Oscillatory coherence becomes maximal as the rat has to make a decision, at the same time, synchronized cell assemblies are formed (Cartoon: courtesy Karim Benchenane)

Our brain is presented with a constant stream of experience: a huge amount of information, too much to be stored in full in our memory. How can we keep only the essential parts of this stream? This is the questions that we have attempted to answer in a paper by Karim Benchenane, Adrien Peyrache, Mehdi Khamassi, Patrick Tierney, Yves Gioanni, myself, and Sidney Wiener, which was published on the June, 24th issue of neuron (you can find the paper here).

The hippocampus and the prefrontal cortex are two brain structures known for their major roles in learning and memory. We previously showed that during sleep, the two structures communicate, possibly favoring the transfer of information from the former to the latter structure: this is the so-called memory consolidation process. In consolidation, new information is reorganized, triaged and becomes part of our knowledge corpus kept in the cerebral cortex. But how exactly can we select relevant information? This seems like a ability crucial to our survival.

Researchers trained rats to learn behavioral rules, apparently simple for us humans, but hard enough for rats, which need tens of trials and errors to acquire them. By following neural activity in the hippocampus and the prefrontal cortex, the researchers showed that, starting from the moment in which the animals understood the task, the two structures synchronize, oscillating in phase, as two pendulums beating at the same tempo. Neurons in the two structures become then active at the same time, which causes the strengthening of the connections between them. This synchronization occurs mainly at the moment in which the rat has to make a decision based on the learned rule. Later on, during sleep, the same neuronal groups become spontaneously active again, because of the privileged links formed during experience. Thus, a memory trace that is relative to the crucial phases in the task is indeed formed and retained.

But what is the signal that flags important memories, triggering synchronization and memory formation? Dopamine is a neuromodulator, a chemical involved in learning, as it increases in concentration in the brain when the animal receives a reward. The researchers tested its role on the hippocampus and the prefrontal cortex. Interestingly, injecting dopamine the prefrontal cortex causes the same effects observed during learning: the two structures synchronize again. Thus, dopamine, whose involvement in the modulation of large networks of neurons was so far poorly understood, could trigger synchronization through the brain after successful learning, so that behaviors that lead to reward can be retained, and the relative brain configuration tagged for spontaneous replay and consolidation.

This work represents a significant advance in our understanding of how different structures communicate between each other, and store important information. This knowledge is necessary to understanding disorders involving a defect in communication between brain structures, such as schizophrenia

Benchenane K, Peyrache A, Khamassi M, Tierney PL, Gioanni Y, Battaglia FP, Wiener SI (2010) Coherent theta oscillations and reorganization of spike timing in the hippocampal- prefrontal network upon learning. Neuron 66:921-936.

During sleep, memories about our previous experience are replayed, as was previously shown in the hippocampus, in the neocortex and in the striatum. This replay is probably important for memory consolidation, the process of stabilization of memories that is thought to be orchestrated by the hippocampus. With Adrien Peyrache, Karim Benchenane, Mehdi Khamassi, and Sidney Wiener at the CNRS, Laboratoire de Physiology de la Perception et de l’Action, Collège de France, Paris, we have shown that replay in the prefrontal cortex is correlated with hippocampal sharp waves. Because sharp waves are when most of the hippocampal replay takes place, this supports the view that information processing in the cortex during sleep is under hippocampal control.  Moreover, in this work, which is the centerpiece of Adrien’s PhD dissertation, we show that not everything about experience is equally replayed. As the figure above shows (please go to the original paper on Nature Neuroscience, just published this week, for details) activity patterns from the “choice points” in the task, and after the rat has learned the task, are the most likely to be replayed. 

Thus, there seems to be a mechanisms determining what the most relevant information is, and what deserves to be replayed, and maybe consolidated. We will get back to you soon – hopefully – with some ideas about what this mechanism may be. 

For the moment, my thanks to Adrien and the other authors!

The Dutch foundation for “fundamental research on physics of the matter” had its meeting in Veldhoven on Januray 20-21. Stan Gielen and Michel Decré organized a symposium on brain and cognition that saw a pretty interested audience.

I was very honored to be invited to speak in this panel, and hope that it will help spurring interest for neuroscience in the Dutch physics coommunity