Retinal mechanisms determine the subadditive response to polychromatic light by the human circadian system

May 16, 2008Neuroscience letters

How eye signals cause weaker combined effects of mixed-color light on the body’s internal clock

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Abstract

Melatonin suppression was significantly lower in the condition with 1/2 unit of blue and 1/2 unit of green light compared to conditions with one unit of each color.

The circadian system is influenced by both blue and green light, with specific inputs from retinal cells.
Conditions with one unit of blue light and one unit of green light produced similar levels of melatonin suppression.
A combined lower intensity of blue and green light resulted in less melatonin suppression than higher intensities.
Findings support a model of circadian phototransduction that includes the role of color opponent retinal neurons.
The results indicate that the human circadian system shows a subadditive response to certain combinations of polychromatic light.

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Light is the major synchronizer of circadian rhythms to the 24-h solar day. The intrinsically photosensitive retinal ganglion cells (ipRGCs) play a central role in circadian regulation but cones also provide, albeit indirectly, input to these cells. In humans, spectrally opponent blue versus yellow (b-y) bipolar cells lying distal to the ganglion cell layer were hypothesized to provide direct input to the ipRGCs and therefore, the circadian system should exhibit subadditivity to some types of polychromatic light. Ten subjects participated in a within-subjects 3-night protocol. Three experimental conditions were employed that provided the same total irradiance at both eyes: (1) one unit of blue light (lambda(max)=450 nm, 0.077 W/m(2)) to the left eye plus one unit of green light (lambda(max)=525 nm, 0.211 W/m(2)) to the right eye, (2) one unit of blue light to the right eye plus one unit of green light to the left eye, and (3) 1/2 unit of blue light plus 1/2 unit of green light to both eyes. The first two conditions did not differ significantly in melatonin suppression while the third condition had significantly less melatonin suppression than conditions 1 and 2. Furthermore, the magnitudes of suppression were well predicted by a previously published model of circadian phototransduction incorporating spectral opponency. As was previously demonstrated, these results show that the human circadian system exhibits a subadditive response to certain polychromatic light spectra. This study demonstrates for the first time that subadditivity is due to spectrally opponent (color) retinal neurons.

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Retinal mechanisms determine the subadditive response to polychromatic light by the human circadian system

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