Radial Asymmetry in the Sensitivity of the Blue Mechanism

In the human retina, the density of blue cones is lower along the nasal horizontal meridian (Curcio et al, 1991 Journal of Comparative Neurology 312 610 – 624) than along the other cardinal meridians, but this finding might reflect interpolation between widely spaced data points rather than actual radial asymmetry. Available psychophysical data on the sensitivity of the blue mechanism do not address this question.

In the present study, subjects had to detect a blue test stimulus (wavelength range at half-amplitude 425 – 480 nm, diameter 24 min arc) presented against yellow background (wavelength above 550 nm, 200 cd m⁻²). The test stimulus was flickering at 5.7 Hz around a mean level of 3 cd m⁻² within a Gaussian temporal window with a time constant of 0.2 s. Foveal and parafoveal sensitivity was measured along the vertical and horizontal meridians. Radial asymmetry in the sensitivity of the blue mechanism was observed, in that the known foveal insensitivity also involved part of the nasal horizontal meridian. The dip in the sensitivity map at the fovea was narrower along the vertical than along the horizontal meridian. While these data are in good agreement with the blue-cone density map, the dip in sensitivity is wider than the depression in the density of blue cones at the fovea. Both screening by yellow pigment and effects of stimulus size might contribute to this discrepancy.