Mouse retinal section: cell nuclei labelled in blue; RNA for the GABA synthesis enzyme Gad2 magenta; and RNA for melanopsin green.

Researchers from Northwestern University in Illinois have found that a subset of retinal neurons sends inhibitory signals to the brain, contradicting the belief that the eye only sends excitatory signals.

This contradictory subset of retinal neurons is involved in subconscious behaviours, such as synchronizing circadian rhythms to light/dark cycles and causing our pupils to constrict in response to intense bright lights, said assistant professor of neurobiology Tiffany Schmidt who led the research.

“This suggests that there is a signal from the eye that actively inhibits circadian rhythms realignment when environmental light changes, which was unexpected. This makes some sense, however, because you do not want to adjust your body’s entire clock for minor perturbations in the environmental light/dark cycle, you only want this massive adjustment to take place if the change in lighting is robust.

“We think that our results provide a mechanism for understanding why our eye is so exquisitely sensitive to light, but our subconscious behaviours are comparatively insensitive to light.”

By better understanding how these neurons function, researchers can explore new pathways by which light influences our behaviour, she said.