DIRECTIONS OF MOTION AFTER-EFFECTS INDUCED BY GRATINGS AND PLAIDS

In three experiments the direction of motion after-effect (MAE) is mea sured following adaptation to two gratings moving in different directi ons presented in alternation (component induced MAEs: CMAEs), and to m oving plaid patterns composed of superimposed pairs of these gratings (plaid-induced MAEs; PMAEs). These MAEs are compared to: (i) the vecto r sum direction of the component gratings; (ii) the IOC-predicted dire ction of the plaids; and (iii) the perceived direction of the plaids a s reported by observers. Contrary to previous findings (Burke D, Wende roth P. Vis Res 1993;33:351-9), directions of PMAEs are shown to appro ximate the vector sum direction of the components, whereas directions of CMAEs are shown to approximate the mean (unweighted) direction of t he components. This difference is attributed to the activity, and adap tation, of an additional population of neurones whose stimulus), or a counterphase moving plaid (a combined Fourier and non-Fourier stimulus ), rules out the possibility that the discrepancy between PMAE directi on and actual plaid direction is due to the use of test stimuli that d o not adequately reflect adaptation by the Fourier and non-Fourier com ponents of the adapting plaids (HR, Ferrera VP: Yo C. Vis Neurosci 199 2;9:79-97). Various explanations of this paradoxical result are discus sed, including: (i) that MAEs produced by Fourier components out-weigh land possibly even mask) MAEs produced by non-Fourier plaid component s; (ii) PMAEs are influenced by adaptation of a population of componen t-selective neurones that do not contribute to plaid perception; and, (iii) PMAEs are influenced by component-specific adaptation effects th at are weighted according to relative component sensitivity, rather th an relative component speed (Pantle A. Vis Res 14;1974:1229-36). We re view psychophysical and neurophysiological evidence consistent with th ese explanations. (C) 1998 Elsevier Science Ltd. All rights reserved.