Meridional differences in the directional sensitivity of the human color mechanisms.
The directional sensitivity (DS) of the red (pi 5), green (pi 4), and blue (pi 1) color mechanisms was measured for both the horizontal and vertical pupil meridians, thus localizing the peak of each pi-mechanism DS function within the two-dimensional pupillary aperture. In the horizontal meridian, there were greater differences among observers in DS function peak locations than there were for any one observer. Along the vertical meridian, peak location differences within observers were similar to those found in the horizontal meridian, whereas among-observer differences were much smaller. Significant differences between horizontal and vertical DS function halfwidths were also found for at least one pi-mechanism of each observer. It is concluded that meridional variations exist in either the morphology or orientational distribution of foveal photoreceptors, and that more than one alignment mechanism is responsible for photoreceptor orientation in the human fovea.