Analysis of the motion of 2-dimensional patterns: Evidence for a second-order process

Abstract

The sum of two differently orientated moving sinusoidal gratings of similar spatial frequency, contrast, and velocity appears as a single coherent “plaid” pattern. The visual system is thought to analyse the motion of plaids in two stages, first analysing the motion of the (1-D) components, and then calculating a speed and direction which is consistent with those 1-D motions. We studied the apparent direction of motion of plaids made by adding two components that had the same spatial frequency and contrast, and were symmetrically oriented about the vertical axis. The gratings moved in jumps, and we studied the effect of varying the size of the jump, the angle between the component gratings, and the temporal interval between the jumps, on the perceived direction of motion. When the size of the jumps was increased to 3/8 of their spatial period, the perceived direction of motion of the plaid pattern reversed, although if one component were presented alone, its direction of movement did not reverse. Reversed motion of this type was consistently obtained if the angle between the components was greater than about 140°, if the interval between jumps was at least 25 msec, and if the spatial frequency of the component gratings was less than about 4 c/deg. When the angle between the components was smaller, or the time between jumps was greater, most observers saw normal motion in the direction predicted by the two-stage hypothesis. When the spatial frequency was raised, observers saw no consistent motion. We interpret these results to show that a “second-order” motion mechanism contributes to the analysis of plaid motion, possibly by providing inputs at the first-stage of the two-stage process hypothesised by Adelson, E. H. and Movshon, J. A. (1982 Nature, 300, 523–525).