When we lose memories, do we lose them forever?

DEAR READER:

In yesterday’s column, I answered a reader’s question about what memories are. The reader asked: “When we lose memories, do we lose them forever?” Today, I want to return to that profoundly important question.

But first, a little reprise. Yesterday, I explained that specific memories are thought to reside inside a small group of nerve cells that “talk” to each other.

Each nerve cell has little projections that, microscopically, look like tiny trees with branches and twigs. The branches and (particularly) the twigs of one nerve cell send chemical signals that reach the branches and twigs of another nerve cell. That’s how nerve cells talk to each other. The “talk” requires the chemicals — and the twigs.

We used to think that when a person lost a memory, it probably reflected irreversible damage to the synapses involved in that memory. When, for example, a patient with Alzheimer’s disease forgets the name of his son, we assumed the connections between the nerve cells that store that name have been damaged beyond repair.

On reflection, there was a very good reason to have questioned that assumption. Anyone who has known people with Alzheimer’s disease knows that, at least early on, memories are fitful. A patient may forget his son’s name one day and remember it the next. Why, even some of us without Alzheimer’s disease will forget someone’s name one day and remember it the next. That all suggests a memory really is still there, but that we just have trouble retrieving it sometimes.

Some remarkable studies from the laboratory of Dr. Susumu Tonegawa, a Nobel Prize-winning scientist at MIT, indicate that memory loss may not be permanent.

Tonegawa’s team studied both normal mice and mice with a form of Alzheimer’s disease. The mice were placed in a cage that then delivered an unpleasant electric shock to their feet. A few hours late, the mice were placed back in the cage: They all froze in fear.

However, days later the mice were placed back in the same cage. The normal mice were fearful. In contrast, the mice with Alzheimer’s-like disease weren’t frightened: They had apparently lost the memory of the shock they had received in that cage.

The scientists had located where in the brains of the mice the memory of the shock was stored. They then used a technique called optogenetics to stimulate that area in the mice with Alzheimer’s-like disease. This time, those mice (like the normal mice) froze in fear when they were placed in the cage. The memory that had been “lost” had really been there all along, and the stimulation had retrieved it.

This research indicates that memories may not really be lost. The optogenetics technique that retrieved memory in mice could not be used in humans in its current form. However, the research offers hope that one day scientists will be able to figure out how to retrieve memories that are there, but inaccessible, in humans.

Don’t hold your breath; that day is almost surely a long way off. But if and when it comes, it will be an important milestone in human history.