S-, M- and L-cone contributions to human pupil responses under dim illumination conditions

Abstract

Cones, rods and melanopsin contribute to pupillary responses. However, how each type of cones contributes to pupil responses has been rarely studied. In our study, we investigated pupil responses to S-, M- and L-cone flicker inputs under dim illumination conditions. A four-primary Ganzfeldphotostimulator (DiagnosysColorDome) controlled rod and cone stimulations at −0.9 log cd/m², which is below the melanopsin threshold but above the cone detection threshold. Four types of sinusoidally modulated stimuli (1 Hz, 20% contrast) were used: 1) rod stimuli (modulated rod excitation with a constant S- M- and L-cone excitation); 2) S-cone stimuli (modulated S-cone excitation; constant rod, M- and L-cone excitation); 3) M-cone stimuli (modulated M-cone excitation; constant rod, S- and L-cone excitation); and 4) L-cone stimuli (modulated L-cone excitation; constant rod, S- and M-cone excitation). Pupil sizes were measured using an Eyelink II eyetracker (SR Research) with a 250 Hz sampling rate. Pupil amplitudes and phases were derived by Fourier analysis. The results showed that, overall, the cone (S-, M- or L-) stimuli produced smaller responses than rod stimuli, suggesting rod dominance at −0.9 cd/m². On the other hand, pupil responses to the M-cone stimuli were larger than for S- or L-cone stimuli. While the M- and L-cone stimuli produced similar response phases (between 0 deg and −50 deg), the S-cone stimuli had a response phase of approximately 180 deg. These results are consistent with L/M-On and S-OFF response characteristics of ipRGCs measured in primate retinas (Dacey et al., 2005).