Visual and Non-Visual Responses to Short-Wavelength Light

Objectives: The human retina contains five photoreceptors: The three spectral classes of cones — the S cones (peak spectral sensitivity λmax ~420 nm), the M cones (λmax ~530 nm), the L cones (λmax ~558 nm) — , rods ( λmax ~495 nm), and melanopsin ( λmax ~480 nm). Short-wavelength light is known have important effects on what is conventionally called “non-image-forming” visual function, including circadian photoentrainment, phase shifting and the pupillary light reflex. Methods: By presenting stimuli which are matched in the activation of a given set of photoreceptors (so-called metamers) but differ in activation of the target photoreceptors, the contribu- tions of the different photoreceptors can be mapped out. We have previously used this method to examine the properties of the hu- man pupillary light reflex, and encoding of melanopsin-targeted stimuli in human visual cortex (using fMRI). Here, we are examin- ing metameric illuminants at five chromaticities along the daylight locus (correlated colour temperatures 25,000K, 10,000K, 6,500K, 5,000K and 4,000K) differing in melanopsin activation by 200% and measuring the pupil size in human observers under naturalistic viewing conditions. Results: While the colour rendering properties of these meta- meric illuminants are generally poor, they elicit physiologically differentiable pupil sizes by up to 20%. This is not a surprising result, as it is known that melanopsin contributes to the pupil under conditions of silent substitution; but it provides evidence that large melanopsin contrast in isolation seen in naturalistic viewing condi- tions has a tangible physiological effect. Conclusions: This research represents a step towards pro- ducing carefully controlled metameric melanopsin-targeted stim- uli under naturalistic (e.g. sleep lab, home setting, office) viewing conditions with a demonstrable physiological effect