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Gerrit W Maus, Samuel B Hutton, Romi Nijhawan, David Whitney, Jamie Ward; Reduction of the flash-lag effect with TMS over MT/V5. Journal of Vision 2009;9(8):646. doi: https://doi.org/10.1167/9.8.646.
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In the flash-lag effect a moving object is seen ahead of a physically aligned flash. Little is known about the contribution of cortical areas to this illusion. We investigated whether disrupting neural activity in area MT/V5 with transcranial magnetic stimulation (TMS) impairs the perceptual forward shift of the moving object. First, we measured the flash-lag effect for each participant without TMS. Participants judged whether a bar moving leftward in the right visual field was to the ‘left’ or the ‘right’ of a flash appearing at variable time points in a fixed position next to the bar's trajectory. All participants mislocalized the bar in the direction of motion. In the TMS runs, the flash was presented before the bar reached the flash position, when about 70% of trials were judged as ‘left’. In each trial, TMS pulses were applied to MT/V5 at asynchronies from 40 ms before to 140 ms after the bar reached the flash position. To control for unspecific TMS effects, V1 or Vertex were stimulated in separate runs. With stimulation of MT/V5 there were fewer ‘left’ responses than with Vertex stimulation, indicating a reduction of the moving bar's forward shift. This reduction was most pronounced when pulses occurred just as the bar reached the flash position, and approximately 60 ms afterwards. Our results demonstrate that area MT/V5 contributes to the perceptual localization of moving objects, and to the illusory forward shift in the flash-lag effect in particular.
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