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 s^–1 and 6.7 s^–1, 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.