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Xiaoming Du, Ke Zhou, Lin Chen; Different temporal dynamics of topological and projective geometrical perceptions in primary visual cortex: a TMS study. Journal of Vision 2011;11(11):863. doi: 10.1167/11.11.863.
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Accumulating behavioral evidence suggested that global topological properties are extracted at the very beginning of visual processing. However, neuroimaging studies found that topological perception mainly activated the anterior temporal lobe (ATL), which lies in the late destination of ventral visual pathway. There seems a contradiction. To reconcile the behavioral findings with neurophysiology and anatomy of topological perception, we hypothesized that topological properties may be processed via a rapid subcortical shortcut to ATL, bypassing V1. We tested this prediction by applying transcranial magnetic stimulation (TMS) over V1 in an odd quadrant task. In the odd quadrant task, all four quadrants each contained a figure, and subjects were asked to report whether a quadrant differs from the other three. The disparate quadrant differs from the rest either in projective properties (e.g., disk vs. solid parallelogram) or in topological properties (‘hole’ vs. ‘no-hole’ figure).The single-pulse TMS was applied over occipital pole at different SOAs (−30∼300 ms). Consistent with previous findings, the data revealed two vital temporal phases for visual discrimination based on projective differences. The discrimination performance (d') declined significantly when V1 was disrupted at SOA of 30 ms (early) and 150 ms (late). This early phase usually reflects that feedforward processing of visual information travel through the early subcortical stages of the retino-geniculo-striate pathway and arrives at V1. The late phase usually reflects the feedback modulation of higher cortical areas to V1 and is vital for arousal and consciousness. However, only a late phase was revealed for all kinds of topological discrimination. Disruption of V1 at early time did not affect topological discrimination. Thus, the results provided the direct evidence that topological perception does not necessarily depend on the initial processing at V1 and may be mediated via a rapid subcortical pathway to ATL, bypassing V1.
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