Perceptual Sampling: The Wagon Wheel Illusion

Ever noticed how wheels or hubcaps can appear to rotate backwards, either in movies, on TV, or even on the highway? Illusory motion reversal is one of the least understood visual illusions, but an ongoing scientific debate asks whether it may actually reflect the temporal resolution of consciousness. If you haven't experienced this illusion - often known as the "wagon wheel illusion" - try it here before reading further.

Up until recently this illusion was thought to be purely a result of subsampling through stroboscopic illumination. And yet, many report experiencing this effect in broad daylight - under continuous illumination. This led Van Rullen, Reddy & Koch to wonder whether the illusion could actually reflect some kind of "discrete perceptual sampling": if the rate of apparent backwards motion in a movie is related to the movie's frame rate, perhaps the rate of apparent backwards motion under continuous light would reflect the temporal resolution of some aspect of cognition.

Interestingly, the authors found that the rate of perceived motion of alternating sunburst patterns was highest at alternation rates of between 10 and 20 Hz. This suggests that some aspect of the visual system is sampling the perceptual stream around 10-20 times per second. But the story doesn't end there: the frequency and duration of motion reversal was dependent on focused attention, such that the average perceived direction of motion at 10Hz was actually worse under directed attention. This is apparently one of the very few known cases where attention actually decreases accuracy in visual performance.

It's not clear exactly what neural mechanism would cause this sampling frequency, because the conventional frequency bands of cortical oscillation do not lie within this range. Some propose that a specific type of motion detector could create this effect; others counter that such "Reichardt" detectors wouldn't be likely show the same temporal sensitivity, and even if they could, there's no way to confirm that they do, since no Reichardt detectors have been discovered in the mammalian visual system. Selected directed attention has effects on both alpha, beta, and gamma band frequencies, but mechanisms by which these could interact with motion perception are unknown.

I've written previously about cortical oscillations in the context of visual attention, but there's still a few more relevant articles to be summarized in the next few days before it's time for (public) hypotheses about the possible relationships between measures of perceptual sampling, measures of processing speed, and visual working memory span. So watch this space ...

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