How does noise influence the estimation of speed?

Local motion signals have to be combined in space and time, to yield a cohe rent motion percept as it is involved in a variety of visual tasks. This co mbination necessarily means to trade-off between loosing spatio-temporal re solution by pooling local signals and maintaining perceptually significant segmentation between them. When signals are pooled to detect the presence o f coherent motion in large amounts of random noise, the question raised is how the noise affects the perceived quality, in particular speed, of the co herent motion. Is there an analogy to the well-known reduction in the perce ived speed of moving gratings at low contrast? Using a two-interval forced- choice procedure, we have investigated the assessment of speed in random-do t kinematograms containing different proportions of noise. Under the condit ions investigated, there is no strong reduction of perceived speed with inc reasing noise, as long as coherence levels remain well above the thresholds for directional judgements. This basic result, which could suggest conside rable but not perfect segregation of signal and noise motion components in the pooling process leading to speed estimation, is discussed in relation t o a model that is designed to decode speed from a population of elementary motion detectors (EMDs) of the correlation type. A strategy to estimate spe ed from a set of EMDs with a variety of spatio-temporal tuning does not onl y provide a velocity predictor unambiguous with the spatial structure of th e stimulus, but also is largely independent of noise. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.