A well-lit street or a dark alleyway? Most of us would prefer the street with a sufficient amount of light and not venture into the unknown of the dark alleyway. But why is that?
Light plays an important role in human physiology. From the regulation of our internal circadian clock to influencing our mood and emotional regulation. Although the exact mechanism of how light influences mood and emotional regulation in humans remains unclear, findings in rodents suggest that light may have an effect on the fear and emotional processing center of the brain, an area called the amygdala.
Researchers from Monash University and Australian Catholic University wanted to learn more about this and so devised a human experiment using Functional magnetic resonance imaging (fMRI) to investigate how light versus dark influences amygdala activity. Publishing their findings in the open-access journal PLOS ONE, they found that moderate light suppressed the activity of the amygdala more so than dim light and hence could provide an explanation to why the dark might be causing us more fear.
The study involved 24 participants that underwent fMRI while being exposed to moderate light (100 lux) or dim light (10 lux). fMRI allows researchers to see activated brain areas in real-time by looking at changes in blood flow in the brain.
The researchers found that when the participates were exposed to moderate light, their amygdala activity on fMRI was lower compared to when they were exposed to dim light.
Furthermore, the amygdala is also connected to a region of the brain called the Ventromedial prefrontal cortex (vmPFC) that is involved in the higher-order processing of risk, emotional responses, and fear. This brain region plays a critical role in the regulation of amygdala activity and is especially important in the suppression of emotional responses. The authors of the latest study also found that not only does light directly suppress the activity of the amygdala, it also seems to increase the connectivity between the amygdala and the vmPFC which may explain how light benefits emotional regulation.
"Light is an effective therapeutic tool for mood problems. We have shown that dim-to-moderate light suppresses amygdala activation and enhances amygdala-vmPFC connectivity. These effects may contribute directly to the mood-elevating effects of light via improved emotional processing, and a reduction in fear-related emotion," the authors wrote in their paper.
More studies will have to be done to elucidate the exact mechanism by which light suppresses amygdala activity and increases connectivity to the vmPFC, but this work seems to demonstrate a reason why humans are more fearful in the dark than in the light.