Memories that have been lost as a result of amnesia can be retrieved by activating brain cells with light, a study suggested.

The findings, published in the US journal Science on Thursday, may help answer a fiercely-debated question in neuroscience as to whether memories lost to amnesia are completely erased or merely unable to be recalled, Xinhua reported.

"The majority of researchers have favoured the storage theory, but we have shown in this paper that this majority theory is probably wrong," Susumu Tonegawa, a professor at the Massachusetts Institute of Technology, said.

"Amnesia is a problem of retrieval impairment."

In the study, mice were first trained to associate a mild foot shock with a specific environment, known as chamber A, eliciting a typical "freezing" behaviour. Eventually, trained mice would "freeze" in chamber A even without the shock.

Brain cells activated during memory formation were genetically labelled with a blue light-sensitive protein to allow their visualisation and re-activation.

Some mice were then given a chemical called anisomycin to induce retrograde amnesia, which follows traumatic brain injury, stress, or diseases such as Alzheimer's in humans.

Other mice received saline as a control.

As expected, amnesiac mice did not "freeze" after returning to chamber A, indicating that they could not recall the memory for the specific association of the chamber and the mild foot shock.

Next, the mice were put in a novel, neutral environment called chamber B and a technology involving using blue light pulses was used to selectively activate brain cells that were genetically labelled during their training in chamber A.

When the cells, collectively called a "memory engram", were activated, the amnesiac mice froze again, just as the control mice, the researchers said.

"Our conclusion is that in retrograde amnesia, past memories may not be erased, but could simply be lost and inaccessible for recall," said Tonegawa.

"These findings provide striking insight into the fleeting nature of memories, and will stimulate future research on the biology of memory and its clinical restoration," he added.