Different patterns of popout for direction of motion and for orientation

We used the visual search paradigm to demonstrate that popout of features in the dimensions of orientation and motion may be different. A series of experiments compared a directional motion task to an orientation task. When the target in the motion task was defined by particular direction of movement and all the distractors were defined by a second, homogeneous direction of movement, search time was minimally affected by the number of distractors. However, when the direction of motion for the distractors was heterogeneous, reaction time was dramatically affected by the number of distractors. By contrast, varying distractor heterogeneity in the orientation task had little effect on performance. Search for a target defined by one orientation was minimally affected by the number of distractors when the orientation of the distractors was heterogeneous. The results suggest that the mechanism that leads to a popout of direction of movement may be different from the one that leads to a popout of orientation. We suggest that the popout for the direction of motion is due to a global analysis of a homogeneity of the display, whereas the popout for orientation is mediated by local analyses of differences between stimuli.