Scientists retrieve 'lost' memories using light

WASHINGTON: Memories that are 'lost' as a result of traumatic injury, stress or diseases such as Alzheimer's can be recalled by activating brain cells with light, MIT scientists, including one of Indian-origin, have found.

Researchers were able to reactivate memories that could not otherwise be retrieved, using a technology known as optogenetics.

The finding answers a fiercely debated question in neuroscience as to the nature of amnesia, according to Susumu Tonegawa, Professor in Massachusetts Institute of Technology (MIT)'s Department of Biology.

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.

Researchers have previously speculated that somewhere in the brain network is a population of neurons that are activated during the process of acquiring a memory, causing enduring physical or chemical changes.

If these groups of neurons are subsequently reactivated by a trigger such as a particular sight or smell, for example, the entire memory is recalled. These neurons are known as "memory engram cells."

Tonegawa's group, including lead authors Tomas Ryan, Dheeraj Roy, and Michelle Pignatelli, used optogenetics -- in which proteins are added to neurons to allow them to be activated with light - to demonstrate for the first time that such a population of neurons does indeed exist in an area of the brain called the hippocampus.

However, until now no one has been able to show that these groups of neurons do undergo enduring chemical changes, in a process known as memory consolidation.

One such change, known as "long-term potentiation" (LTP), involves the strengthening of synapses, the structures that allow groups of neurons to send signals to each other, as a result of learning and experience.

To find out if these chemical changes do indeed take place, the researchers first identified a group of engram cells in the hippocampus that, when activated using optogenetic tools, were able to express a memory.

When they then recorded the activity of this particular group of cells, they found that the synapses connecting them had been strengthened.

"We were able to demonstrate for the first time that these specific cells -- a small group of cells in the hippocampus -- had undergone this augmentation of synaptic strength," said Tonegawa.

The study was published in the journal Science.