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Neurobiology

Enhancing Memorization, Erasing Trauma

By discovering the brain structure that processes social information in humans and successfully erasing fearful memories in rodents, CNRS scientists are looking at how to enhance our brain performance.

Discovering ways to maximize our learning abilities or erase disturbing memories–in short, controlling the mind–are dreams (or nightmares) that have long inspired science-fiction novels. But if in reality we are still in the early stages of uncovering memory processes, recent interdisciplinary research involving cognitive psychology, neuropsychology, and neurobiology has resulted in significant discoveries.

Memory is now believed to be made up of several subsystems. Neuroscientists speak of different types of memory–semantic, episodic, or procedural–each one relying on distinct neuronal networks.

Among these subsystems, episodic memory can be defined as the process enabling individuals to remember past events (a birthday celebration or a trip abroad). When such events occur, the various types of information (visual, spatial, emotional, etc.) are first 'encoded'–transformed into memory–which is then stored long-term and can be retrieved.

Two recent studies involving CNRS researchers have further advanced our knowledge of two of these phases: encoding and retrieval. In the first study,1 researchers from McGill University in Montreal2 and CNRS researcher Philippe Fossati,3 identified the structure specialized in encoding social information–i.e., one that involves other human beings. “Just as we know that the amygdala is the structure specialized in dealing with emotions, we showed that the medial prefrontal cortex (mPFC) is specialized in treating social information,” says McGill researcher Philippe-Olivier Harvey.

 

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© P. O. Harvey

Regions of the prefrontal cortex responsible for encoding social information.


The team used functional magnetic resonance imaging (FMRI) to measure the candidates' brain activity. Volunteers were shown two sets of pictures: one displaying people, the other devoid of social content. Their recordings showed that the mPFC is specifically activated when observing social pictures. The images that triggered the most activity inside the mPFC at first sight were also the ones volunteers remembered in later experiments. “We are not sure why some pictures stimulated the mPFC more than others. It's clearly not linked to their emotional component or to the number of people in the pictures,” says Harvey. “It's possible that the volunteers felt stronger about the activities in those images by personal association.” In parallel, past studies have shown that the stronger the emotional and social content of an event, the better the chances of remembering it. “Acting directly on the amygdala and on the mPFC could possibly help enhance memorization,” suggests Harvey.

In the second study,4 CNRS researcher Valérie Doyère,5 working in collaboration with American scientists, managed to erase specific fear memories in rats by acting directly on the amygdala. “When memories are brought back to conscience, there is a period during which they are vulnerable and can be modified by pharmacological agents, before being stored again,” notes Doyère. The team trained rats to fear two sounds, which they associated to electric shocks. Then they administered an amnesia-causing drug inside the amygdala just before replaying one of the sounds. Their goal was to act on the memory of this sound in its vulnerable state, during memorization. When the researchers later replayed both sounds, rats no longer feared the one they had heard while on the drug. Electrophysiological recordings inside the amygdala confirmed the drop in emotional activity associated with this sound. “But the rats still feared the other sound, which shows that the erasure is extremely selective,” says Doyère. “It doesn't overlap with other stimuli, even if they are very similar.”

These findings could help manage post-traumatic stress disorder (PTSD) in human beings, an ailment in which recurring traumatic memories assail patients. According to Doyère, existing beta-blocking medications such as propranolol are currently being tested to evaluate whether they could erase specific stimuli. But there is still much to be learned. “There are many aspects to investigate before we can act on the mind. Traumatic events are made up of millions of bits of information, and for the moment it's inconceivable to act on all of them,” says Doyère. “But we might be able to improve PTSD condition by targeting specific fears.”

 

Clémentine Wallace

Notes :

1. P.O. Harvey et al., “Modulation of memory formation by stimulus content: Specific role of the medial prefrontal cortex in the successful encoding of social pictures,” J Cogn Neurosci. 19: 351-62. 2007.
2. The Douglas Mental Health University Institute (McGill University).
3. Laboratoire de Vulnérabilité, Adaptation et Psychologie (CNRS / Université Paris-VI).
4. V. Doyère et al., “Synapse-specific reconsolidation of distinct fear memories in the lateral amygdala,” Nature Neuroscience. 10: 414-6. 2007.
5. Laboratoire de neurobiologie de l'apprentissage, de la mémoire et de la communication (CNRS / Université Paris Sud).

Contacts :

> Philippe-Olivier Harvey
McGill University, Montreal.
philippe-olivier.harvey@douglas.mcgill
> Valerie Doyère NAMC, Orsay.
valerie.doyere@u-psud.fr


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