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J. D. Fesi, R. O. Gilmore; The spatial distribution of VEP responses to temporal modulations of motion contrast in human adults. Journal of Vision 2010;10(7):930. doi: 10.1167/10.7.930.
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Single unit and fMRI evidence suggests that extrastriate cortical regions process motion contrast. In this study, we employed high-density steady-state visual evoked potential (SSVEP) recordings to study the tuning curves of cortical areas sensitive to the separation of a figure from its background using different classes of motion contrast information--direction and global coherence. Participants (n=21; 11 female) viewed moving dot displays (7 cd/m2 size; 8% density) in which four square “figure” regions (9° wide) emerged from and disappeared into the background at a specific frequency (1.2 Hz: 1F1), based on differences in dot direction and motion coherence. We found responses at 1F1 that increased monotonically to both types of motion contrast, observed over midline channels near the occipital pole. Responses at the second harmonic (2F1) were strongest over lateral channels; there the response curves saturated once a minimal threshold of motion contrast magnitude was reached. We interpret the midline activity at 1F1 to reflect the processing of the magnitude of motion contrast information in early visual association areas, possibly V2 or V3A/D. The lateralized activity at 2F1 appears to reflect a non-linear thresholding operation associated with extracting the figure from background, perhaps engaging lateral occipital cortex (LOC), among other areas. Source modeling techniques will allow us to locate the precise coordinates of the processing centers of these functionally distinct evoked responses in the brain.
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