Meaning Humans perceive changes in ambient lighting, luxotonic properties , unrelated to the vision of shapes, and these changes influence a wide range of functions, including circadian rhythms , visual reflexes, mood, and probable cognitive processing. While imaging pathways in the primate brain detect minute changes in illumination, it is unclear how light intensity signals reach and are processed in the brain structures involved in basic mood states and their response. dysfunction, pathways that likely arise from intrinsically photosensitive retinal ganglion cells . Here we show that prefrontal regions of the human brain have luxotonic signals . These signals have properties similar to intrinsically photosensitive retinal ganglion cells, and may underlie the effects of light intensity on complex behaviors. Summary Experimental animal studies have revealed a mood-regulating neural pathway linking intrinsically photosensitive retinal ganglion cells (ipRGCs) and the prefrontal cortex (PFC), implicated in the pathophysiology of mood disorders. Since humans also have ipRGCs that encode light intensity , we wondered if a similar pathway exists in humans. Here, fMRI was used to identify regions of PFC and other areas that exhibit light intensity-dependent signals. We report 26 human brain regions that have activation that monotonically decreases or increases with light intensity. Luxotonic- related activation occurred in the cerebral cortex, various subcortical structures, and the cerebellum, which encompasses regions with functions related to visual image formation, motor control, cognition, and emotion. Light suppressed PFC activation, which monotonically decreased with increasing light intensity. The sustained time course of light-evoked PFC responses and their susceptibility to prior light exposure resembled those of ipRGCs. These findings offer a functional link between light exposure and PFC-mediated cognitive and affective phenomena. |
Comments
Researchers discover brain pathway that helps explain the effect of light on mood
From changes in daylight throughout the seasons to artificial lighting options in workplaces, it’s clear that the amount and quality of light a person encounters can significantly affect mood. Now, scientists at Brown University think they know why.
In a new study published in the Proceedings of the National Academy of Science , 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 show sensitivity to light intensity.
The discovery has implications for understanding mood problems such as seasonal affective disorder and major depressive disorders , as well as how to treat them, said the study’s senior author, Jerome Sanes, a Brown neuroscience professor affiliated with the Carney Institute for Brain Science University.
“Identifying this pathway and understanding its function could directly promote the development of approaches to treat depression, whether through pharmacological manipulations or non-invasive brain stimulation at select nodes of the pathway or with targeted bright light therapy,” Sanes said.
The findings build on previous research by study co-author David Berson, a Brown neuroscience professor, 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 what is known as "object vision" or "shape vision ," Sanes said, but instead function primarily to detect intensity . of the light .
Previous research, some of it by Berson, found that some animals have a mood-regulating neural pathway that links these photosensitive retinal cells to areas of the prefrontal cortex involved in mood disorders. Sanes said the new study was designed to determine whether a similar pathway existed in humans and whether they could find evidence that the pathway had a functional similarity to light-sensitive retinal ganglion cells.
To determine whether a light intensity coding pathway modulates the human prefrontal cortex, the researchers used functional magnetic resonance imaging to explore 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 light and eliminated visual shapes, colors, and other objects from the environment. Participants saw light intensities ranging from dark to bright, for 30 seconds each. To keep them alert, they simultaneously performed an auditory task that required them to point out the difference between two tones.
By evaluating fMRI images taken during exercise, the researchers identified 26 regions of the human brain where activity decreased or increased according to light intensity. This "luxotonic-related activation" occurred in the cerebral cortex, various subcortical structures, and the cerebellum, which encompasses regions with functions related to visual image formation, motor control, cognition, and emotion.
They found that light suppressed activity in the prefrontal cortex in proportion to the intensity of the light. Light-evoked responses in the prefrontal cortex and their alteration by prior light exposure resembled responses of intrinsically photosensitive retinal ganglion cells.
It is well known that changes in ambient lighting that do not necessarily have anything to do with the shape or vision of the object influence several basic functions, such as circadian rhythms, visual reflexes, mood and probable cognitive processing, Sanes said. However, it was not clear how these light intensity signals reached the relevant areas of the human brain.
In this study, the researchers showed that prefrontal regions of the human brain have light-sensitive signals, and that these signals are similar to intrinsically photosensitive retinal ganglion cells, which together, Sanes said, can explain the effects of Light intensity on complex emotional emotions and cognitive behaviors.
“The findings of our study offer a functional link between light exposure and cognitive and affective responses mediated by the prefrontal cortex,” Sanes said.
A logical next question, Sanes said, concerns how light affects these same pathways and regions of the brain in people with mood disorders such as seasonal affective disorder or major depressive disorders.
"How does that compare to a control group of healthy people who have not been diagnosed with these disorders?" she 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 research,” she said, adding that the answers could inform the development of therapeutic treatments for mood disorders.
Michael Worden of Brown’s Department of Neuroscience and the Carney Institute for Brain Science also contributed to this research, 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 Division of Biology and Medicine at Brown University, the Israel National Institute of Psychobiology, and a Banting Postdoctoral Fellowship from Canada.
