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Rufin VanRullen, Leila Reddy, Christof Koch; Attention-dependent discrete sampling of motion perception. Journal of Vision 2005;5(8):237. doi: 10.1167/5.8.237.
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Temporal subsampling of the perceptual stream can cause illusory reversals of the perceived motion direction. This “Wagon Wheel Illusion”, most apparent in movies or on television, can also be observed under continuous illumination, suggesting that our visual systems too might sample motion in a sequence of discrete epochs (Purves et al, Proc. Nat. Acad. Sci. USA, 1996). This phenomenon is bistable by nature, with the actual motion direction generally dominating perception (Kline et al. Vis. Res, 2004). Previously we reported that a motion energy model subsampling visual inputs at a rate between 10 and 20 Hz can quantitatively predict the relative durations of real and illusory percepts during continuous viewing of the illusion (VanRullen et al, Society for Neuroscience, 2004). Here we used pairs of gratings drifting in opposite directions to investigate this effect. By way of these “counterphase” gratings of slightly different contrasts, one can directly manipulate the ambiguity of motion direction, and enhance the relative strength of the illusory percept. We found that motion direction judgments for these stimuli were selectively impaired around 10 Hz, as predicted by the temporally subsampled motion energy model. Interestingly, this impairment vanished when focal attention was directed away from the motion stimulus. We used a dual-task paradigm to draw spatial attention to a stream of rapidly presented randomly rotated letters at the center of the grating. Under these conditions, simultaneous motion direction judgments were in fact better at 10 Hz than when attention was directed to the motion itself. This is one of very few known instances were focal attention is found to impair performance. These results support the idea that, at least in some circumstances, the visual system represents motion in discrete epochs, and that this effect is mediated by focal attention.
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