Relation of split apparent motion to metacontrast.

The apparent suppression of the target in metacontrast is often accompanied by "split" apparent motion. In Experiments 1, 4, 5, and 6 "neighboring stimuli" (similar to and flanking the mask stimuli) were added to the display, and subjects rated both metacontrast and split motion. Under some conditions, both split motion and metacontrast were completely eliminated (Experiment 1), supporting the assumption that apparent motion is necessary for metacontrast. However, under other conditions, neighboring stimuli caused a much stronger depression of metacontrast than of split motion (Experiments 4 and 5), sometimes even enhancing the latter (Experiment 6), supporting the assumption that the mechanisms underlying the two phenomena are essentially independent. Further, peripheral presentation and close spacing of target and mask with no neighboring stimuli (Experiments 2 and 3) gave strong metacontrast while completely eliminating split motion, showing clearly that apparent motion is not necessary for metacontrast. Results are interpreted in terms of a "fusion" process underlying metacontrast and a "direction-sensitive unit" underlying apparent motion. Interactions between these two processes that might account for the common co-occurrence of motion and metacontrast are proposed.