Impaired perception of isoluminant contrast modulation stimuli: Evidence for a magnocellular pathway mechanism

Contrast modulation (CM) stimuli have been previously used to reveal nonlinear contributions of Y-like retinal ganglion cells (RGCs) such as parasol cells to cortical responses and perception. To test whether CMs are selectively processed within the magnocellular pathway, we assessed envelope motion discrimination and detection for achromatic (yellow-black) and chromatic (red-green) CMs in the presence of luminance masking noise to disrupt luminance-based mechanisms of motion processing. Compared to achromatic CMs, perception of chromatic CMs was more sensitive to luminance masking noise, suggesting that CM envelope motion perception relied predominantly on luminance signals. Specifically, envelope motion discrimination performance was better maintained for achromatic CMs than chromatic CMs, even at high masking noise levels. Notably, luminance masking noise greatly impaired envelope direction discrimination for chromatic CMs but had minimal impact on their detection, suggesting that chromatic aberrations may enhance envelope motion perception for chromatic CMs by introducing luminance signals. These findings collectively emphasize that CM stimuli selectively activate Y-like/parasol RGCs of the retino-geniculate magnocellular pathway, underscoring their potential to specifically target this pathway. This might have clinical advantages for early diagnosis in disorders such as glaucoma or dyslexia, where magnocellular pathway dysfunction is predominant.