Cortical magnification factor and contrast sensitivity to luminance-modulated chromatic gratings

Contrast sensitivity to luminance-modulated blue, green, red, and neutral gratings was measured at different spatial frequencies and eccentricities within 0-86 deg in the temporal visual field. Contrast sensitivity to gratings of constant area and spatial frequency was independent of wavelength composition but decreased with increasing eccentricity. When the gratings were scaled by the magnification factor of the human striate cortex to produce cortically similar stimulus conditions at different eccentricities (M-scaling), contrast sensitivities became independent of visual field location irrespective of grating colour. Using colour naming we found, in accordance with previous studies, that hue changed and saturation decreased when the eccentricity of a constant-size grating field increased. In contrast, the hue and saturation of M-scaled grating fields were independent of eccentricity. The results suggest that the effects of eccentricity on photopic colour vision can largely be counteracted by M-scaling which adjusts the spatial aspects of stimuli with respect to the decrease in ganglion cell density and increase in receptive field size towards the periphery of the visual field.