Abstract
We investigated how the electrophysiological signature of contour integration is changed by the context in which a contour is embedded. Specifically, we manipulated the orientations of Gabor elements surrounding an embedded shape outline. The amplitudes of early visual components over posterior scalp regions were changed by the presence of a contour, and by the orientation of elements surrounding the contour. Differences in context type had an effect on the early P1 and N1 components, but not on the later P2 component. The presence of a contour had an effect on the N1 and P2 components, but not on the earlier P1 component. A modulatory effect of context on contour integration was observed on the N1 component. These results highlight the importance of the context in which contour integration takes place.
Highlights
Our visual system provides us with a stable and coherent representation of the external world
Our study provides evidence that context modulates the electrophysiological signature of contour integration at early stages of visual processing
The effect of context was measurable at the time of the P1 peak. It was large at the time of the N1 peak, but absent at the time of the P2 peak
Summary
Our visual system provides us with a stable and coherent representation of the external world. An important intermediate step to achieve this stability is to determine which parts of the retinal input belong together, a process known as perceptual grouping. Vision takes advantage of statistical regularities in the input image to guide perceptual grouping processes. Adjacent elements of a shape outline are usually locally aligned. Detection of this collinearity might serve as a cue to the presence of a contour [1]. The importance of collinearity as a cue for contour integration is illustrated in the snake detection or pathfinder paradigm (for a review, see [2]), in which participants have to detect a contour in a cluttered background. The strength of contour grouping depends on the spacing and orientations of elements relative to the path orientation [3], [4]. More global stimulus properties, seem to influence the binding of local contour elements: Closed contours are more readily detected than open ones [7], [8], and symmetric contours are easier to detect than asymmetric ones [9]
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