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Most simple cells in the striate cortex of the cat are direction selective, firing to broadside movement of an optimally oriented edge or bar in one direction of motion and not the other. The smallest stimulus displacement for which a direction-selective discrimination can be made cannot be smaller than the threshold for the detection of the displacement itself. It is shown that retinal image quality is an important limiting factor in respect to the thresholds both for stimulus displacement and direction selectivity.
Goodwin et al. (Sat,) studied this question.