Do you feel differently when you enter a fast-food restaurant with bright lights from the way you feel in a restaurant with low lighting? Research has shown that the quality and quantity of light affects our mood, especially in seasonal affective disorder (SAD) and depressive disorders, but why that happens hasn’t been clear. Scientists at Brown University may have the answer. In a new study published in the Proceedings of the National Academy of Science and reported in ScienceDaily, “Researchers discover a brain pathway that helps to explain light’s effect on mood.” The research team used functional MRI to reveal how light-intensity signals reach the brain, and how brain structures involved in mood process those signals. The study showed that some regions of the cerebral cortex involved in cognitive processing and mood, and the prefrontal cortex in particular are sensitive to light intensity.
A small percentage of retinal ganglion cells contribute little or nothing to vision but are themselves photosensitive. These cells have long axons that extend into the brain and contribute to circadian rhythms, irregularities of which have been shown to affect mood and pupillary light reflex. Previous animal studies revealed a mood-regulating neural pathway linking the photosensitive retinal ganglion cells and the prefrontal cortex involved with mood disorders. The new research investigated whether this pathway exists in humans.
Researchers identified 26 human brain regions having activation that either decreases or increases with light intensity. Activation occurred across the cerebral cortex, in the cerebellum, which has a role in emotion regulation and impulse, and in regions associated with visual image formation, motor control, cognition and emotion. They showed that the prefrontal regions of the human brain have light-sensitive signals, and that these signals are similar to the photosensitive retinal ganglion cells. Jerome Sanes, a Brown professor of neuroscience affiliated with the University’s Carney Institute for Brain Science and lead study author, said that together, these signals may explain the effects of light intensity on complex emotional and cognitive behaviors.
The research was conducted on healthy adults and showed a link between light exposure and prefrontal cortex cognitive and mood responses. For Sanes, the next question is how light affects these same brain pathways and regions in people with mood disorders such as SAD or major depressive disorders. More study is needed but determining whether light activates the same brain regions and with the same levels of sensitivity in individuals with these disorders could contribute to the development of therapeutic treatments. There is so much more to light, the eye and the brain than we ever thought possible!
• Linda Conlin
Pro to Pro Managing Editor