High temporal frequency synchrony is insufficient for perceptual grouping.

We used textures of randomly moving grating patches to assess the role of fine-grain temporal synchrony in texture segregation. In the target area, patches reversed direction simultaneously. In the surround, patches changed direction at random times. Thus, phase changes in the target area were precisely synchronous, whereas those in the surround were not. In agreement with work carried out by Lee and Blake, we found that the target area was frequently visible, and that observers could discriminate its shape (horizontal versus vertical) at frame rates of 100 Hz in brief exposures (200 ms). Further experiments suggested that the length of unidirectional motion sequences in the target area, rather than synchrony, determined its visibility. To eliminate completely contrast and motion cues, we made all the background elements identical to the target elements, but with a random starting phase. Despite the presence of synchrony in the target area but not the background, the target was generally very hard to see. Targets that remained visible contained low temporal frequency modulations of direction. We conclude that the human observer can detect synchrony, but only at modest temporal frequencies once motion and contrast artefacts have been eliminated.