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TT Tanskanen, J Saarinen, R Hari; Human Cortical Responses to Contours . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4726.
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Purpose: Perception of global contours and shapes requires integration of information from the early, spatially limited processes. We used magnetoencephalography (MEG) to characterize the time course and cortical location of this integration. In addition, we studied the effects of contour properties on such processing. Methods: The stimuli were a modification of the path paradigm (Field, Hayes, & Hess, 1993) used to study contour integration psychophysically, i.e. arrays (7.6 x 7.6 deg2) of 140 high-contrast (57%) Gabor patches (sf 4.8 c/deg), in which a proportion of the patches either formed or did not form a contour. We presented three types of contours: a full circle with local elements oriented tangentially to the global contour, a full circle with elements orthogonal to the contour, and a quadrant of a circle with tangential local elements. The stimuli appeared abruptly on an average gray (90 cd/m2) background once every 2.5 s and remained visible for 0.5 s. Subjects did not respond to the stimuli but catch trials were included to maintain alertness. We recorded whole-scalp neuromagnetic responses from 7 adults and calculated Minimum Current Estimates to identify cerebral activity underlying these signals. We also collected reaction times (RTs) for discriminating contour and no-contour stimuli in a separate session. Results: The first cortical responses, exceeding 2 SD of the prestimulus noise level at 69 ± 2 ms (mean ± SEM) and peaking at 85 ± 3 ms, did not differ between contour and no-contour stimuli. Earliest contour-specific activity, i.e. significant difference between the responses to contour and no-contour stimuli, emerged at 130 ± 9 ms for the tangential circle, at 164 ± 17 ms for the orthogonal circle, and at 149 ± 13 ms for the quadrant path. This contour vs. no-contour difference, observed in the medial posterior brain areas, returned to zero 100-200 ms after the offset of the stimulus. The shortest discrimination RTs, about 550 ms, were obtained to the tangential circles, whereas RTs to the two other conditions were ∼50 ms longer (p < 0.01). RTs to contour vs. no-contour stimuli did not differ significantly in any category. Conclusions: The results demonstrate a sustained modulation in the cortical responses to contour vs. no-contour stimuli, emerging after the early non-selective activity. This modulation resembles the contextual effects observed in monkey V1 (e.g. Zipser, Lamme, & Schiller, 1996). Both cortical responses and RTs showed that contour processing is facilitated when local orientation is tangential to the global path orientation.
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