Manufacturing Memory Templates In Advance: The Hippocampal Theta Cycle in Novelty

One of the interesting findings of the rat "place cell" research into hippocampal memory of places during a rat's exploration of its surroundings is that, as the rat retraces a known route, hippocampal memory neurons fire in an order consistent with the order the rat traces its journey, in a cycle of looping memories which reflect an array of past, present, and future locations. The window of the rat's current journey is reflected in the "theta" (about 8/sec, low range alpha in humans actually) rhythms of the hippocampus as the rat follows its route.

What happens if the rat is exploring a new area? memory of the area just traveled may follow the same pattern as in the well trodden route. But what of the future areas? Do they too have a representation in hippocampal neuron firings? It seems that they do: hippocampal cells are placed in the theta sequence before the experiences they are soon to encode even occur. Furthermore, the recruitment of anticipated memory cells varies according to the anticipated distance tothe goal, as if the buffer allocated for future memory is set in size by the anticipated duration before the planned goal is expected to be reached. One wonders if people might remember better if that buffer could be made to default to larger sizes? Or perhaps such sizing is already optimal. Who knows?

Thus, it appears that the hippocampus may be continually making a framework for memory of our journeys before we finish them, with pre-made memory templating for events that we have not yet experienced, but somehow anticipate. The hippocampus thus may create a speculative version of memory, anticipating that journey's sequences before we get there. Are journeys in our imagination made, in a concrete kind of representation, in similar locations to where we make real memories of our experiences? Probably. How then do we tell the difference between memory and imagination? That has yet to be established, but it's likely that parts of the brain outside the temporal lobe, possibly the forebrain, would be involved in reality testing to determine when goals are met.

One observation regarding the role of the 8-Hz rat hippocampal "theta cycle" in spatial memory processing: the rhythmic theta here is a default cycle, a framework for the carrying of more specific information. This is in keeping with a concept of EEG waves as having a role, not as the specific information being processed by the brain, but rather as a default or carrier wave framework for synchronizing the actual use of information perceived and actions performed by the organism.

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ABSTRACT

Hippocampal theta sequences reflect current goals

Andrew M Wikenheiser & A David Redish

Nature Neuroscience (2015) doi:10.1038/nn.3909

Received 12 August 2014 Accepted 25 November 2014 Published online 05 January 2015

Hippocampal information processing is discretized by oscillations, and the ensemble activity of place cells is organized into temporal sequences bounded by theta cycles. Theta sequences represent time-compressed trajectories through space. Their forward-directed nature makes them an intuitive candidate mechanism for planning future trajectories, but their connection to goal-directed behavior remains unclear. As rats performed a value-guided decision-making task, the extent to which theta sequences projected ahead of the animal's current location varied on a moment-by-moment basis depending on the rat's goals. Look-ahead extended farther on journeys to distant goals than on journeys to more proximal goals and was predictive of the animal's destination. On arrival at goals, however, look-ahead was similar regardless of where the animal began its journey from. Together, these results provide evidence that hippocampal theta sequences contain information related to goals or intentions, pointing toward a potential spatial basis for planning.

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