The detection of direction-defined and speed-defined spatial contours: one mechanism or two?

It is now accepted that the visual system integrates local orientation information across space to define spatial contours [Vision Research 33 (1993) 173]. More recently, it has been shown that similar integration occurs for the direction of local motion signals, in different parts of the visual field, if they are aligned along the axis of a spatial contour [Vision Research 42 (2002) 653]. Here we ask whether similar spatial-linking rules hold for contours comprised of local elements that share only a common speed (but not direction), in the presence of background elements which collectively have the same mean speed as the contour but considerable random variation in the speeds of the individual elements. Furthermore we investigate the detection of spatial contours that are defined by a common speed that is different (both locally and globally) from that of the background elements. The results show that there is a significant, albeit relatively weak, speed-association field with preferential linking between spatially proximal elements that have similar speeds. Although a salient speed difference between the contour and the background elements enhances detection performance for motion-defined contours, it does so primarily via a different route to that of direction linking. We suggest that for motion-defined contours the Gestalt notions of "common fate" and "good continuity", that describe the parsing of local velocity information into objects, boundaries and contours, are mediated via separate underlying perceptual mechanisms.