It’s clear that the amount and quality of light a person is exposed to can have a significant impact on mood, from seasonal changes in daylight to the choice of artificial lighting in workplaces.
Now, scientists at Brown University think they know why.
In a new study published in Proceedings of the National Academy of Sciences, the research team used functional MRI to reveal how light intensity signals reach the brain and how brain structures involved in mood process these signals. The study found that certain areas of the cerebral cortex involved in cognitive processing and mood show sensitivity to light intensity.
The discovery has implications for understanding mood problems, such as seasonal affective disorder and major depressive disorder, and how to treat them, said lead study author Jerome Sains, Brown Professor of Neurology at the University’s Carney Brain Science Institute.
“Identifying this pathway and understanding its function may directly inform the development of approaches to treat depression, either through pharmacological manipulation, noninvasive brain stimulation at individual nodes in the pathway, or through targeted bright light therapy,” Sanes said.
The findings build on previous research by study co-author David Berson, a professor of neurology at Brown, who in 2002 discovered special light-sensitive cells in the eye. Unlike rods and cones, these “intrinsically photosensitive retinal ganglion cells” are not involved in so-called “object vision” or “shape vision,” Sanes said, but mainly function to sense light intensity.
Earlier research, some of it by Berson, found that some animals have a neural pathway that regulates mood, connecting these light-sensitive cells in the retina to regions of the brain’s prefrontal cortex involved in mood disorders. Sanes said the new study was designed to determine whether a similar pathway exists in humans and whether they could find evidence that the pathway shares functional similarities with light-sensitive retinal ganglion cells.
To determine whether the light intensity coding pathway modulates the human prefrontal cortex, the researchers used functional MRI to examine whole-brain activation patterns in 20 healthy adults.
In a relatively simple experiment, according to Sanes, participants viewed four different levels of light intensity through glasses that diffused the light and eliminated visual shapes, colors and other objects in the environment. Participants watched the intensity of the light go from dark to bright for 30 seconds each. To keep them alert, they simultaneously performed an auditory task that required them to tell the difference between two tones.
By evaluating functional MRI images taken during exercise, the researchers identified 26 regions of the human brain where activity decreased or increased depending on light intensity. This “luxotonic-related activation” occurred in the cerebral cortex, various subcortical structures, and the cerebellum, covering areas with functions related to visual image formation, motor control, cognition, and emotion.
They found that light suppressed activity in the prefrontal cortex is proportional to light intensity. Light-evoked responses in the prefrontal cortex of the brain and their change as a result of previous exposure to light resembled the responses of light-sensitive retinal ganglion cells.
It’s well known that changes in ambient light, which don’t necessarily have anything to do with an object’s shape or vision, affect a variety of basic functions, such as circadian rhythms, visual reflexes, mood and possibly cognitive processing, Sanes said. However, it remained unclear how these light intensity signals reach the relevant areas of the human brain.
In this study, the researchers showed that the prefrontal region of the human brain has light-sensitive signals, and that these signals are similar to the intrinsically light-sensitive retinal ganglion cells — which together, Sains said, may explain the effect of light intensity on a complex of emotional and cognitive behaviors.
“The findings of our study show a functional link between light exposure and cognitive and affective responses mediated by the prefrontal cortex,” Sanes said.
The next logical question to ask, Sains said, concerns how light affects these same brain pathways and regions in people with mood disorders such as seasonal affective disorder or major depressive disorder.
“How does it compare to a control group of healthy people who have not been diagnosed with these disorders?” he asked. “Does light activate the same regions, and if so, are these regions more or less sensitive to light activation? What is the magnitude of the difference in effect? This is an area of ongoing investigation,” he said, adding that the answers could inform development therapeutic methods of treatment of mood disorders.
Michael Worden of Brown’s Department of Neuroscience and the Carney Brain Science Institute also contributed to this study, as did researchers at the Hebrew University of Jerusalem.
The research was funded by the National Institutes of Health (R01EY12793, P20GM103645, S10OD025181), an Alcon Research Institute Award, the Department of Biology and Medicine at Brown University, the Israel National Institute of Psychobiology, and a Bunting Graduate Fellowship in Canada.