Visual feature binding: the critical time windows of V1/V2 and parietal activity.

Interactions between the posterior parietal cortex and the early visual cortex have been proposed to play a central role in the binding of visual features into coherent objects. Here we investigated the importance of these interactions by contrasting the time windows at which the early visual cortex (V1/V2) and the angular gyrus (AG) play a causal role in visual feature binding. Transcranial magnetic stimulation was applied over these areas at different latencies during a visual task requiring binding. The contribution of V1/V2 was critical for feature detection 90-120 ms after the onset of the stimulus. When visual binding was required, an additional late time window (lasting until 240 ms after stimulus onset) was observed. The contribution of AG was found to be necessary for visual binding between 90 ms and 180 ms, overlapping with the "early" feature detection stage in V1/V2 and peaking around 180 ms. That the late V1/V2 time window overlaps and even extends beyond the peak time window of AG is consistent with the view that reentrant processing between higher areas and early visual cortex is necessary for visual binding.