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

We measured, as a function of exposure duration, contrast sensitivity and c oherence sensitivity for discerning the direction of motion of random dot p atterns moving in circular, radial or translational directions. Contrast se nsitivity for these patterns increased linearly with exposure duration, up to about 200-300 ms, consistent with previous estimates of temporal summati on of early motion units. Coherence sensitivity, however, showed much longe r 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 li nearly, as expected from an ideal integrator. When presented without the no ise 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 embe dded noise (about 3 s). For translation, however, the curve was steeper tha n the theoretical prediction (nearly linear), and the summation estimates o f around 1000 ms. When the duration of the target was constant at 200 ms, b ut that of the flanking noise varied, sensitivity decreased with total dura tion over a similar interval. We interpret our results to reflect at least two stages of analysis, a threshold-limited early stage of local-motion ana lysis, with a time constant of 200-300 ms, and a later global-motion integr ation stage with a much longer time constant, around 3000 ms. There may als o exist an intermediate stage, with an integration time of around 1000 ms. (C) 2001 Elsevier Science Ltd. All rights reserved.