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Abstract:
We determined two-dimensional motion discrimination contours in the spatio-temporal
frequency plane to characterize the mechanisms underlying velocity perception. In particular, we
wanted to determine whether there exist mechanisms tuned specifically to velocity, rather than sepa-
rable mechanisms tuned to spatial and temporal frequency. A 4-AFC paradigm was used to determine
spatio-temporal frequency discrimination thresholds for sinewave gratings defined by luminance
contrast. Three of the grating patches used were defined by the same spatial and temporal frequency
(standard), the other (test) differed by a fixed proportional change in spatial and temporal frequency.
Subjects had to indicate which grating differed most from the others and the thresholds determined
for varying proportions of change in spatial and temporal frequency were used to trace out complete
threshold contours in the plane spanned by these attributes. Some of the contours, primarily at speeds
above 1 deg/s, were noticeably oriented along lines of constant velocity. To further isolate these
mechanisms, spatio-temporal noise was added to the standard stimuli either along a line of constant
velocity or in the direction orthogonal to it. When spatio-temporal noise of constant velocity was
added to the standard stimuli, threshold contours became elongated only along the direction of the
noise. The same amount of noise in the orthogonal direction produced an overall increase in thresh-
olds without changing the shape of the contour, presenting clear evidence for velocity tuned mecha-
nisms. In further experiments we discovered that velocity tuned mechanisms interact with separable
mechanisms to produce optimal discriminability. Analogous experiments with isoluminant stimuli
failed to exhibit evidence for velocity tuning, supporting the notion that the human color vision sys-
tem is impaired in its coding of stimulus speed, despite excellent sensitivity to direction of motion.
