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Harish Vasudevan, Neil Halelamien, Shinsuke Shimojo; Visual replay effect: objective evidence from a masking paradigm. Journal of Vision 2009;9(8):972. doi: 10.1167/9.8.972.
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Transcranial magnetic stimulation (TMS) is a non-invasive method employing a magnetic coil to generate an electrical signal in the human brain. When applied directly to the visual cortex, TMS induces either scotomas (blind spots) or phosphenes (flashes of light) depending on coil type and stimulation conditions. Interestingly, if TMS is applied at a particular timing after a visual stimulus, an “instant replay” of the original stimulus is perceived, typically within the phosphene area (Halelamien, et al, VSS 2007). This replay effect potentially offers unique insight into the formation and storage of mental representations. To obtain objective evidence of replay's perception and determine if replay shares the same early perceptual circuit as retinally-triggered stimuli, we use a masking paradigm to examine replay's spatial and temporal specificity. Subjects were presented a physical mask followed by a letter, which they were asked to report. In the TMS condition, a double pulse of TMS was administered after mask presentation to facilitate “replay” of the mask, obscuring letter detection. The timing was carefully selected such that there was no direct forward masking by the physical mask but a potential masking effect by the replayed mask. Three conditions were tested with and without TMS: 1) no physical mask presented, 2) physical mask and letter in different positions, and 3) physical mask and letter in the same position. We found that the replay effect was most pronounced when the delay between mask and TMS pulse was 150–250 ms. Further, our data shows statistically significant performance degradation with the addition of TMS only when the physical mask and letter are presented in the same visual location, suggesting a replay induced masking effect. These results provide evidence for the ability of TMS induced replay to interact with the same perceptual circuits as retinal input in a spatiotopically organized manner.
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