Researchers have finally found a specific region in the brain — medial temporal lobe — that plays a key role in rapidly forming memories about everyday events.
The medial temporal lobe is associated with episodic memory — the brain’s ability to recall situations like meeting a friend in, say, a mall. Episodic memory logs such happenings and very rapidly forms new associations in the brain.
People suffering from Alzheimer’s and other neurological disorders have this type of memory affected. Hence, the results of a study published today (July 2) in the journal Neuron has great clinical significance.
The study also found that firing of a single neuron led to new memories being formed in the human brain. Till date, the role of a single neuron in memory formation has been studied only in animals. And only limited information of episodic memory formation in animals has come out owing to the nature of such studies. As a result, it has remained unclear how individual neurons enable the rapid encoding of new episodic memories in natural settings.
To study the single-neuron underpinnings of episodic memory formation, Matias J. Ison, the lead author from the University of Leicester, U.S., studied the activity of more than 600 neurons in the medial temporal lobe of 14 epileptic patients who had electrodes implanted in the brain region of interest.
The experiment was carried out in two stages. In the first stage, the patients were shown images of people — family members and famous personalities — and places like the Eiffel Tower and the White House. This was done to analyse the encoding activity of the individual neurons in the medial temporal lobe as the images were registered by the patients.
In the second phase, the patients were shown composite pictures of the same people in front of the landmarks shown before. The use of composite pictures was done to “mimic the experience of meeting an individual in a particular place.” The activity of individual neurons that were previously studied was followed while the patients learned the contextual associations.
The neurons that were activated when photos of people (but not landmarks) were shown in the first phase of the experiment started abruptly firing when composite photos were shown.
In effect, the volunteers (patients) had new memories formed when they learnt the association between each person and the landmark. “Our results provide a plausible neural substrate for the inception of associations, which are crucial for the formation of episodic memories,” the authors write.
The neurons changed the firing properties right at the moment when the patients formed new memories of contextual associations and after they were shown the composite picture just once.
“This study looks at the single neuron correlates of the learning of new contextual associations in the human brain, and we were able to show for the first time that the speed at which complex associations are encoded is compatible with the basic mechanisms of episodic memory creation,” Dr. Ison said in a press release. “We had hypothesized that we'd be able to see some changes in the firing of the neurons. These changes were dramatic, in the sense of neurons being very silent or very active, and that it occurred at the exact moment of learning.”