Abstract
Abstract: :
Purpose: To test cortical models of chromatic/achromatic processing with a new VEP method. Methods: Both the spatial interaction (similar to the pattern onset) VEP and the temporal interaction (similar to the pattern reversal) VEP were recorded simultaneously with a new multifocal VEP paradigm and 4 normal observers. The display was an 18–sector dartboard. The stimuli were either purely achromatic, purely chromatic (i.e. red/green isoluminant) or mixed (i.e. a combination of the achromatic and chromatic stimuli.). The amplitudes of both the achromatic and chromatic components were estimated by decomposing a mixed achromatic and chromatic response into separate "pure achromatic" and "pure isoluminant" components. The results were compared to a model that assumed that a mixed achromatic and chromatic response is a linear combination of pure achromatic and pure isoluminant components [1, 2]. Results: The VEP contrast response functions were fitted well by the Michaelis–Menten eq. and the achromatic temporal interaction VEP had a much lower saturation contrast than the achromatic spatial interaction VEP as previously described [3]. Although the amplitudes of the achromatic temporal interaction VEP were almost identical for both 16% and 32% Michelson contrasts, the amplitudes of the isoluminant component to the mixed stimuli (isoluminant chromatic and 16% or 32% achromatic) differed dramatically. The equivalent (achromatic) contrasts of pure isoluminant stimuli can be determined according to the interaction between isoluminant and achromatic responses. When plotted against the equivalent contrast, the isoluminant contrast response function and the achromatic contrast response function are similar. For both the spatial and the temporal interaction VEP, the equivalent contrasts of the isoluminant stimuli are similar, being about 20% Michelson contrast for our red–green isoluminant stimulus. Conclusions: These results do not support a model in which independent pathways (e.g. M and P), specifically sensitive to either achromatic or chromatic stimuli, shape the waveform of high contrast VEPs. Rather, they require a model in which the interaction between achromatic and chromatic responses occurs before a compressive nonlinear stage (i.e. a global compressive model of achromatic and chromatic VEP). Equivalent contrast, as defined here, provides an objective means for evaluating the strength of an isoluminant stimulus and unites the achromatic and isoluminant contrast response functions. 1. Derrington and Krauskopf et al. (1984); 2. Valberg and Lee et al. (1987); 3. Regan. Human Brain Electrophysiology (1988);
Keywords: color vision • electrophysiology: non–clinical • visual cortex