Linear analysis of eccentricity-dependent changes in the null-based isoluminant plane

Abstract

The percept of motion that is elicited by moving gratings is lost at isoluminance when the stimulus is viewed continuously. This loss of motion perception was used to evaluate variations with retinal eccentricity in the spectral sensitivity of the motion mechanism, nulled at isoluminance. Subjects viewed 2.3° square, 1.3 cpd, 3.75 Hz moving gratings generated by a color cathode-ray tube. These stimuli were used to estimate the motion null plane foveally and at 2,4, and 8°. The orientations (in three-dimensional color space) of the best-fitting null planes were determined by principal component analysis. The orientation of the foveal isoluminant plane differed from that obtained at 2, 4, and 8°. The variation in orientation with eccentricity was further analyzed by expressing the null planes as linear combinations of two eccentricity-dependent vectors: one calculated fora macular pigment-absorption template of variable density and the other calculated for variable linear combinations of L- and M-cone absorption spectra. The analysis suggests that most of the eccentricity-dependent differences in spectral sensitivity should be attributed to variations in macular pigment density across the retina and not to changes in relative L-and M-cone contributions to the motion mechanism nulled at isoluminance.