Coherence sensitivity of cortical responses to global form J Wattam-Bell, F Corbett, V Chelliah |
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Department of Developmental Science, University College London, United Kingdom
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| Ventral extrastriate areas respond better to coherently organised than to scrambled (zero coherence) global patterns. Here, we used event-related potentials (ERPs) to measure cortical responses to intermediate levels of coherence. The stimuli were arrays of short line segments aligned into concentric or radial global patterns. Coherence was varied by randomising the orientation of a subset of the lines. 128-channel ERPs were recorded while adult subjects viewed a sequence of one-second trials in which the different global patterns and coherence levels were presented in random order. At the end of each trial subjects made a forced-choice judgement about which organisation (radial or concentric) had been presented. The main coherence-sensitive occipital ERP was a bilateral negative response with sources in ventral extrastriate cortex (eg LOC/V4). Its amplitude increased linearly (ie became more negative) with increasing coherence. These results differ sharply from the ERP to motion coherence, which is dominated by a midline posterior response originating in V1/V2, whose amplitude decreases non-linearly with increasing coherence (Corbett 2012, Perception 41, 1515). We conjecture that the motion ERP reflects modulation of V1 activity by feedback from extrastriate areas, and that feedback to V1 plays a much less prominent role in cortical processing of global form. |
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