CHANGES IN THE PERCEIVED DIRECTION OF DRIFTING PLAIDS, INDUCED BY ASYMMETRICAL CHANGES IN THE SPATIOTEMPORAL STRUCTURE OF THE UNDERLYING COMPONENTS

When a plaid pattern with symmetrical velocity components (Type I) is changed to a plaid pattern with asymmetrical velocity components (Type IA), the overall direction of drift appears to undergo a rotation wit hout any other change to the spatial parameters of the components. Thi s change in the perceived drift direction can be induced by altering e ither the temporal frequency of the components or by altering their sp atial frequency. In separate experiments, we have estimated the magnit ude of the temporal and spatial frequency thresholds that are necessar y to create a liminal change in direction of this type. The results fr om both temporal and spatial frequency experiments are closely similar . We find that liminal rotations can be induced by changes in the spat io-temporal structure of the sine-wave grating components that are und etectable when these components are presented in isolation. Further, w e find that the ''velocity threshold for direction'' is not a constant factor, but critically depends on the relative orientation of the two elements that form the plaid. Forced-choice experiments were also con ducted to estimate the extent of the apparent rotation of the plaid pa ttern for differing levels of asymmetry in the spatial frequency and t emporal frequency of the components. The magnitude of the pattern rota tion is predicted by a model of motion direction that encodes the succ essive displacements of the intersections of the gratings. Finally, we demonstrate that the velocity thresholds for perceived rotation exhib it a meridional anisotropy that depends on the direction of drift of t he overall pattern and not on the orientation of the components, Taken together, the results support the view that, unlike speed discriminat ion, the main limiting factor in the perception of motion direction is not at the level at which the components are extracted, but is at or beyond the level of pattern analysis. The two constituents of the patt ern motion vector, speed and direction, are computed independently.