Psychophysically measuring the efficiency of rods

Recent studies suggest that the efficiency of cones to detect photons can be evaluated by measuring the equivalent input noise (EIN; derived from contrast thresholds measured in the presence and absence of visual noise) under specific conditions in which the contrast threshold is limited by the variability in the number of photons detected by photoreceptors (i.e., photon noise). These conditions can be identified based on the known properties of photon noise: spatially and temporally white and inversely proportional to the luminance intensity. The present study aims to adapt this psychophysical paradigm to evaluate the efficiency of rods to detect photons. A motion direction discrimination task was used to evaluate the EIN over a wide range of luminance intensities for various spatial and temporal frequencies when the display was blue or red (to which rods have little sensitivity). The target was either a Gabor patch presented at 20 degrees of eccentricity (first experiment) or a rotating sine-wave annulus with a radius of 10 degrees of eccentricity (second experiment). In both experiments, the EIN was found to be inversely proportional to luminance intensity over a limited range of luminance intensities for both display colors. At these luminance intensities, the EIN was roughly independent of the spatial and temporal frequencies, matching the properties of photon noise. Furthermore, under these conditions, contrast thresholds were lower (i.e., better) when the display was blue rather than red, which suggests that vision was mediated by rods when the display was blue. We conclude that the efficiency of rods to detect photons can be evaluated by measuring contrast thresholds in the presence and absence of visual noise over a limited range of luminance intensities with a blue display.