Detecting low shape-frequencies in smooth and jagged contours

It is often assumed that a two-stage filter-process is involved in the coding of visual contours. In the first stage the contour is coded by localized luminance filters selective for, among other dimensions, orientation and spatial frequency. A second stage then integrates the outputs of these local luminance filters over space. In the experiments described here we address the issue of which spatial scales of early luminance filters are involved in the detection of a contour's deviation from linearity ('co-linearity failure'), especially at low contour frequencies, for both smooth contours and jagged edges. We also address the question whether it is the orientation or position of the first-stage luminance filters that is used by the second stage. We report two main conclusions. Firstly, across a wide range of shape frequencies, we find that detection thresholds are relatively independent of the spatial scale of the luminance information present in the contour, indicating that detection of co-linearity failure can be effectively mediated by luminance filters tuned to a range of spatial scales. Secondly, we find that detection of co-linearity failure in low shape-frequency contours is primarily based on the local positions, not orientations, of the first-stage luminance filters. As our results suggest that the contour's local orientation may nevertheless play a role, we hypothesize that the local orientation of the contour is not signaled by luminance filters directly but rather by second-order filters acting on the local positions of the contour.