Saving your memories save your brain

The brain’s ability to preserve memories lies at the heart of our human experience and also of our identity. This is why forgetting is one of our most dangerous enemies. Diseases like Alzheimer delete our identity and make our life impossible. But we don’t have to get some of these debilitating illnesses to have problems. We are our memories and anytime we are not able to remember some experiences we lived in the past we feel like losing parts of our life. Recent findings show the fate of memories and also suggest the possible role of psychotherapy. And more…

But where in the brain do those old memories go? Despite decades studying how the brain transforms memories over time, neuroscientists remain divided over the answer. This month, researchers from Johns Hopkins University have come up with a new theory that might clear up this controversy. They claim that what we do with a memory determines where it is stored in the brain. Their theory, called Competitive Trace Theory (or CTT), suggests that what really matters is how often we revisit the memory. Memories they say are transformed each time we revisit them. A memory is first encoded by the activity of neurons from one part of the brain called the hippocampus. The hippocampus acts like as the brain’s director, telling the cortex which particular neurons to activate (see in the bellow image the structures presented in green – hippocampus and cortex). Each time we recall that memory, a similar but not identical set of neurons are activated. Neurons that are frequently activated became part of the permanent memory trace in the cortex, while the rarely activated ones are lost. Every reactivation re-encodes the memory, and depending on what cortical neurons are engaged, can strengthen, weaken or up-date particular memory features. With each memory reactivation, some features are reinforced while others disappear, explaining why the memory seems to get fuzzy over time. And the more details that are lost, the less “episodic” and the more “semantic” the memory becomes, explaining the sense of personal detachment often associated with very old memories. As memories get older, they are decontextualized due to competition among partially overlapping traces and become more semantic and reliant on cortex storage. Consolidation that leads to the strengthening of memories enhances conceptual knowledge (and becoming semantic memories) at the expense of vivid contextual details (or episodic memories).

Therapists working with traumatic patients know this very well – there is a clear distinction between recent traumas and distant traumas. Actually psychotherapy helps in this way : when a memory is recalled often it will more rapidly become stored in the cortex, become less episodic and independent of the hippocampus while a memory that’s rarely revisited will remain dependent on the hippocampus. As a result, remote memories are more likely to have a stronger semantic representation but also to be less vivid and more likely to include illusory details. And this is how it can incorporate a new perspective, a less emotional and more rational one, built by the joined efforts of the patient and the therapist.

Above I pointed out that each time we recall a memory a  set of neurons are activated, and neurons that are frequently activated became part of the permanent memory trace in the cortex, while the rarely activated ones are lost. In a recently published review, I present evidence that learning based on hippocampus seems to trigger the DNA repair pathways inside those hippocampal neurons, if the subject is re-exposed to the previously learnt information. So every time we encounter (or generally recall) some previously experienced contextual memories (a memory linked with a specific situation) the neurons involved in that memories seems to repair themselves and become healthier. And hence live longer. The new theory described above suggests that this process also save the memories stored in those neurons, the hippocampus activating the cortex and sending that data to it. Maybe the DNA repair process associated with this data transfer helps to clean up the hippocampus in order to be ready to learn something new. Something like a refresh process. Refresh that doesn’t happened in Alzheimer patients and other forms of dementia.

So revisiting your past can save your memories and also your brain.