Memories lost due to traumatic brain injury, stress, or diseases such as Alzheimer’s may be retrieved by activating brain cells with light, shows a promising study by Massachusetts Institute of Technology (MIT) researchers, including one of Indian-origin. The findings by Mr Dheeraj Roy and colleagues may help answer a fiercely-debated question in neuroscience.

Neuroscience researchers have for many years debated whether retrograde amnesia—which follows traumatic injury, stress, or diseases such as Alzheimer’s—is caused by damage to specific brain cells, meaning a memory cannot be stored, or if access to that memory is somehow blocked, preventing its recall. “The majority of researchers have favoured the storage theory, but we have shown in this paper that this majority theory is probably wrong,” said Mr Susumu Tonegawa, professor at the Massachusetts Institute of Technology.

“Amnesia is a problem of retrieval impairment,” Mr Tonegawa said. 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 labeled 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. 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,” said Mr Tonegawa. The study was published in the journal Science.