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Abstract:
Transcranial magnetic stimulation (TMS) at V5 has been found
to lead to disruptions of motion perception in humans (Beckers
& Zeki, 1995; Hotson et al., 1994; Walsh et al., 1998). There
is some controversy, though, about the precise moment at which TMS
is effective. Potentially, this is due to the fact that the
critical site for stimulation is selected for only a single delay
between presentation of the visual motion stimulus and the magnetic
stimulation. Our objective was to investigate in more detail
spatio-temporal characteristics of motion perception in V5. A
visual stimulus consisted of a window at the right visual
hemifield, in which dots moved coherently to the left or right. The
background consisted of randomly moving dots. Within a matrix of 2
X 2 cm around the T5 electrode 9 locations were stimulated. Three
subjects were instructed to indicate the direction of the
coherently moving dots. Significant decreases in performance
occurred at delays -25 ms, 0 ms, 25 ms, and between 75 ms and 200
ms (p<0.05). These delays are comparable to those reported in
other TMS studies on motion perception. The results suggest that
our methodology is more sensitive for finding the delay at which
motion perception is impaired. Furthermore, the area in which TMS
might effect motion processing seems not restricted to a single
location, but is spatially distributed.
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