Local variations in L/M ratio influence the detection and color naming of small spots.

The distribution of long (L)-, middle (M)-, and short (S)-wavelength sensitive cones in the retina determines how different frequencies of incident light are sampled across space and has been hypothesized to influence spatial and color vision. We examined how the detection and color naming of small, short-duration increment stimuli (λ = 543 or 680 nm) depend on the local spectral topography of the cone mosaic. Stimuli were corrected for optical aberrations by an adaptive optics system and targeted to locations in the parafovea where cone spectral types were known. We found that sensitivity to 680-nm light, normalized by sensitivity to 543-nm light, grew with the proportion of L cones at the stimulated locus, although intra- and intersubject variability was considerable. A similar trend was derived from a simple model of the achromatic (L+M) pathway, suggesting that small spot detection mainly relies on a non-opponent mechanism. Most stimuli were categorized as achromatic, with red and green responses becoming more common as stimulus intensity increased and as the local proportion of L and M cones became more balanced. The proximity of S cones to the stimulated region did not influence the likelihood of eliciting a chromatic percept. Our detection data confirm earlier reports that small spot psychophysics can reveal information about local cone topography, and our color naming findings suggest that chromatic sensitivity may improve when the L/M ratio approaches unity.