Chromatic center-surround antagonism revealed by the Westheimer paradigm

Visual sensitivity to an object is dependent on its surrounding context. The Westheimer paradigm quantifies the effects of surround pedestal size on sensitivity to a centrally located spot of light. The resultant curve is thought to reveal the center-surround interactions of underlying neural mechanisms. While the achromatic Westheimer function has been well characterized, chromatic mechanisms remain relatively unexplored. The goal of this study was to characterize spatial interactions for stimuli defined by the cardinal axes of DKL color space. Chromatic cone contrast thresholds to a spot of light were measured as a function of the size of a surrounding pedestal in four subjects. Stimuli were presented on persistent pedestals randomly to the left or right of fixation at 5° retinal eccentricity, with detection indicated by a button press corresponding to the side that contained the target. The target and pedestal were defined along the same direction in color space, with the pedestal set at a lower contrast. The achromatic background had a luminance of 10 cd/m2. For chromatic conditions, luminance noise was added to potential target locations to minimize non-chromatic cues. Results were well described by a ratio of gaussians (ROG) model. Pedestal-dependent amplitudes were consistent within visual mechanisms and between subjects. Perceptive field center size was defined by the peak of desensitization, and surround size by the asymptote where increasing pedestal size no longer affected thresholds. For achromatic conditions, mean center and surround sizes were 0.36° (±0.04) and 4.4° (±1.0) respectively, 0.98° (±0.16) and 3.8° (±0.36) for +LM direction, 1.2° (±0.10) and 5.5° (±0.71) for the -LM direction, 1.3° (±0.13) and 6.4° (±0.57) for the +S direction, and 1.49° (±0.12) and 6.4° (±1.0) for the -S direction. The large sizes suggest a cortical locus and provide insight into the spatial grain of low-level chromatic mechanisms.