Apparent speed of type I symmetrical plaids.

The apparent speed of plaids made up of two gratings having the same spatial frequency and the same speed was evaluated (symmetrical type I). The plaids were moving vertically as defined by the intersection-of-constraints (IOC) rule with a mean duration of 300 msec. The comparison-stimulus was a horizontal line moving vertically. The main goal of the study was to test the effect of the spatial frequency of the Distortion Product (DP). Here the DP velocity is identical to the IOC velocity. The main effect is that reducing the DP spatial frequency from 4 to 1 c/deg decreases apparent speed. A smaller effect is due to the speed of the components: when this speed becomes relatively smaller than the IOC speed, apparent speed of the plaid decreases. Finally, the DP temporal frequency seems to determine the upper limit (about 16 Hz) beyond which the plaid appears as a non-rigid moving and flickering pattern.