Impaired perception of isoluminant red-green contrast modulation stimuli: Evidence for a magnocellular pathway mechanism.

Parasol retinal ganglion cells form the magnocellular pathway and support perception of achromatic visual stimuli at high temporal frequencies (TF) and low spatial frequencies (SF). Parasol cells also exhibit nonlinear behavior like that of Y-type retinal ganglion cells in the cat, giving a second harmonic response at high spatiotemporal frequencies, and therefore can be considered "Y-like" cells. Previous neurophysiology showed that responses to contrast modulation (CM) stimuli, composed of a high SF grating (carrier) whose contrast is modulated by a low SF sinewave (envelope), are driven by the nonlinear subunits of Y-like cells. Recent human psychophysics has shown that direction discrimination of CMs at high spatiotemporal carrier frequencies may reflect nonlinear processing of Y-like cells (Ramirez et al, 2022). Since Y-like cells do not process color, CM motion direction discrimination should be impaired at isoluminance. It is commonly known that motion is absent or reduced at isoluminance. However, residual motion performance might be mediated by the luminance pathway, as indicated by impaired chromatic motion when adding luminance noise. Healthy normal subjects monocularly viewed luminant yellow-black (Y-B) or isoluminant red-green (R-G) CMs presented at the center of a CRT monitor while fixating eccentrically. They reported the direction of the moving envelope. Within each block of trials, envelope contrast for Y-B and R-G CMs was varied with the method of constant-stimuli. Superimposed 1-D luminance (Y-B) noise was added at different contrast levels. We found that subjects could discriminate motion direction for both kinds of stimuli in the absence of masking noise. Luminance (Y-B) noise maskers affected performance of Y-B CMs only when above the masker's own detection threshold, while R-G CMs were impaired at substantially lower noise contrasts. These results are consistent with our hypothesis that CM responses are driven by Y-like neurons in the magnocellular pathway.