Abstract
Purpose: The perception of radial deformations of circular contours (radial frequency patterns) involves global form analysis (Wilkinson et al., 1998 Vision Res. 38, 3555–68). The purpose of the present work is to assess whether contextual information affects the perception of form, by evaluating the effect of a surround pattern (mask) on a smaller central (test) pattern. Methods: Thresholds for detecting deviations from circularity were measured for a test radial frequency (RF) pattern with 5 cycles (RF5) and a radius of 1.14 deg. Three conditions were tested: measures were made for RF5 test patterns presented alone, with a surrounding circular mask, and with a surrounding RF5 mask. Mask radius was 1.71 deg and RF5 mask amplitude was approximately 10 times detection threshold. Three relative phase combinations between test and mask were used (in-phase, 90 out-of-phase, and 180 out-of-phase). Observers (3) were required to discriminate deviations from circularity in a 2IFC paradigm using the method of constant stimuli; and thresholds were taken at 75% correct responses. Results: Thresholds were similar for the RF5 test presented alone and with the circular mask. Thresholds were elevated by a factor of approximately 1.5 when the RF5 mask was present, and effects were obtained even at a contour separation of 0.57 deg. In addition, effects of phase were apparent.
Conclusions: A surround circular pattern (mask) does not affect the perception of a smaller deformed test pattern, whereas a deformed mask alters the form representation of this test pattern. These findings suggest that contextual information affects the perception of global form.