Spatial versus temporal grouping in a modified Ternus display.

The Ternus display can induce a percept of 'element motion' or 'group motion'. Conventionally, this has been attributed to two different motion processes, with different spatial and temporal ranges. In contrast, recent studies have emphasised spatial and temporal grouping principles as underlying the apparent motion percepts in the Ternus display. The present study explored effects of spatial and temporal grouping on the apparent motion percept in a novel Ternus display of oriented Gabor elements with no inter-frame interval. Each frame of this stimulus could be further divided into 'sub-frames', and the orientation of the carriers was changed across these sub-frames. In four experiments transitions were found between the motion percepts with changes in orientation across time (Experiment 1) and space (Experiment 2), and with a temporal offset in the orientation change of the outer element (Experiment 3) to the extent that group motion was not perceived even with large orientation changes over time that previously led to group motion (Experiment 4). Collectively, these results indicate that while spatial properties have an influence in determining the percept of the Ternus display, temporal properties also have a strong influence, and can override spatial grouping. However, these temporal effects cannot be attributed to spatio-temporal limits of low-level motion processes. Some aspects of the observed spatial grouping effects can be accounted for in terms of a modified association field, which may occur through connectivity of orientation selective units in V1. The temporal effects observed are considered in terms of temporal integration, the transitional value at a temporal offset of 40ms being remarkably similar to psychophysical and neurophysiological estimates of the peak temporal impulse response. These temporal responses could be detected at a higher level in the system, providing a basis for apparent motion perception.