Contour integration and scale combination processes in visual edge detection.

Contours in the natural visual environment consist mainly of edges which are spatially broad-band and whose (cosinusoidal) components have arrival phases close to +/-90 deg. Because early visual processing is thought to be based on a local Fourier description, the representation of edges requires two forms of filter combination: scale integration (filter combination across spatial frequency) and contour integration (filter combination across space). In order to determine how these two types of combination fit together, we determined spatial-frequency tuning for the detection of contours composed of broadband edge elements, alternating with narrow-band Gabor elements. A contour integration system operating independently at a number of spatial scales should be able to ignore the distracting influence of edge structure in such patterns. However, subjects cannot ignore edge structure indicating that local phase-alignment across spatial scale is coded prior to, or concurrent with, contour integration. Moreover, unlike contours composed of Gabors, the bandwidth of local elements is important for edge integration; the coding of element bandwidth seems to be dependent on the phase alignment of features across spatial frequency.