During the total eclipse, when the Moon slid in front of the Sun, casting large swathes of the United States into darkness, many who observed the incredible astronomical event noticed another strange effect.

Despite the fact that the Sun was completely blotted out, creating darkness equivalent to that of night time, many people who witnessed the eclipse could still see people and objects with some clarity. Researchers have now figured out exactly what was going on in their eyes, explaining how the eclipse managed to give them “superhuman” night vision.

It all comes down to a certain receptor that covers particular cells in your retina. The receptor responds to a neurotransmitter known as gamma-aminobutyric acid or "GABA" for short. This molecule is responsible for communication between cells, particularly when sending information around the brain.

Reporting in the journal Current Biology, the researchers suggested how the activity of the GABA receptor in the retina could be behind people's enhanced ability to see in the dark during the total solar eclipse.

During sunny days, the GABA receptor is generously represented in the retina, helping us view the edges of objects and refining our ability to see in detail. But as the Sun starts to set, and light slowly ebbs away, the receptor also starts to disappear from the retina. By the time it's pitch black, it is gone. However, when light is lost suddenly like it was in the eclipse, the GABA receptor sticks around. 

The researchers carried out further experiments on rabbits in the laboratory and found that in the process of build-up and fallback of the GABA receptor, the neurotransmitter dopamine also plays a role. Dopamine enhances the receptor's ability to detect GABA in the first place.

“On bright days, dopamine levels are high and signaling is strong, enhancing the detection of spatial details and edges,” explained Ohio State University’s Stuart Mangel, in a statement. “On moonless nights, however, dopamine levels are low and the GABA signal is minimal, decreasing our ability to see those details.”

Yet when the US was plunged into almost complete darkness within just a few minutes, the cells in people's retinas were still covered by the GABA receptor and dopamine levels were still high. This allowed onlookers to pick out the detail and edges of the people and things surrounding them, in effect enhancing their vision.