It is well known that light has a significant effect on mood: “If you’re looking for a new apartment, you go for the one with the nice big windows and not the basement with only artificial light,” Professor of Neuroscience David Berson said.
A new study by University researchers now sheds light on the brain pathway responsible for this behavior. Their paper, published in July, suggests that light exposure suppresses activity in parts of the prefrontal cortex, an area of the brain associated with mood and cognitive processing.
The researchers’ findings may have implications for the study of depressive disorders, though future work is still being explored.
Berson’s interest in this pathway started nearly 20 years ago, when he discovered a new type of retinal ganglion cell — a cell that transmits information about light in the retina. He found that unlike most retinal ganglion cells, this new class of cells had pigment. Since then, he has been on a quest to find where these neurons project to and what these projections mean.
For this study, the researchers built off previous work that demonstrated a pathway from the retina that conveys light intensity to areas of the brain that don’t typically carry out visual functions, Professor of Neuroscience Jerome Sanes, an author of the paper, wrote in an email to The Herald.
“We know that light is implicated in mood disorders both in humans and animals, and we know that there are pathways from the retina of the eye to the prefrontal cortex in experimental animals,” said Assistant Professor of Neuroscience Michael S. Worden, another author of the paper. “But we have never seen those pathways in humans, so we were interested to know whether or not we could find evidence for those neural pathways in humans through this experiment.”
Some work suggests that this pathway plays a role in regulating behavior, Sanes wrote, citing a study of the phenomenon in mice.
“Mice were very sensitive to having their day-night cycle warped by putting them in a housing situation where they have three-and-a-half-hour-long days and three-and-a-half-hour-long nights,” Berson explained. “After a couple of weeks, they showed signs of depression.”
The researchers hypothesized that this same pathway exists in humans. Using functional MRI, they were able to test this hypothesis by looking at global activity in the human brain in response to changes to light in the environment, Berson said. Participants were exposed to four different light intensities ranging from nearly dark to moderately bright.
In this way, the “stimuli were devoid of any kind of color or form information,” Worden said. “The participants were wearing teflon goggles, so all they could see was how bright or how dark it was, but they couldn't see any kind of images.”
By measuring the blood oxygen levels of the brain, fMRI allowed the researchers to see which brain areas were activated during light exposure, Worden said.
While the researchers found responses to changes in light intensity across the brain, these responses were temporary in areas of the brain traditionally considered to process visual information. But the changes were sustained in other areas of the brain, including the prefrontal cortex.
“As the light got brighter, the sustained brain activation in prefrontal regions went down … in regions that have been previously associated with mood and cognitive processing,” Worden said.
“The paper makes an exciting discovery about brain areas that are influenced by light and that are independent of the conventional pathways for conscious visual perception,” Felice Dunn, associate professor in the department of ophthalmology at the University of California, San Francisco, wrote in an email to The Herald.
According to Dunn, who has previously worked with Berson, these findings identify regions in the prefrontal cortex that may participate in the maintenance and regulation of circadian rhythms, learning, mood regulation and other brain functions influenced by light.
“Some of the frontal cortical areas that tracked light intensity are the same areas that show different responses in patients with mood disorders,” Sanes wrote.
As a follow up to this study, the researchers are studying how changes in light levels affect patients with various depressive disorders, such as major depression or seasonal affective disorder,” Worden added.
“From a clinical standpoint, it's too early to tell how the results will translate to our understanding and treatment of seasonal affective disorder (SAD) and non-seasonal depression, but (the researchers) have definitely been talking about next steps in this line of research to better understand the clinical implications,” Professor of Psychiatry and Human Behavior Linda Carpenter wrote in an email to The Herald.