Temporal integration of optic flow, measured by contrast thresholds and by coherence thresholds

We measured, as a function of exposure duration, contrast sensitivity and coherence sensitivity for discerning the direction of motion of random dot patterns moving in circular, radial or translational directions. Contrast sensitivity for these patterns increased linearly with exposure duration, up to about 200–300 ms, consistent with previous estimates of temporal summation of early motion units. Coherence sensitivity, however, showed much longer summation periods, about 3 s. When the stimulus was embedded within 10 s of noise, sensitivity increased with duration up to 2–3 s, approximately linearly, as expected from an ideal integrator. When presented without the noise period, sensitivity also increased, but in a different way. For radial and circular motion the increase tended towards the theoretically predicted square root relationship for the same duration as that found with the embedded noise (about 3 s). For translation, however, the curve was steeper than the theoretical prediction (nearly linear), and the summation estimates of around 1000 ms. When the duration of the target was constant at 200 ms, but that of the flanking noise varied, sensitivity decreased with total duration over a similar interval. We interpret our results to reflect at least two stages of analysis, a threshold-limited early stage of local-motion analysis, with a time constant of 200–300 ms, and a later global-motion integration stage with a much longer time constant, around 3000 ms. There may also exist an intermediate stage, with an integration time of around 1000 ms.