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Fig. 2. The effect of contrast frequency on the front and rear field turning
responses. The mean response amplitudes ± S.E.M. for front
(open symbols; N=12) and rear (black symbols; N=13) field
motion reach a maximum at a contrast of 10 s1 and 6.7
s1, respectively. Doubling the spatial period of the pattern
(gray symbols; N=8) results in a shift in the contrast frequency
optimum. No response reversal indicative of aliasing was found within the
tested range of spatial and temporal frequencies.