Guang Yang¹, David Yu¹, Chris Paffen², Frans Verstraten¹; ¹The University of Sydney, ²Utrecht University

The motion aftereffect has intrigued many for ages. Although reported already by Aristotle, it still has outstanding questions that need answers. Following Barlow & Hill’s famous 1963 Nature paper (see their figure 1), it is often assumed that the response characteristics of direction selective neurons reflect the percept of illusory motion in humans. It led for example Sekuler & Pantle (1967) to suggest that “the duration of the MAE is a function of the time to recover baseline level” as a valid model assumption. The question we address here is whether the percept of illusory motion reflects this recovery from adaptation process. In a series of experiments, we tested whether the recovery from the adaptation period reflects the perceived illusory motion. We focus on unidirectional motion. Individual MAE durations were measured assuming that subsequent adaptation to an orthogonal motion direction combines with subthreshold activity of previous presented motion, if present. That is, in the case the recovery process is still ongoing, the part below perceptual threshold can be made visible again by integrating the residual activity with a supra threshold MAE signal of a second adaptation pattern, which then is reflected in the direction of the combined aftereffect (see Verstraten et al., VisRes, 1994). This direction is in between the aftereffect directions of both adapting patterns, if there is subthreshold residual adaptation. The results show that the perceived illusory motion reflects only about half of the recovery from adaptation period. We conclude that the motion aftereffect is better defined as ‘the visible part of the recovery from adaptation period’. This means that part of this process is below perceptual threshold and should be taken into consideration when using the MAE as the psychophysicist’s micro-electrode (John Frisby, 1979). Not perceiving the aftereffect doesn’t mean there is no recovery from adaptation anymore.