Cue combination anisotropies in contour integration: The role of lower spatial frequencies

The combination of local orientation collinearity and spatial frequency contrast in contour integration was studied in two experiments using a 2AFC contour detection and discrimination task. Target contours were defined by local orientation collinearity, spatial frequency contrast between contour and background elements, or both cues. Experiments differed in the source of spatial frequency contrast by manipulating the spatial frequency of either contour or background elements. Cue summation gains, defined as the performance benefit of double cue conditions over single cue conditions, were evaluated and tested against the predictions derived from probability summation and linear summation. Summation gains were generally stronger than linear summation and tended to increase with the single-cue performance level until limited by ceiling effects. Cue summation was particularly large when contour elements exhibited a lower spatial frequency than background elements, regardless of the absolute spatial frequency ranges. The highly effective integration of lower spatial frequency contours in cluttered surrounds is discussed in the context of recent findings on high-level neural representations of contour integration as well as feature synergy.