Piéron's law relates human reaction times to the intensity of a sensory stimulus by a power function. The neural processes responsible for this nonlinear behavior are not understood. A simple neural model based on the Brownian motion of spikes and information theory is presented. The model shows that Piéron's law is a transformation function in time. The shape of Piéron's law is invariant and scales into the intensity-response function of single neurons in a fractal-like process. The model also shows that Piéron's law gives rise to 1/falpha noise together with a high-frequency thermal noise limit. It is proposed that the biophysical origin of reaction time variability is related to a form of noise-induced synchronization in weakly coupled neurons. The implications in visual-motor transduction are discussed.

(Doc. ID 62073) Simple visual-reaction times (VRT) were measured for a variety of stimuli selected along red-green (L − M axis) and blue-yellow [S − ͑L+M͒ axis] directions in the isoluminant plane under different adaptation stimuli. Data were plotted in terms of the RMS cone contrast in contrast-threshold units. For each opponent system, a modified Piéron function was fitted in each experimental configuration and on all adaptation stimuli. A single function did not account for all the data, confirming the existence of separate postreceptoral adaptation mechanisms in each opponent system under suprathreshold conditions. The analysis of the VRT-hazard functions suggested that both color-opponent mechanisms present a well-defined, transient-sustained structure at marked suprathreshold conditions. The influence of signal polarity and chromatic adaptation on each color axis proves the existence of asymmetries in the integrated hazard functions, suggesting separate detection mechanisms for each pole (red, green, blue, and yellow detectors).

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Detection of a Gabor pattern is impaired in the presence of a similar pattern of orthogonal orientation, a phenomenon known as cross-orientation masking (XOM). Here we investigate the role of color in cross-orientation masking. We measured contrast detection thresholds to horizontally oriented Gabors overlaid by similar Gabors of a different orientation. Red-green chromatic masking was compared to achromatic masking for a wide range of spatial and temporal frequencies, orientations, and masks contrasts. We find that cross-orientation masking is significantly greater for chromatic than achromatic contrast. We also find it is invariant with the spatio-temporal conditions used, unlike achromatic cross-orientation masking that is known to have a spatio-temporal dependence (greatest for low spatial frequencies at high temporal frequencies). Furthermore, chromatic masking is isotropic (invariant across the orientation difference between test and mask), whereas the achromatic version of the masking effect displays orientation tuning, a phenomenon that was originally used to indicate the presence of orientationally selective mechanisms in human vision. We conclude that the P cell pathway or its projections can support cross-orientation masking. We propose distinct physiological origins for chromatic and achromatic masking, with a predominantly cortical site for chromatic masking in contrast to the M cell subcortical influences on achromatic masking suggested by previous studies.

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