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Researchers have discovered that the arrangement of existing memories in the brain is altered when we embed new memories

The coactivation structure of hippocampal neurons

Brain architecture is sophisticated enough to integrate new information while allowing new and old memories to interact, rather than having to forget old experiences to make room for new ones.

In a study published in Nature Neuroscience, a team from the University of Oxford and Imperial College London devised an experiment using graph theory to study this mechanism of memory integration in mice. They set up a task in which the animals learned that a particular compartment in a new environment contained sucrose. The mice also explored a familiar environment before and after forming this novel place-reward association.

This allowed the scientists to observe how the laying down of the new memory affected the network formed by patterns of co-activity among neurons in the hippocampus, a brain area that plays a major role in learning and memory. They found that the network topology in the hippocampus – the functional structure describing the patterns of coordinated neuron firing that occur when old memories are recalled – changed as the mice embedded new memories. 

The team also found that during learning, the patterns of co-activity among neurons unfolded along particular directions in the ‘neuronal activity space’. This showed that novelty, spatial location, and reward experience were key factors involved in the process of integrating new memories. In addition, they discovered that high activity cells formed the core of each memory, while low activity cells contributed to the patterns of co-activity ‘on demand’, in order to segregate individual experiences. This finding highlights an important division of labour among hippocampal neurons.

Lead researcher Professor David Dupret at the Medical Research Council (MRC) Brain Network Dynamics Unit at the University of Oxford said: ‘This research sheds new light on the network mechanisms underlying the continual storage and recall of multiple memories in the hippocampus. Neuroscientists are now implementing new methods, such as in vivo imaging, to monitor large-scale neuronal populations over days and weeks of learning experience, to understand more about this process’.

Professor Simon Schultz from Imperial College London said: ‘This research highlights the value of an interdisciplinary approach to understanding the brain – by bringing a viewpoint from engineering and the mathematical sciences to bear upon the problem, we were able to tease out insights that would not have been possible based on a traditional approach to the study of memory.’

 

This research was funded by the Medical Research Council and the Engineering and Physcial Sciences Research Council.

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