Attention and non-retinotopic feature integration.

Features of moving objects are non-retinotopically integrated along their motion trajectories as demonstrated by a variety of recent studies. The mechanisms of non-retinotopic feature integration are largely unknown. Here, we investigated the role of attention in non-retinotopic feature integration by using the sequential metacontrast paradigm. A central line was offset either to the left or right. A sequence of flanking lines followed eliciting the percept of two diverging motion streams. Although the central line was invisible, its offset was perceived within the streams. Observers attended to one stream. If an offset was introduced to one of the flanking lines in the attended stream, this offset integrated with the central line offset. No integration occurred when the offset was in the non-attended stream. Here, we manipulated the allocation of attention by using an auditory cueing paradigm. First, we show that mandatory non-retinotopic integration occurred even when the cue came long after the motion sequence. Second, we used more than two streams of which two could merge. Offsets in different streams were integrated when the streams merged. However, offsets of one stream were not integrated when this stream had to be ignored. We propose a hierarchical two stage model, in which motion grouping determines mandatory feature integration while attention selects motion streams for optional feature integration.