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Ilona Kovács, Márta Zimmer, Gyula Kovács; Electrophysiological correlates of contour integration in human visual cortex. Journal of Vision 2005;5(8):974. doi: 10.1167/5.8.974.
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Integration of local features into global shapes has been studied in a contour integration paradigm. We investigated the neural bases of contour integration with the help of event related potentials (ERP). Observers had to either detect an egg-shaped contour (DET), or discriminate between to directions that the egg-shaped contours were pointing at (DIS). In both conditions the same stimuli were used: closed contours composed of Gabor patches on a background of randomly positioned and oriented Gabor patches. Task difficulty was varied by gradually rotating the contour patches from the predetermined path of the contour, that resulted in six levels of difficulty, and undetectable contours in about half the trials. We repeated both tasks at high and low contrast values for the Gabors. While subjects performed 360 trials for each condition we obtained ERPs (recorded from 23 channels, positioned according to the 10–20 system). DET and DIS trials were recorded in separate blocks. Difference waves were constructed by subtracting ERPs for undetectable from that of for detectable contours. Contour integration (as reflected in the difference wave) was characterised by a more negative wave between 200 and 300 msec. This difference is generated by a smaller P2 (at around 200–220 msec) and an enhanced N2 (at 260–280 msec) at occipito-temporal electrodes. Increased attention to the shape of the figure in the DIS condition only slightly increased the magnitude of this effect. Reducing the contrast of the images also led to an increase of the effect between 200 and 300 msec, and extended the difference to the N3 (at 350– 380 msec) component of the ERP. The time course of these results is consistent with earlier findings in the monkey cortex (e.g.Zipser et al, 1996, Bauer and Heinze 2002), suggesting the relevance of a later, “tonic” response phase within the early visual cortex in the integration of orientation information across the visual field.
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