Mechanisms specialized for the perception of image geometry.

Angle discrimination thresholds were obtained for V-shaped targets with a base angle of 90 deg at four different pattern orientations (0, 45, 90 and 135 deg). A comparison of these thresholds with the orientation discrimination thresholds for the single lines from which the patterns had been constructed, revealed that angle acuity cannot be predicted from component acuity. Angle acuity is finer than the corresponding orientation acuity in all cases and does not exhibit the pronounced oblique effect that is found for orientation discrimination. Other experiments showed that acuity for pattern angle depends critically on base angle, with minima close to 0, 90 and 180 deg. The shape and amplitude of this function are independent of pattern orientation. It was found that the angle acuity was unaffected by excluding a large portion of the target in the region of the vertex, and that the pattern of dependence of acuity on angle changed radically when the target was reduced ultimately to three blobs that defined the cardinal points of the stimulus. The data suggest that when the target comprises line segments, angle discrimination is not limited by noise that arises at early levels of processing and that angle perception is mediated by mechanisms that are specialized for the perception of image geometry. An opponent process model, that is based on the combined outputs of just two types of filter, is proposed as the basis for the perception of image geometry. This type of system is appropriate for computing one of the differential invariants in an optic flow field.