and RTs to occipital-induced phosphenes but not parietal-induced phosphenes. These results indicate that the ventral stream may also be involved with processing size-contrast illusions arising from TMS of the occipital cortex, but not for percepts induced in the parietal cortex. Meeting abstract presented at Vision Science Society Meeting 2015.
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