The motion aftereffect

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Abstract

The motion aftereffect is a powerful illusion of motion in the visual image caused by prior exposure to motion in the opposite direction. For example, when one looks at the rocks beside a waterfall they may appear to drift upwards after one has viewed the flowing water for a short period—perhaps 60 seconds. The illusion almost certainly originates in the visual cortex, and arises from selective adaptation in cells tuned to respond to movement direction. Cells responding to the movement of the water suffer a reduction in responsiveness, so that during competitive interactions between detector outputs, false motion signals arise. The result is the appearance of motion in the opposite direction when one later gazes at the rocks. The adaptation is not confined to just one population of cells, but probably occurs at several cortical sites, reflecting the multiple levels of processing involved in visual motion analysis. The effect is unlikely to be caused by neural fatigue; more likely, the MAE and similar adaptation effects provide a form of error-correction or coding optimization, or both.

Section snippets

Methods of measurement

The MAE is not easy to measure. After prolonged inspection of a moving `adapting' stimulus, a static `test' stimulus appears to move in the opposite direction. Observers can report the duration of the MAE, which seems to be a square-root function of the duration of the adapting motion[11]. Attempts to null the MAE by moving the test stimulus slowly in the opposite direction to the MAE (i.e. in the same direction as the adapting motion) fall foul of the fact that the MAE produces a sensation of

Higher-order aftereffects

MAEs are thought to originate not only at early stages but also at later (higher) levels of visual motion processing (see Appendix C). For example, the MAE is reduced in duration when the observer's attention is distracted by a difficult letter-reading task[25]. Culham and Cavanagh[26]adapted their observers to a radial counterphase-flickering grating, rather like a flickering rotating wheel. This is a directionally ambiguous stimulus that either just flickers or sometimes moves in random

Physiological substrate

Barlow and Hill[8]explained the MAE in terms of the discharge characteristics of single neurons in the visual system. In their experiment, as described earlier, the firing rate of the ganglion cells dropped below its baseline level when the motion stopped, recovering gradually over 30 s. No rebound effect was found; following a continuous adapting motion in the null direction, spontaneous activity did not change. The transient reduction in spontaneous activity was regarded as corresponding to an

Theoretical models

Models are designed to link the MAE with neural adaptation to a particular direction of motion, which consequently reduces the sensitivity of cells tuned to that direction relative to cells tuned to other directions. The responses of the adapted and unadapted cells are then compared to generate a motion percept.

Sutherland[45]proposed that the direction of seen motion depends upon the firing ratios of cells sensitive to movement in opposite directions. After exposure to prolonged movement in one

A function for aftereffects?

Does the MAE have a functional value or is it simply a design fault in the visual system? The naive view that neurons can `fatigue' rather like over-exercised muscles, perhaps owing to depletion of neurotransmitters, is almost certainly incorrect, since some neurons appear to resist adaptation altogether. Although cortical neurons in the cat certainly do adapt to motion[49], retinal and geniculate cells do not[50]. If some visual neurons do not fatigue, why should any? Furthermore, the time

Conclusions

Even more than 2000 years after Aristotle's report of the MAE it is difficult to make firm statements on the actual nature of this illusion. There are good reasons to suggest that the MAE is not caused by neural fatigue alone, because the timing is wrong and because some visual neurons do not even show fatigue. It could be that the MAE provides a form of error correction or coding optimization, or even both. We now also know that adaptation occurs at several cortical sites, reflecting a range

Acknowledgements

Thanks to Hiroshi Ashida, Peter Bex and Mike Swanston for comments on the manuscript. S.A. is supported by NEI Grant E-10241, F.V. by the Royal Netherlands Academy of Arts and Sciences and G.M. by the EPSRC.

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