Spatial Vision, Vol. 17, No. 3, pp. 235–248 (2004)  VSP 2004. Also available online - www.vsppub.com When motion is not perceived: Evidence from adaptation and dynamical stability HOWARD S. HOCK ∗ , DAVID F. NICHOLS and JESSICA ESPINOZA Department of Psychology, Florida Atlantic University, Boca Raton, FL 33431, USA Received 11 January 2004; accepted 12 February 2004 Abstract—Adaptation was used to probe the perceiver’s activation state when either motion or nonmotion percepts are formed for bistable, single-element apparent motion stimuli. Although adaptation was not observed in every instance, when it was observed its effect was to increase the probability of both motion-to-nonmotion and nonmotion-to-motion switches, the time scale of adaptation corresponding to neurophysiological observations for directionally selective cortical cells (Giaschi et al. 1993). This susceptibility to de-stabilizing adaptation effects indicated that the nonmotion percept was not the result of inadequate stimulation producing subthreshold levels of motion detector activation; if that were the case, activation-dependent adaptation would have decreased the nonmotion-to-motion switching rate by reducing activation further below threshold. Above-threshold activation levels are therefore associated with both nonmotion and motion perceptual states, and the failure to perceive motion despite the presence of adequate motion detector stimulation can be attributed to inhibitory competition between detectors activated by motion-specifying stimulus information and detectors activated to similar levels by motion-independent stimulus information, consistent with the dynamical quality of single-element apparent motion. Keywords: Adaptation; motion; dynamics; stability; nonmotion. INTRODUCTION Research in motion perception generally has been concerned with the measurement of motion detection thresholds. The overall message from studies of apparent motion of discrete elements (e.g. Korte, 1915; Kolers, 1972), continuous motion of discrete elements (e.g. Nakayama and Tyler, 1978), and continuously drifting sine gratings (e.g. Kulikowski, 1978; Nakayama and Silverman, 1985) is that when motion is not perceived, it is because motion detectors are inadequately stimulated. These results notwithstanding, Hock et al. (1997a) have shown that adequate stimulation of motion detectors does not guarantee that motion will be perceived. ∗ To whom correspondence should be addressed. E-mail: hockhs@fau.edu