The Processing of Contours in Human Visual Cortex

Contour processing and figure-ground detection within human retinotopic areas were investigated, using event-related functional magnetic resonance imaging (fMRI) (Part 1 and 2) and VEP (part 3). An isoeccentric square (6°) was defined by either i) texture, ii) motion, iii) color or iv) gray-level, while stimuli were otherwise virtually identical. A demanding foveal letter discrimination task prevented subjects from noticing the peripheral contour stimulus, thus avoiding attentional bias in activation of early visual areas. Customized retinotopic mapping and cortical flattening techniques were used to identify the retinotopic localization of the contour related responses and to investigate contextual responses. BOLD-modulations related to contours were mostly restricted to the subpart of visual cortex corresponding to the retinotopic representation of the contours. The activation patterns in different visual areas varied in respect to the contour defining modality. All modalities activated areas V2 and V4, but in other retinotopic areas the responses differed. Therefore, we suggest that these specific activation patterns reflect different neuronal populations engaged in the processing of different contour modalities. Here, V2 may serve as the main contour processing area, while higher retinotopic areas contribute modality specific information. Using VEP we found a clear negativity, specific for contour segregation with highly similar latency (210ms at O1 and O2) and comparable amplitude for both versions. The amplitude of the motion-defined contour segregation negativity was of comparable latency but significantly smaller. In a stimulus combining the motion- and the texture-defined contour the negativity was present, but the amplitude was not significantly larger than in the experiments using only the orientation-defined contours. The discrepancies between these electrophysiological results and the previously reported fMRI-results are discussed