Perceived speed of moving two-dimensional patterns

When two cosine gratings drifting in different directions are superimposed they can form a coherently moving two-dimensional pattern (plaid) whose resultant speed is related to the component velocities by a geometric construction known as the intersection-of-constraints (IOC). When measured against a standard which has the same spatial frequency as its components, a plaid always appears to move slower than the IOC prediction. However, the perceived speed is generally faster than would be predicted if speed were judged based on the temporal frequency of either the components or the nodes of the plaid. On the other hand, when the standard has the same spatial period as the nodes, the plaid appears to move at the same rate as the predicted IOC resultant. Furthermore, a grating with the same spatial period as the nodes appears to move slower than a grating at the component spatial frequency, just the plaid does. It is therefore likely that speed is encoded similarly for both gratings and plaids, and that the perceived speed of both is determined by the spatial periodicity of the pattern. We have previously classified 2D moving patterns as either type I (resultant lies between component directions) or type II (resultant outside of components). We find that the perceived speed of both types can be accounted for on the basis of the nodal spatial period. Finally we present a model for velocity coding which is based on the responses of spatio-temporal mechanisms.