How does light affect the human brain

How we perceive light beyond sight

The human eye perceives light stimuli through its rods and cones and supplies the brain with information: about light and dark and about colors. But there are other cells in our retina that are sensitive to light and play a crucial role in calibrating our internal clock. Researchers have now taken a closer look at these special ganglion cells. So we have at least three different types of these light sensors. They could play an important role in our sleep-wake cycle - and explain why some blind people literally feel light.

Sunlight is of crucial importance for our health: It plays an important role as a clock for the human body clock, stimulates the production of vitamins and even has a positive effect on our thinking skills and our psyche. But artificial light, office work, night shifts and other phenomena in modern society mean that the natural influence of daylight is less and less important. "We have now become an indoor species that is increasingly withdrawing from the natural cycle of daylight during the day and almost total darkness at night," says Satchidanada Panda from the Salk Institute for Biological Studies in La Jolla. With noticeable consequences: disturbances in the normal day-night rhythm can promote sleep problems, obesity and even diseases such as cancer and depression.

To counteract these problems, researchers want to better understand how our eyes and brain perceive light. Because these findings could enable the development of “intelligent” lighting that contributes to maintaining a healthy circadian rhythm. It is known that in addition to rod and cone cells, certain ganglion cells in the retina are sensitive to light. In mice, these cells play a role in the day-night cycle and control sleep, mood and alertness, among other things. "In humans, however, the functioning of these cells has never been examined in more detail," explain Panda and his colleagues around first author Ludovic Mure.

Sensitive to blue

This is exactly what the scientists have now made up for. For their study, they used retinal donations from deceased people who had been kept healthy and functional outside of the body using a special method. Mures' team then used these samples to examine how the ganglion cells of the retina respond to light stimuli. They discovered that a small group of these cells actually start to fire when exposed to light stimuli. These special ganglion cells are particularly sensitive to light in the blue spectrum. This range of visible light is typical of daylight - and often shines on us from the screens of our laptops and smartphones.

Further experiments revealed that the light-sensitive ganglion cells in our retina can be assigned to at least three different types. These types react differently to light stimuli, stay longer or shorter “switched on” or only become active at certain brightness levels. The researchers also found that these light sensors are not only connected to the brain. In some cases, they also work closely with the other cells of the retina. In this way, they can add additional information about contrast and brightness to the images conveyed by the rods and cones.

Blind people feel light

As Mure and his colleagues report, the sensitivity of the ganglion cells to light could also explain a phenomenon known to some blind people: Despite non-functioning cones and rods and thus de facto no vision, the body clock of the person affected adapts to the natural rhythm of light and dark . So these people have to feel the light somehow. The light-sensitive ganglion cells in the retina, which transmit the corresponding signals to the brain, could be responsible for this - without creating a conscious visual impression.

In the future, the scientists want to investigate more closely how the ganglion cells react to different light influences and whether they work differently in young people than in older people. In the long term, they hope that this will lead to new approaches for the development of lights for offices and hospitals, as well as therapeutic lamps for depression, migraines and other ailments. “It could also improve the screens of televisions, computers and smartphones,” concludes Panda.

Source: Ludovic Mure (Salk Institute for Biological Studies, La Jolla) et al., Science, doi: 10.1126 / science.aaz0898

5th December 2019

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