The properties of the motion-detecting mechanisms mediating perceived direction in stochastic displays

Previous studies [e.g. Baker & Hess, 1998. Vision Research, 38, 1211-1222] have shown that perceived direction in displays composed of multiple, limit ed-lifetime, Gabor micropatterns (G) is influenced by movement both at the fine spatial scale of the internal luminance modulation (first-order motion ) and the coarse spatial scale of the Gaussian, contrast window (second-ord er motion). However it is presently indeterminate as to whether this patter n of results is indicative of the processes by which first-order and second -order motion signals interact within the visual system per se or those by which motion information, irrespective of how it is defined, is utilised ac ross different spatial scales. To address this issue, and more generally th e properties of the mechanisms that analyse motion in such displays, we emp loyed stochastic motion sequences composed of either G, G added to a static carrier (G + C) or G multiplied with a carrier (G*C). Crucially G*C, unlik e both G and G + C, micropatterns contain no net first-order motion and sec ond-order motion only at the scale of the internal contrast modulation. For small displacements perceived direction in all cases showed a dependence o n the internal sinusoidal spatial structure of the micropatterns and charac teristic oscillations were typically observed, consistent with models in wh ich first-order motion and second-order motion are encoded on the basis of similar low-level mechanisms. Importantly for larger displacements, and als o when the internal spatial structure was randomised on successive exposure s (so that motion at this spatial scale was unreliable), performance tended to be veridical for all types of micropattern, even though under these con ditions displacements of the G*C micropatterns should have been invisible t o current, low-level, motion-detecting schemes. This suggests that both low -level motion sensors and mechanisms utilising a different motion-detecting strategy such as high-level, attentive, feature-tracking may mediate perce ptual judgements in stochastic displays. (C) 2000 Elsevier Science Ltd. All rights reserved.