Lost memories have been retrieved with the help of light in a study that could further the understanding and treatment of patients with severe amnesia following injury or disease.

A study on laboratory mice has found that a lost memory is still stored within the brain cells even though the animals had apparently lost their ability to retrieve it, scientists said.

Scientists have long debated the nature of amnesia, with many researchers suggesting that the problem is due to the storage of memory rather than the inability to retrieve it from long-term memory banks of the brain.

In a series of experiments on mice involving a technique called optogenetics – when light is used to activate specific nerve cells in the brain – the researchers found that it was possible for the mice to remember a memory that had previously been lost.

“The majority of researchers have favoured the storage theory, but we have shown in this [study] that this majority theory is probably wrong. Amnesia is a problem of retrieval impairment,” said Susumu Tonegawa of the Massachusetts Institute of Technology in Cambridge, Massachusetts.

A condition known as retrograde amnesia, when memory loss occurs after traumatic injury, stress or diseases such as Alzheimer’s, is probably caused by damage to the brain cells involving memory retrieval rather than memory storage, he said.