DETECTION OF 2ND-ORDER STRUCTURE IN OPTICAL-FLOW FIELDS

We investigate how well the direction of the expansion gradient and th e direction of a spatial step in expansion can be extracted from a com plex optical flow field. The expansion gradient is an invariant of the second-order structure under rotations of the image plane. Our stimul i consist of sparse random dot patterns in a circular window with a di ameter of 20 degrees. We find that translational velocity masks the di rection of both the expansion gradient and the step in expansion. Thre sholds for both the expansion gradient and the step in expansion are f ound to be similar for a wide range of expansions. We introduce two si mple models that use the thresholds for the step in expansion to predi ct thresholds for the expansion gradient. Results of these models are comparable with the results of our subjects, showing that we do not ne ed a hypothetical separate mechanism for the detection of second-order structure to explain the data. (C) 1997 Optical Society of America.