Stimulus configuration determines the detectability of motion signals in noise

We measured the detectability of moving signal dots in dynamic noise to det ermine whether local motion signals are preferentially combined along an ax is parallel to the direction of motion. Observers were asked to detect a si gnal composed of three dots moving in a linear trajectory among dynamic noi se dots. The signal dots were collinear and equally spaced in a configurati on that was either parallel to or perpendicular to their trajectory. The pr obability of detecting the signal was measured as a function of noise densi ty, over a range of signal dot spacings from 0.5 degrees to 5.0 degrees. At any given noise density, the signal in the parallel configuration was more detectable than that in the perpendicular configuration. Our four observer s could tolerate 1.5-2.5 times more noise in the parallel configuration. Th is improvement is not due merely to temporal summation between consecutive dots in the parallel trajectory. Temporal summation functions measured on o ur observers indicate that the benefit from spatial coincidence of the dots lasts for no more than 50 ms, whereas the increased detectability of the p arallel configuration is observed up to the largest temporal separations te sted (210 ms). These results demonstrate that dots arranged parallel to the signal trajectory are more easily detected than those arranged perpendicul arly. Moreover, this enhancement points to the existence of visual mechanis ms that preferentially organize motion information parallel to the directio n of motion. (C) 2000 Optical Society of America [S0740-3232(00)02009-3] OC IS codes: 330.4150, 330.4060.