New kind of cell in the eye that perceives brightness discovered by researchers

These cells don't directly help us 'see' the world around us, their role is more in "sensing" light intensity.

Researchers in Japan are throwing light on the key role a newly-discovered light-sensitive cell plays in vision. Popularly referred to as 'melanopsin cells', they were identified less than two decades ago. Alongside their better-known siblings — rods and cones — melanopsins make up a small number of innately photosensitive cells in the body.

Researchers from Yokohama National University in Japan have helped describe the function and larger role that this new class of retinal cells plays in visual perception. While melanopsin cells, coined 'intrinsically photosensitive retinal ganglion cells', or ipRGCs, don't directly help us see the world around us, they do help us "sense" light. When they sense light, the cells produce melanopsin, a molecule that signals parts of the brain to stay awake and alert. Melanopsin also has a role to play in regulating and setting the body's circadian rhythm.

The study's findings, published in Scientific Reports, could help produce therapies that are most effective in treating diseases and complications relating to the eye. They can also serve as a basis for developing custom lighting and display systems, since screens and lighting affects melanopsin levels (and thus, the circadian rhythm) in the body.

The amount of light that enters the eyes is controlled by the dilating and contracting pupil — the coloured part of the eye.

The amount of light that enters the eyes is controlled by the dilating and contracting pupil — that coloured part of the eye.

"Until now, the role of retinal melanopsin cells and how they contribute to the perception of the brightness of light has been unclear," Katsunori Okajima, one of the authors of the study, from Yokohama National University, said in a statement.

The retina is a collection of cells that line the back wall of the eye. It is composed of various cell types (collectively referred to as photoreceptors), each of which responds differently to different amounts and wavelengths of light. 'Cones' process the most light, while 'rods' process lower levels of light. Till recently, researchers were under the assumption that rods and cones were the only cells that responded to light when it hit the retina. Unlike rods and cones though, ipRG cells have and produce melanopsin, a pigment that is sensitive to light. While ipRGCs are known to participate in syncing the brain's internal (or biological) clock with daylight and changes in light intensity, their role in detecting the amount of light isn't understood all too well.

The two-cell diagram of the kinds of photosensitive cells in the retina needs an update!

The two-cell diagram of the kinds of photosensitive cells in the retina needs an update!

According to Okajima, melanopsin cells play a crucial role in the ability to gauge how well-lit an environment really is. They also challenge the notion of a conventional two-variable system of light detection — namely, brightness and the amount of incoming light.

The study's results suggest that the perception of brightness should rely on a third variable — intensity. Specifically, the intensity of a stimulus targeting the melanopsin cells. Cones and melanopsin appear to work together for the perception of brightness, as per the study's findings.

To better understand the link between melanopsin and detection of light, signals of melanopsin were isolated from cones and rods. The researchers designed and placed visual stimuli carefully to stimulate melanopsin alone. Using tracking software, researchers measured how wide the pupils of the subjects diameters widened under each visual stimulus. The relationship between the perception of brightness and the actual intensity of an object on the retina could then be charted.

The researchers found that a melanopsin response when the intensity was varied. The response to this change, however, was generated by 'cones'. The results also suggest that melanopsin isn't a minor, but a major, contributor in the perception of brightness.

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