THE ANALYSIS OF MOTION OF 2-DIMENSIONAL PATTERNS - DO FOURIER COMPONENTS PROVIDE THE 1ST-STAGE

Human observers were required to report the direction of motion of sim ple two-dimensional (2-D) ''plaid'' patterns made by adding together t wo sinusoidal gratings of identical contrast (0.5 or 1.5 log units abo ve threshold), spatial frequency (1 or 5 c/deg) and orthogonal orienta tions (horizontal and vertical, or + 45 deg). The patterns were made t o move either by moving both gratings at the same speed (pattern motio n) or by moving one component with the other stationary (component mot ion). In one task (direction discrimination) the observer knew the axi s 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 moti on and the axis of motion. In both tasks the discrimination of pattern motion was consistently better than the discrimination of component m otion, contrary to the predictions of the ''two-stage'' model of motio n 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 variat ion in direction discrimination of pattern motion with speed did not h ave the form predicted under the assumption that the direction of moti on of the pattern could be discriminated using the motion of either of its two component gratings. Finally, an elaborated version of the Ade lson and Movshon [(1982) Nature, 300, 523-525] two-stage model, in whi ch noise affects the two stages fails to predict the performance in id entifying the pattern motion of the plaid pattern, except for 1 c/deg low contrast plaids. These results suggest that when 2-D patterns cont ain moderately high contrasts or high spatial frequencies observers ma y use other attributes, instead of, or in addition to Fourier componen ts, to analyse their 2-D motion.