Three Cell Types in the Eye that Help the Brain Differentiate Day from Night Discovered
Thu, April 22, 2021

Three Cell Types in the Eye that Help the Brain Differentiate Day from Night Discovered

Recently, credible researchers have discovered optical cells that may distinguish day and night / Photo Credit: Javier Sanchez Mingorance via 123rf


A team of researchers from nonprofit Salk Institute for Biological Studies in California, University of Utah’s John A. Moran Eye Center, and The Scripps Research Institute’s Department of Molecular Medicine has discovered the three cell types in the human eyes that help our brain tell the difference between day and night. 

Intrinsically photosensitive retinal ganglion cells (ipRGCs)

Their study published in the journal Science refer to these cells as the intrinsically photosensitive retinal ganglion cells (ipRGCs). They said that ipRGCs have been recognized before in the retinas of mice but never reported to exist in humans until their discovery. For their research, they developed a new method to keep the retina samples functional and healthy even after the donors’ death. They then used an electrode grip to place the retina samples and examine how they reacted to the light.

They discovered that a small group of cells started just after the 30-second pulse of light. By pulse of light, they mean an optical pulse or a flash of light. After the light was turned off, the cells also took a few seconds before they stopped firing. This led the research team to test various colors of light. They discovered that the intrinsically photosensitive cells were sensitive to blue light. In the visible light spectrum, blue light rays have the shortest wavelengths and the highest energy. This type of light is popular in LED lights and our devices, such as laptops and smartphones.


Study shows that discovery of the 3 types of ipRGCs can help people design better digital gadgets / Photo Credit: Javier Sanchez Mingorance via 123rf


The discovery of the three types of ipRGCs

Their follow-up experiments likewise revealed the three types of ipRGCs. Type 1 ipRGCs responded relatively quickly to light although it took a long time to turn off. Type 2 ipRGCs took longer to turn on and off. On the other hand, Type 3 ipRGCs responded only when the light was very bright but turned on faster. The type 3 also switched off faster the moment the light was turned off.

Ludovic S. Mure and the team said that understanding the three types of ipRGCs in the human eye as well as their respective functions may help other researchers design better therapeutics and lighting that can help turn the cell activity off and on.

Their findings also explain the phenomenon discovered in past studies involving blind people that, despite not being blind, could still alight their cardiac rhythms or sleep-wake cycle to the day and night cycle of people who can see. Thus, blind people may have ways of sensing light. Now, the Salk Institute study suggests that ipRGCs are the cells found in the eyes that send the light signal to the brain even if there are individuals who lack the cone and rod cells needed to pass on the contrast information and brightness to what the eyes see.



Other purposes of their findings

The Salk Institute researchers also said via medical research platform Medical Xpress that their discovery of the three types of ipRGCs help people design better smartphone screens, computer monitors, and televisions. For example, inventors may alter the proportion of the blue light coming from the screens to trick the brain into thinking that it is seeing a dim or bright image.

Co-author Satchidananda Panda states that the next step of their study is to research the net output of the three cells under different colors of light, duration, and intensity. For instance, they will compare how the cells react to the short light pulses compared to a longer duration in a few minutes. The group is likewise interested to know how the three cells will react to light sequences, such as a blue light that turns orange and vice versa. This is meant to mimic other lights we see in nature at dusk or dawn.

The researchers went on to say that repeating their experiments in the donor retina preparations of different ages will further help them understand to what extent older and young individuals differ in their function of ipRGCs. Consequently, it can help design better indoor light for better night and day synchronization of the body's circadian rhythms. It can probably also help others design applications of light for mood improvement, which will be particularly helpful for patients with dementia or who are older.



Facts about the biological clock and eyes

The circadian rhythms are the behavioral, mental, and physical changes that follow a 24-hour cycle and respond primarily to darkness and light in the environment. Medical and health information provider OnHealth shares that the body clocks of newborn babies induce them to sleep an approximate 16 to 20 hours every day. Then, their need for sleep will decrease to 11 to 12 hours per day. Teenagers, on the other hand, approximately need 9 to 10 hours of sleep daily. An adult may feel well-rested by getting 7 to 9 hours of sleep every night. 

Meanwhile, Southwestern Eye Center shares that the human eye can differentiate roughly one million different colors.