The research is published in the Proceedings of the National Academy of Sciences.

The lead author of the study, Dr Jimo Borjigin, of the University of Michigan, said: "A lot of people thought that the brain after clinical death was inactive or hypoactive, with less activity than the waking state, and we show that is definitely not the case.

"If anything, it is much more active during the dying process than even the waking state."

Consciousness

However, studying this in humans is a challenge, and these visions are little understood.

To find out more, scientists at the University of Michigan monitored nine rats as they were dying.

In the 30-second period after the animal's hearts stopped beating, they measured a sharp increase in high-frequency brainwaves called gamma oscillations.

These pulses are one of the neuronal features that are thought to underpin consciousness in humans, especially when they help to "link" information from different parts of the brain.

In the rats, these electrical pulses were found at even higher levels just after the cardiac arrest than when animals were awake and well.

Dr Borjigin said it was feasible that the same thing would happen in the human brain, and that an elevated level of brain activity and consciousness could give rise to near-death visions.

"This can give us a framework to begin to explain these. The fact they see light perhaps indicates the visual cortex in the brain is highly activated - and we have evidence to suggest this might be the case, because we have seen increased gamma in area of the brain that is right on top of the visual cortex," she said.

"We have seen increased coupling between the lower-frequency waves and the gamma that has been shown to be a feature of visual awareness and visual sensation."

However, she said that to confirm the findings a study would have to be carried out on humans who have experienced clinical death and have been revived.

Commenting on the research, Dr Jason Braithwaite, of the University of Birmingham, said the phenomenon appeared to be the brain's "last hurrah".

"This is a very neat demonstration of an idea that's been around for a long time: that under certain unfamiliar and confusing circumstances - like near-death - the brain becomes overstimulated and hyperexcited," he said.