Discrimination of opposite directions measured with stroboscopically illuminated random-dot patterns

The aim of the present experiments was to investigate the maximum displacement over which coherent motion can be perceived (Dmax) for a stroboscopically illuminated moving random-dot pattern, both as a function of the size of the pattern and as a function of the spatial-frequency content. Perception of coherent motion was defined operationally as the ability to discriminate opposite directions. The maximum velocity (Vmax) for which the observer could discriminate the direction of motion for a given strobe rate was obtained by using a forced-choice staircase procedure. Vmax was measured as a function of the strobe rate for four different sizes of the pattern. For each pattern size Vmax was limited by the spatial asynchrony between the flashes (Dmax). For a given size of the pattern, Dmax did not depend on the temporal asynchronies tested (10-100 msec). Dmax values were increased with the size of the pattern. Bandpass rectangular filtering was used to change the spatial-frequency content of the pattern. There was a gradual increase in Dmax as the spatial-frequency content shifted to low spatial frequencies. Interestingly, the Dmax that was obtained with the unfiltered pattern was larger than that obtained with the high-pass pattern but also smaller than that obtained with the low-pass pattern. The present results are in agreement with results obtained with a two-frame display. Dmax is not a fixed value: it depends on the size of the pattern as well on the spatial-frequency content.(ABSTRACT TRUNCATED AT 250 WORDS)