Abstract
It has been shown that early visual areas are involved in contour processing. However, it is not clear how local and global context interact to influence responses in those areas, nor has the interarea coordination that yields coherent structural percepts been fully studied, especially in human observers. In this study, we used functional magnetic resonance imaging (fMRI) to measure activity in early visual cortex while observers performed a contour detection task in which alignment of Gabor elements and background clutter were manipulated. Six regions of interest (two regions, containing either the cortex representing the target or the background clutter, in each of areas V1, V2, and V3) were predefined using separate target versus background functional localizer scans. The first analysis using a general linear model showed that in the presence of background clutter, responses in V1 and V2 target regions of interest were significantly stronger to aligned than unaligned contours, whereas when background clutter was absent, no significant difference was observed. The second analysis using interarea correlations showed that with background clutter, there was an increase in V1–V2 coordination within the target regions when perceiving aligned versus unaligned contours; without clutter, however, correlations between V1 and V2 were similar no matter whether aligned contours were present or not. Both the average response magnitude and the connectivity analysis suggest different mechanisms support contour processing with or without background distractors. Coordination between V1 and V2 may play a major role in coherent structure perception, especially with complex scene organization.
Highlights
Contour integration involves grouping local features across several levels of abstraction and a range of spatial scales
The early visual areas V1, V2, and V3 were manually defined according to the polar angle (Figure 4A) and eccentricity phase maps acquired in separate scanning sessions
Using a two-way Analysis of variance (ANOVA) model within each visual area (Alignment and Background as fixed effects, and subjects as a random effect), a significant interaction between Alignment and Background was observed in tgV2 Regions of interest (ROIs), F(1, 59) 1⁄4 7.83, p 1⁄4 0.014, and a similar trend was seen in tgV1 ROI, F(1, 59) 1⁄4 5.85, p 1⁄4 0.030
Summary
Contour integration involves grouping local features across several levels of abstraction and a range of spatial scales. Similar elements positioned closely along an invisible smooth path are perceptually organized as due to a continuous contour. This grouping process is enhanced if the elements have orientations that align with the path (Field, Hayes, & Hess, 1993; Hess & Field, 1999; Kovacs, 1996; Li & Gilbert, 2002). Knowledge of the global form of the path contributes to local integration, such as the form closing (Kovacs, 1996; Kovacs & Julesz, 1993, 1994) and smoothness (Pettet, 1999; Pettet, McKee, & Grzywacz, 1998), and the global knowledge is often necessary to disambiguate competing local groupings in cluttered scenes (Ullman & Sha’ashua, 1988).
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