Dissociable effects of element-lifetime and stimulus-duration on local and global motion processing: An equivalent noise study

Human visual motion processing is thought to involve two stages: local motion processing and global pooling of local motion signals. We used an efficient equivalent noise paradigm to explore temporal aspects of motion processing, specifically manipulating the lifetime of moving elements and overall stimulus-duration, to examine (respectively) local and global processing. Participants judged whether the overall direction of a dot pattern was clockwise or anti-clockwise of a reference direction. An adaptive staircase determined a) the minimum direction offset required for reliable judgment of direction when dots moved in a similar direction and b) the maximum directional noise (range of directions) that still allowed participants to tell if a stimulus was moving either ±45° from the reference direction. The two sets of thresholds allowed us to infer both a) the precision of participants' judgment of the direction of a single dot (i.e., local noise) and b) the number of samples they were effectively averaging (i.e., global sampling). Dots moved at 10 deg/s and we tested various dot lifetimes (33.36, 66.72, 133.4, and 266.8 ms) with a stimulus-duration of 1,000 ms, and various stimulus-durations (62, 125, 250, 500, 1,000, and 2,000 ms) with a dot lifetime of 133.4 ms. Longer element-lifetimes increased local directional precision but did not influence global integration (number of samples effectively averaged). Conversely, longer exposures did not greatly influence local precision, but increased global averaging by an average of 13% for every doubling of stimulus-duration. Thus, element-lifetime and stimulus-duration have dissociable effects on motion perception improving local and global motion processing, respectively.