Human velocity and direction discrimination measured with random dot patterns

In the present experiments three different motion discrimination tasks were studied using a random dot pattern as stimulus: velocity discrimination, direction discrimination and discrimination of opposite directions. The analysis of the motion of random dot patterns is based on motion sensitive mechanisms without the confounding interference of position sensitive mechanisms (Nakayama and Tyler, 1981). Furthermore, since isotropic random dot patterns contain no dominant orientation, a change in the direction of motion does not parallel a change in orientation. Hence the use of a random dot pattern as stimulus allows velocity and direction discrimination to be compared. Human velocity discrimination displays a U-shaped dependence on the stimulus velocity: the JNDs, expressed as Weber-fractions, are minimal for velocities ranging from 4 to 64 deg.sec-1. The Weber-fractions in velocity, determined with a staircase procedure tracking a 84% correct response level, were about 7% at the optimal speeds. The velocity discrimination curve obtained with the random dot pattern is similar to that obtained with light bars. Human direction discrimination, defined as the smallest difference in direction which can be resolved, also displays a U-shaped dependence on the stimulus velocity. Direction discrimination thresholds decrease up to a velocity of 4 deg.sec-1, they then stay at a constant level up to 128 deg.sec-1. Beyond this velocity the thresholds increase again. The mean direction discrimination threshold was 1.8 deg at optimal speeds. Discrimination of opposite directions, determined for the same conditions as those for which velocity and direction discrimination thresholds were determined, was better than the 90% response level at all speeds. However at low contrast, opposite directions are reliably discriminated only at intermediate speeds. Perceiving a coherent moving random dot pattern is supposed to be based on a cooperation between a large number of local motion detectors. In order to evaluate the importance of detector output pooling, the influence of the size of the pattern and of the presentation time on the three discrimination tasks was measured. The results indicate that the pooling requirements are task dependent. A somewhat larger pooling is required for velocity discrimination than for direction discrimination, whereas for discrimination of opposite directions only a few local motion detectors are involved.