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Hippocampal replay of neuronal activity patterns promotes the retention of salient experiences

We do not remember all our experiences equally long nor equally vividly. It appears intuitive, and has been experimentally demonstrated, that more salient and relevant experiences are preferentially remembered for a longer period of time. However, the mechanisms underlying the selective retention of memories are still under active investigation. Cognitive functions are thought to be supported by the coordinated activity of neuronal populations distributed in different brain regions. The formation of memory and its transfer into a long-lasting state, known as memory consolidation, engages the interaction between neuronal ensembles in the cortex and the hippocampus.

The spatio-temporal context of an experience is associated with a unique pattern of neuronal activity in the population of hippocampal neurons. During periods of quiet wakefulness or during sleep, the activity patterns observed during experience are reactivated. Such replay occurs at a faster time scale than during experience and co-occur with fast oscillations measured in the local field potential called sharp wave ripples (SWRs). Each single replay event in the hippocampus relates to the context of one particular past experience and thereby could specifically support the consolidation of the associated memory. Thus, it is also conceivable that the most relevant experiences are preferentially replayed during sleep, leading to selectively enhanced consolidation of the associated memory.

Rats trained to associate two contextually distinct locations with different amounts of reward, showed stronger memory for the location associated with the larger reward after a delay. The effect appeared to be strongest for locations demanding the highest level of spatial integration. The reward-related enhancement of memory was accompanied by increased replay activity of the highly rewarded context, an effect that was more prevalent for locations requiring higher spatial integration, and it was specifically sensitive to disruption of all replay events during the delay after learning. These results demonstrate a causal link between the increased occurrences of replays during sleep and the selective enhancement of memory consolidation for highly rewarded experiences.

A better understanding of the mechanisms underlying the formation and consolidation of memory will require the simultaneous recordings of population of neurons in the different brain regions involved. The technologies used today, wire electrodes loaded into micro-drive arrays and electrode-dense silicone probes respectively lack the number of electrodes or their flexible and accurate positioning to perform such recordings.

Newly engineered flexible silicone probes enabled to combine the advantages of the two approaches, allowing to load a micro-drive array with up to 16 independently movable silicon probes. The use of the device was demonstrated by performing simultaneous chronic recordings of populations of cells in the cortex and the hippocampus. This prototype offers an alternative solution towards large scale recordings of distributed neuronal ensembles.

Date:17 Jan 2012 →  1 Jul 2019
Keywords:memory, behavior, electrophysiology
Disciplines:Neurosciences
Project type:PhD project