Two experiments were performed to examine whether the perceptual grouping by shape similarity affected visual stream segregation (VISS), in which four lights appear to be a set of two apparent motion (streams) when flashed at a moderately fast rate. Sixteen undergraduates found the upper limits of the stimulus-onset asynchrony (SOA) for producing VISS under various conditions of arrangement of shape in four vertical positions. The experiments showed that the SOA was longer where the grouping by shape similarity was compatible with the original grouping of VISS than where there was no grouping due to shape. They also showed that the SOA was shorter where the grouping by shape was not compatible than where there was no grouping due to shape. The results indicate that, in addition to the flashing rate of the lights, the perceptual grouping is another important factor to produce VISS. They also show that VISS is a type of apparent motion affected by the perceptual organization among stimuli in the display.

Key words: visual apparent motion, visual stream segregation, perceptual grouping, shape