Discrimination of the speed and direction of global second-order motion in stochastic displays

The ability to integrate local second-order motion signals over space and time was examined using random-dot-kinematograms (RDKs) in which the dots were defined by spatial variation in the contrast, rather than luminance, of a random noise field. When either the speeds or the directions of the individual dots were selected at random from a range of possible values, globally the stimulus appeared to drift either in a single direction or at a single speed in a manner analogous to that reported previously for first-order (luminance-defined) RDKs. To quantify the precision with which observers could extract the global stimulus motion, speed- and direction-discrimination thresholds were measured using pairs of RDKs, one of which (the comparison) comprised dots whose speeds or directions were assigned stochastically and the other (the standard) comprised dots that all had the same drift direction and speed. Speed-discrimination thresholds were of the order of 8% and changed little as the range of dot speeds (bandwidth) of the comparison increased, in that performance was almost as good when the individual dot speeds were selected at random from a range spanning 3.84 deg/s as when all the dots moved at the same speed. There was a tendency for the perceived global speed of the comparison RDK to decrease as the speed bandwidth was increased and perceived speed tended to coincide with the geometric mean speed of the dots rather than the arithmetic mean speed. Direction-discrimination thresholds were lowest (approximately 4 degrees) when the range of dot directions was less than 90 degrees but increased markedly thereafter. Observers were able to perform both discrimination tasks when the lifetimes of the dots comprising the RDKs was reduced from 25 to 2 frames, a manipulation that prevented observers from determining the overall speed or direction of image motion from the extended trajectories of individual dots within the display. Thresholds under these conditions were somewhat higher but were otherwise comparable to those obtained with a dot lifetime of 25 frames. The similarities between the present results and those of previous studies that have employed first-order RDKs suggest that the extraction of the global speed and direction of each type of motion is likely to be based on computationally similar principles.