The perception and discrimination of speed in complex motion

Random dot kinematograms were used to simulate radial, rotational and spira l optic flow. The stimuli were designed so that, while dot speed increased linearly with distance from the centre of the display, the density of dots remained uniform throughout their presentation. In two experiments, subject s were required to perform a temporal 2AFC speed discrimination task. Exper iment 1 measured the perceived speed of a range of optic flow patterns agai nst a rotational comparison stimulus. Radial motions were found to appear f aster than rotations by approximately 10%, with a smaller but significant e ffect for spirals. Experiment 2 measured discrimination thresholds for pair s of similar optic flow stimuli identical in all respects except mean speed . No consistent differences were observed between the speed discrimination thresholds of radial, rotational and spiral motions and a control stimulus with the same speed profile in which motion followed fixed random trajector ies. The perceived speed results are interpreted in terms of a model satisf ying constraints on motion-in-depth and object rigidity, while speed discri mination appears to be based upon the pooled responses of elementary motion detectors. (C) 1999 Published by Elsevier Science Ltd. All rights reserved .