Changes (but not differences) in motion direction fail to capture attention

In this study we investigated under what conditions motion direction changes pop out in continuously moving target/distractor environments. Participants were presented with vertically oriented Gabor patches whose carrier components drifted at a constant speed from left to right and then reversed direction. On any given trial, one of these elements was nominated as the target and the remaining elements were distractors. Distractor elements all changed direction simultaneously. The distractors either moved in a homogeneous manner (i.e. all moved in the same direction), or in a heterogeneous manner (i.e. direction was randomized). The target moved with a similar spatio-temporal trajectory as the distractors from left to right (or vice versa), but changed direction asynchronously with respect to the distracting elements. The participants' task was to locate this deviant (target) Gabor patch. We show that a motion direction change pops out (as indicated by the absence of a set size effect) when the surrounding distractors move in a homogeneous direction. When the distractors moved in heterogenous directions, a similar pop out effect was observed when the set size was small (≤5 elements), but not when it was large. This suggests that motion direction changes capture attention only when the change results in a unique direction of motion. Consistent with this finding we also show that a moving target (without direction change) captures attention in cases in which all distractors recently changed direction. This corroborates the idea that, in addition to direction cues, the temporal uniqueness of a change in an object's direction (or absence, thereof) relative to surrounding objects is a cue capable of capturing attention.