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How mammals perceive light changes between night and day. Researchers at Aalto University and the University of Helsinki discovered that mice were better at finding a dim light in pitch-darkness in experiments done at night time compared to ones done during the day time. The scientists were surprised to find that this effect had very little to do with any changes in the eye itself, and was instead controlled by how the mice actually searched for light in the dark, and how their brains processed signals at night vs. day. The results are exciting for neuroscientists interested in how animals and humans can see, and biologists interested in how the time of day alters our bodies, including sensory processing.

Many types of animals behave differently during day and night time. While the “body clock” has long been known, its effect on how different parts of the body function still aren’t fully understood. For instance, researchers knew that the retina, the part of the eye that detects light, has its own circadian rhythm. The team at Aalto and Helsinki Universities were interested in seeing if the eye’s internal clock affected vision, so they modified one of their previous experiments to find out.

‘Our research group is able to link if a mouse can find a dim light in the dark to the mouse’s underlying retinal nerve signals at the sensitivity limit of vision.’ said Professor Petri Ala-Laurila, the research group leader. ‘This allowed us to explore how the day/night cycle changes the mouse’s visual capability, both down in the neural circuit level and all the way up to behaviour responses at the sensitivity limit of vision’

Finding your way in a pitch-dark maze

Earlier this year, Professor Ala-Laurila’s group demonstrated how the eyes of mice detect faint light in near-total darkness. This allowed them to link the mammalian visually-guided behaviour to individual neural impulses, an important world-first for neuroscience. The experiment involved placing a mouse in a maze in total darkness, with a faint light next to the exit of the maze. The mouse is trained to know that the light leads to the way out. They repeated this experiment for this new study, doing some of the tests during the day, and some during the night. They observed that the behaviour change – the mice were better at spotting the light at night than during the day. They were expecting this result, but they also observed that the nerve impulses leaving from the retina themselves did not cause the difference, which was a surprise.