Perception of stationary plaids: the role of spatial filters in edge analysis

Orientation-tuned spatial filters in visual cortex are widely held to act as "orientation detectors", but our experiments on the perception of stationary two-dimensional (2-D) plaids require a new view. When two sinusoidal gratings at different orientations (say 1 c/deg, +/- 45 deg from vertical) are superimposed to form a standard plaid they do not, in general, look like two sets of oblique contours (diamonds) but more like a blurred checkerboard (squares) with vertical and horizontal edges, although the Fourier components are oblique. The pattern of edges seen in this plaid and others corresponds to the zero-crossings (ZCs) in the output of a circular filter, but adaptation and masking experiments suggest that oriented filters are being summed to emulate circular filtering, before ZC analysis. At low contrasts or after adaptation to an intermediate orientation, the combining of filters can fail or be "broken", and the diamond structure of the components is seen instead. Adding a low contrast third harmonic to one component in square-wave phase also changed the plaid's appearance from squares to diamonds, but adapting to the third harmonic enhanced the square appearance. Filters can evidently switch from combining across orientation to combining across spatial frequency. The combination stage of edge detection may involve variably weighted summing of oriented filters in monocular pathways, followed by a process that makes explicit the locations and orientations of features.