Transient apparent-motion stimuli, consisting of single ?-wavelength steps applied to square-wave gratings lacking the fundamental (?missing fundamental stimulus?) and to sinusoidal gratings, were used to elicit ocular following responses in humans. As previously reported, the earliest ocular following responses were strongly dependent on the motion of the major Fourier component, consistent with early spatio-temporal filtering prior to motion detection, as in the well-known energy model of motion analysis. We have now shown that introducing inter-stimulus intervals of 10-200 ms, during which the screen was gray with the same mean luminance, reversed the initial direction of the ocular following response, the peak reversed responses (with inter-stimulus intervals of 20-40 ms) being substantially greater than the non-reversed responses (with an inter-stimulus interval of 0 ms). When the mean luminance was reduced to scotopic levels, reversals now occurred only with inter-stimulus intervals greater than 60 ms and the peak reversed responses (with inter-stimulus intervals of 60-100 ms) were substantially smaller than the non-reversed responses (with an inter-stimulus interval of 0 ms). These findings are consistent with the idea that initial ocular following responses are mediated by 1st-order motion-energy-sensing mechanisms that receive a visual input whose temporal impulse response function is strongly biphasic in photopic conditions and almost monophasic in scotopic conditions.