The visual discrimination of velocity is considered in terms of an observer’s response to its direction at different speeds. The hypothesis is advanced that the direct perception of motion in a middle range of speeds involves a single sensory event dependent on the intensity-time relation. To test this hypothesis, four subjects discriminated velocity at different speeds for each of eight durations of exposure. The minimal luminance required for correct judgements of direction was measured by the method of limits.
The data indicate that velocity discrimination occurs at intermediate speeds when the energy of the stimulating flash is constant (It=C). The reciprocity relation does not hold for exposures longer than a critical duration of 0.1 sec. The luminance of the flash tends to be the sole limiting factor on velocity discriminations at longer durations of exposure (I=K). Implications of these results are discussed with reference to other experiments.
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