3/07/2006

Illusory Motion Reversal: Rivalry or Perceptual Sampling?

During a previous discussion of the wagon wheel illusion, I endorsed the perceptual sampling hypothesis put forth by Van Rullen et al. to explain the phenomenon of illusory motion reversals of spinning wheels under continuous illumination (try it here). An incredibly clever experiment reported in Vision Research suggests that this illusion is not due to discrete sampling of the visual stream by attentional mechanisms (similar to movie's frame rate) but is instead due to rivalry between competing visual representations.

By showing subjects the wagon wheel illusion under continuous illumination, Kline, Holcombe, and Eagleman replicated the "illusory motion reversals" which seemed to support the perceptual sampling hypothesis. However, they also did something special: they also required participants to also report the reversals occuring in the illusion's mirror image. The authors reasoned that if these "motion reversals" actually indicate perceptual sampling, the mirror image should be seen to reverse direction simultaneously. Accordingly, ratings of motion reversal in the illusion or its mirror image did not correspond (and this same effect was also found when the illusion and its mirror image are presented to the same hemifield), suggesting that another process may be at work.

However, this conclusion rests on some tenuous assumptions. First, illusory motion reversals should be synchronized only if the various regions of the visual field are sampled with uniform frequency - this is an interesting prediction, but is not likely to be true given the nature of signal processing and its tradeoffs in temporal/spatial resolution. Second, attention is known to modulate the number of illusory motion reversals; to my mind at least, this is the fact that really suggests the wagon wheel illusion reflects higher cognitive processes. Since participants are unlikely to be able to pay attention to a visual illusion and its mirror image simultaneously, the different effects found in this study could be due to attention factors, as opposed to perceptual rivalry.

Instead, these authors propose that the illusory motion reversals are caused by competing Reichardt detectors: detectors for the illusory direction could become spuriously activated through temporal aliasing, which "happens because S1 occupies the receptive field on the left and, soon after, S2 moves into the receptive field on the right. The detector has no way of knowing that S1 and S2 are different stimuli." This explanation can be criticized on the basis of two facts: first, no Reichardt detectors have ever been observed in the mammalian visual system, and second, such an explanation does not actually argue against "perceptual sampling" but just moves it into the realm of temporal aliasing by a Reichardt detector, as opposed to temporal aliasing at a cognitively higher level.

EDIT: I have just learned that the following paper makes many of these same points:

Rojas, D., Carmona-Fontaine, C., Lo´pez-Caldero´n, J., & Aboitiz, F. (2006). Do discreteness and rivalry coexist in illusory motion reversals? Vision Research, 46(6–7), 1155–1157.

Related Posts:
Perceptual Sampling: The Wagon Wheel Illusion

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