Speed and time discrimination with bouncing disk stimuli

Our previous work showed that a bouncing disk, a moving stimulus that accelerates and decelerates, improved precision on a time discrimination task relative to when time intervals were defined by discrete visual flashes. However, time interval and speed were perfectly correlated, so participants could have used either information to respond correctly. To disentangle the relationship between speed and time with our stimuli, we measured participants’ speed and time discrimination performance with bouncing disk stimuli akin to those used in our prior research. In this study, participants completed two tasks. In the speed task, participants reported whether a test stimulus moved faster or slower relative to the standard stimulus (15 deg/s). Since the stimuli accelerated and decelerated, their average speed was used as the criterion for the speed discrimination. In the time task, participants reported whether a test time interval, defined as the time between the disk’s bounces, was shorter or longer relative to the standard time interval (550 ms). Crucially, all bouncing disk stimuli shown to participants were identical in both tasks. The speed and time dimensions were sampled relatively equally by varying the height of the disk’s trajectory. For all participants, precision (threshold) in the speed discrimination task was at least a factor of three worse than in the time discrimination task. This result suggests that participants in our previous experiment were indeed making time judgments, not speed judgments. Further, the bouncing disk improved time discrimination despite it not providing good information for speed discrimination. A high quality time representation from a moving stimulus was achieved without a high quality speed representation.