Grouping local directional signals into moving contours

We consider how local motion signals are combined to represent the movements of spatially extensive objects. A series of band-pass target dots, whose collective motion defined a moving contour, was positioned within a field of randomly moving noise dots. The visibility of the contours did not depend on the direction of movement relative to local contour orientation unless the contour was constrained to pass through fixation, suggesting that a previously reported advantage for collinear motion trajectories depends on the probability of detecting any of the target elements rather than the integrated contour. Contour visibility was invariant of the spatial frequency of the elements, but it did depend on the speed, number and spacing of elements defining it, as well as the angle and spatial frequency difference between adjacent elements. Local averaging of directional signals is not sufficient to explain these results. The visibility of these moving contours identifies narrow-band grouping processes that are sensitive to the shape defined by the directions of the elements forming the contour. (C) 2003 Elsevier Ltd. All rights reserved.