Trends in Cognitive Sciences
The motion aftereffect
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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