Fig. 2.

Perceptual effects of transparency. A: psychometric curves for 5 human subjects. In the uni-uni condition, the subjects compared 2 patches of unidirectional motion, one with a fixed (reference) speed of 10°/s and the other with the variable speed shown on x-axis. In the uni-bi condition, the subjects compared a unidirectional test speed (x-axis) to a bidirectional reference moving at 10°/s. The clear rightward shift of the point of subjective equivalence (PSE) in the uni-bi conditions represents a strong overestimation of the perceived speed of the bidirectional reference. The intersection of the dotted black lines indicates the PSE for veridical speed perception. B: comparison of PSE in the uni-uni and uni-bi conditions for a range of reference speeds. Each data point represents data from a single subject for a single reference speed. Reference speeds (°/s) are color coded according to the key. This plot confirms that the effect shown in A (overestimation of bidirectional speed) was found consistently, and across the range of reference speeds. C: comparison of the sensitivity (the slope of the psychometric function in A at the PSE) in the uni-uni and uni-bi conditions for a range of reference speeds. Sensitivity was consistently higher in the uni-uni condition. D: behavioral data from 1 monkey subject (monkey M). The uni-uni and uni-bi conditions were identical to those of the human subjects. In the bi-bi condition monkey M compared the speed of a bidirectional test stimulus with the speed of a bidirectional reference. The data were averaged over reference speeds, and therefore the test speed on the x-axis is expressed as % of the reference speed. The data show that monkey M was less sensitive for the bi-bi comparison than the uni-uni comparison. Just like the human subjects, monkey M overestimated the speed of bidirectional stimuli compared with unidirectional reference patterns (uni-bi, red asterisk; error bars representing the 95% confidence limits have a length of 4%, which is smaller than the marker). The red dotted line is a psychometric function whose PSE was chosen to match with the uni-bi performance, while the slope was estimated jointly from the uni-uni and bi-bi trials. The intersection of the dotted black lines indicates the PSE for veridical speed perception.