Temporal asymmetry in motion masking: a shortening of the temporal impulse response function

Morgan and Chubb observed a striking temporal asymmetry in motion masking (Vis. Res. 39 (1999) 4217). Motion was produced with a two-frame sequence of gratings presented in spatial quadrature phase; the second grating (100 ms) was presented immediately after the first grating (100 ms), with no temporal overlap. The contrast threshold for detecting the direction of motion of the stimulus pair was facilitated when the first grating was of low-contrast and the second grating was of high-contrast, but strong masking occurred when the order was reversed, so the high-contrast grating came first. We replicated this result, but showed that the masking mostly disappeared when the two gratings temporally overlapped only slightly. The high sensitivity to the precise temporal pattern of the stimulus can be explained by a small temporal 'shortening' of the temporal impulse response function (IRF) as stimulus contrast is increased. The IRF is biphasic with a negative inhibitory lobe. When the first grating has high-contrast, its flash response (owing to the shortening of the IRF) may be in a fairly strong negative phase by the time that the positive response to the second, lower-contrast grating has reached appreciable strength--this reduces the magnitude of the motion signal generated by the two flashes and can account for the masking. A shortening of the IRF with increased contrast (a nonlinearity) is supported by psychophysical studies in humans and by recordings of magnocellular retinal ganglion cells in macaque, and the present results bolster this concept.