“Phase capture” in the perception of interpolated shape: cue combination and the influence function

This study was concerned with what stimulus information observers use to judge the shape of simple objects. We used a string of four Gabor patches to define a contour. A fifth, center patch served as a test pattern. The observers' task was to judge the location of the test pattern relative to the contour. The contour was either a straight line, or an arc with positive or negative curvature (the radius of curvature was either 2 or 6 deg). We asked whether phase shifts in the inner or outer pairs of patches distributed along the contour influence the perceived shape. That is, we measured the phase shift influence function. We found that shifting the inner patches of the string by 0.25 cycle results in almost complete phase capture (attraction) at the smallest separation (2 lambda), and the capture effect falls off rapidly with separation. A 0.25 cycle shift of the outer pair of patches has a much smaller effect, in the opposite direction (repulsion). In our experiments, the contour is defined by two cues--the cue provided by the Gabor carrier (the 'feature' cue) and that defined by the Gaussian envelope (the 'envelope' cue). Our phase shift influence function can be thought of as a cue combination task. An ideal observer would weight the cues by the inverse variance of the two cues. The variance in each of these cues predicts the main features of our results quite accurately.