Lighting in Design Q1 2019

N ews

Study identifies brain area responsible for mood changes from light

Researchers at the National Institute of Mental Health (NIMH) in the US have proved what has been common knowledge among lighting design- ers for years: light is important for more than just vision, and directly impacts mood and learning. In a new study, the researchers traced the brain pathways responsible for the effects of light on learning and mood. The findings revealed that these effects are brought about by two different and distinct pathways from the retina into the brain. One of these pathways includes a previously unrecognised cluster of neurons in the thalamus, a part of the brain responsible for relaying sensory information to other brain areas. Samer Hattar, the author of the study and chief of the Section on Light and Circadian Rhythm at the NIMH, says that the research finally assigns a brain region where light can have an impact on mood, providing evidence for the positive effects of light therapy in treating depression in humans. It also challenges current thinking that changes in mood associated with alterations in day length preceded learning problems. In 2012, researchers discovered that the effects of light on learning and mood begin in the retina with a group of cells called intrinsically photosen- sitive retinal ganglion cells (ipRGCs). These are naturally sensitive to light, and use neural connec- tions to provide information used in non-image- forming visual functions such as those involved in circadian photoentrainment, which is the training of an internal body clock and biological processes to correspond with light and dark periods, as well as sleep regulation. However, nothing was known about the brain pathways through which these effects occur – until now. The researchers used mice to examine the effects of ipRGCs on a part of the brain called

the suprachiasmatic nucleus (SCN), which houses a central pacemaker that co-ordinates circadian- related functions across the body. The mice were genetically altered so that the ipRGCs that stimu- lated the SCN survived, but those that projected to places outside the SCN did not. The mice were exposed to normal light and dark cycles or to abnormal light and dark cycles. The mice and an unaltered control group, were then tested on several tasks that model learning and cognitive functions and mood. When the researchers looked at learning and cognition in the mice, they found that both the unal- tered mice and the genetically altered mice showed normal behaviours in the normal light condition and abnormal behaviours in the abnormal light condi- tion.These findings suggest that the pathways that lead from ipRGCs to the SCN are sufficient for light to impact learning and cognition. When the mood responses in the mice were examined, they found that both unaltered mice and the genetically altered mice showed no differ- ences when exposed to the normal light condition. However, when these mice were exposed to the abnormal light condition, the unaltered animals showed altered mood responses, whereas the genetically altered mice did not. These findings suggest that pathways from the ipRGC to areas outside of the SCN are required for driving light- based regulation of mood. The researchers also found that the SCN con- nects to parts of the prefrontal cortex involved in mood regulation, and to other brain areas involved in affective-emotional processing. Remarkably, chroni- cally activating the SCN in animals living under nor- mal light conditions inducedmood alterations, while inhibiting this region in animals living under abnormal light conditions blunted the mood changes.

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LiD Q1 - 2019