Brain responses to violet, blue, and green monochromatic light exposures in humans: prominent role of blue light and the brainstem

Abstract

Background: Relatively long duration retinal light exposure elicits nonvisual responses in humans, including modulation of alertness and cognition. These responses are thought to be mediated in part by melanopsin-expressing retinal ganglion cells which are more sensitive to blue light than violet or green light. The contribution of the melanopsin system and the brain mechanisms involved in the establishment of such responses to light remain to be established.

Methodology/principal findings: We exposed 15 participants to short duration (50 s) monochromatic violet (430 nm), blue (473 nm), and green (527 nm) light exposures of equal photon flux (10(13)ph/cm(2)/s) while they were performing a working memory task in fMRI. At light onset, blue light, as compared to green light, increased activity in the left hippocampus, left thalamus, and right amygdala. During the task, blue light, as compared to violet light, increased activity in the left middle frontal gyrus, left thalamus and a bilateral area of the brainstem consistent with activation of the locus coeruleus.

Conclusion/significance: These results support a prominent contribution of melanopsin-expressing retinal ganglion cells to brain responses to light within the very first seconds of an exposure. The results also demonstrate the implication of the brainstem in mediating these responses in humans and speak for a broad involvement of light in the regulation of brain function.

Figure 1. Experimental design.

a. General timeline. Time relative to scheduled wake time (hrs). Arrows: subjective sleepiness assessment (SS 1-7). b. Timeline of the fMRI period and light condition organization. Black bars indicate occurrence of the different conditions. Note that the combination of light 1 and 2 changes from one session to the other. S1-3: sessions 1 to 3 during which 3 combinations of light are employed (combination order is given as example). Time in minutes after entering the scanner. Arrows: subjective sleepiness assessment (SS 5-7).

Figure 2. Significant differences between the blue and violet light conditions during the performance of the 2-back task.

Left panels: statistical results overlaid to the population mean structural image (p_uncorrected<0.001). Right panels: Mean parameter estimates of the blue and violet light conditions during the 2-back task (arbitrary units±SEM). a. left thalamus–b. left MFG–c. right brainstem–d. left brainstem.

Figure 3. Significant differences between blue and green light conditions at light onset.

Left panels: statistical results overlaid to the population mean structural image (p_uncorrected<0.001). Right panels. Mean parameter estimates of the blue and green light conditions at light onset (arbitrary units±SEM). a. left hippocampus–b. right amygdala–c. left thalamus.