The analysis of motion of two-dimensional patterns: do Fourier components provide the first stage?

Human observers were required to report the direction of motion of simple two-dimensional (2-D) "plaid" patterns made by adding together two sinusoidal gratings of identical contrast (0.5 or 1.5 log units above threshold), spatial frequency (1 or 5 c/deg) and orthogonal orientations (horizontal and vertical, or +/- 45 deg). The patterns were made to move either by moving both gratings at the same speed (pattern motion) or by moving one component with the other stationary (component motion). In one task (direction discrimination) the observer knew the axis of motion, and was required to discriminate the direction of motion along that axis in a temporal two-alternative forced-choice paradigm; in the other task (direction identification) the observer did not know the axis of motion and was required to identify the direction of motion and the axis of motion. In both tasks the discrimination of pattern motion was consistently better than the discrimination of component motion, contrary to the predictions of the "two-stage" model of motion analysis, in which it is assumed that the motion of a 2-D pattern is calculated from the 1-D motions of its Fourier components. The variation in direction discrimination of pattern motion with speed did not have the form predicted under the assumption that the direction of motion of the pattern could be discriminated using the motion of either of its two component gratings. Finally, an elaborated version of the Adelson and Movshon [(1982) Nature, 300, 523-525] two-stage model, in which noise affects the two stages fails to predict the performance in identifying the pattern motion of the plaid pattern, except for 1 c/deg low contrast plaids. These results suggest that when 2-D patterns contain moderately high contrasts or high spatial frequencies observers may use other attributes, instead of, or in addition to Fourier components, to analyse their 2-D motion.