Energy versus Synchrony in Perceptual Grouping

H. Farid and E.H. Adelson

Vision Sciences (VSS), Sarasota, FL, 2002

It has been proposed that the human visual system can use temporal synchrony for perceptual grouping. In a compelling demonstration of this theory a stochastic motion display purportedly driven solely by temporal synchrony was shown to promote grouping. It was then argued that these effects point to the role of synchrony-based mechanisms and processes. We have previously argued that the displays contain a traditional form of contrast energy and thus the grouping phenomena might be attributed to traditional mechanisms.

To further study this topic we devised new stimuli rich in temporal synchrony but devoid of contrast energy. These stimuli allow aspects of synchrony and spatio-temporal energy to be independently manipulated. We find that the energy, and not the synchrony, predicts the results.

The stochastic displays consist of a sea of drifting elements. On each frame every element moves according to a random process. Different random processes drive all the elements in the central and surrounding regions. One might argue that the resulting form cue is defined solely by the fine-grained temporally synchronous motion reversals. We observe, however, that there are moments when all elements in one region repeatedly reverse directions, while in the other region all elements have a run with no reversals. We show that a classic spatio-temporal energy model consisting of a spatial lowpass filter and a temporal bandpass filter can convert these relatively large-scale temporal change differences into a contrast cue.

This simple model is consistent with the psychophysical results of Lee and Blake (Science, 1999), Kandil and Fahle (Euro. J. Neuro., 2001), Farid and Adelson (Nat. Neuro., 2001), and Morgan and Castet (Proc. Roy. Soc., 2002). A model based on temporal synchrony alone can not explain all of these results. We conclude that the proposed synchrony-based mechanisms and processes are neither necessary nor sufficient to explain the phenomena.