The ups and downs of global motion perception: A paradoxical advantage for smaller stimuli in the aging visual system

Abstract

Recent evidence suggests that normal aging is typically accompanied byimpairment in the ability to perceive the global (overall) motion of visualobjects in the world. The purpose of this study was to examine the interplaybetween age-related changes in the ability to perceive translational globalmotion (up vs. down) and important factors such as the spatial extent (size)over which movement occurs and how cluttered the moving elements are(density). We used random dot kinematograms (RDKs) and measured motioncoherence thresholds (% signal elements required to reliably discriminateglobal direction) for young and older adults. We did so as a function of thenumber and density of local signal elements, and the aperture area in whichthey were displayed. We found that older adults' performance was relativelyunaffected by changes in aperture size, the number and density of localelements in the display. In young adults, performance was also insensitive toelement number and density but was modulated markedly by display size,such that motion coherence thresholds decreased as aperture area increased(participants required fewer local elements to move coherently to determinethe overall image direction). With the smallest apertures tested, youngparticipants' motion coherence thresholds were considerably higher (~ 1.5times worse) than those of their older counterparts. Therefore, when RDK sizeis relatively small, older participants were actually better than youngparticipants at processing global motion. These findings suggest that thenormal (disease-free) aging process does not lead to a general decline inperceptual ability and in some cases may be visually advantageous. Theresults have important implications for the understanding of the consequencesof aging on visual function and a number of potential explanations areexplored. These include age-related changes in spatial summation, reducedcortical inhibition, neural blur and attentional resource allocation.© 2014 Hutchinson, Ledgeway and Allen.