Temporal properties of the visual detectability of moving spatial white noise

We obtained movement detection thresholds for two-dimensional random speck-patterns ("Julesz" patterns) homogeneously moving over the whole target field (5.21 x 5.31 degrees of visual angle). We alternated between two uncorrelated but otherwise similar patterns, one moving with velocity leads to V1, the other with velocity leads to V2, such that each pattern was on for T ms. We masked this pattern (signal) with spatio-temporal white noise ("snow"). The total r.m.s. contrast was kept constant, whereas the ration of the r.m.s. contrasts of signal and noise was varied. The square of this ratio was designated SNR. At low SNR values the pattern was not perceptually different from the snow alone. At high SNR values the subject detected spatio-temporal correlation (e.g., movement). In these experiments we determined the threshold SNR values as a measure of the detectability of spatio-temporal correlation as a function of the parameters T, leads to V1, and leads to V2. When leads to V1 and leads to V2 were sufficiently dissimilar one of three percepts occurred: for very large T the alternation could be followed, for very small T two transparent, simultaneously moving sheets of noise-pattern with different velocities could be seen. For intermediate T-values no systematic movement at all could be observed. At these T-values the threshold SNR was maximal. This "'critical" T-value decreased with increasing velocity. We found that it was possible to have more than one percept of uniform smooth movement at a single location in the visual field if these movements had velocity vectors with an angular difference of at least 30 deg or if their magnitudes differed by at least a factor of 4.