Journal Club

Our Journal Club highlights high-profile work from our young scientists-in-training and discusses the impact of the Neuroscience Institute's research on the field, our understanding of the brain, and potential breakthroughs that may translate into improved patient care.

Reading the hippocampal cognitive map

The hippocampus is thought to construct a ‘cognitive map’ of experience. How such an abstract map is utilized by other brain regions to guide behavior remains unexplored. This is a general problem in neuroscience: is the relationship between measured brain activity and an external variable observed by the experimenter actually read out by downstream observer mechanisms in the brain? In our work, we examined how the place cell activity in the hippocampus is utilized by its main output, the lateral septum (Tingley and Buzsáki, 2018). We chose this structure because, of all brain regions, it receives the densest hippocampal projections and also has direct projections to the hypothalamus and brainstem; potentially allowing for the selection of action programs. To examine how neuronal firing patterns in the hippocampus are transmitted and transformed, we simultaneously recorded neurons in both structures during a spatial navigation task.

Firing patterns of the lateral septal neurons did not provide information about the animal’s position (Figure 1A). However, when spiking activity was related to the phase of the hippocampal theta oscillations, the preferred theta phase of firing shifted continuously from the beginning to the end of the journey (Figure 1B).  Thus, we found a firing rate-independent ‘phase code’ for spatial position.

With additional analyses, we identified that the rate-independent phase code is not inherited from the phase or rate information of individual hippocampal place cells, rather it reflected a computation upon the continuously changing relationship between CA1 and CA3 place cell assemblies. From the perspective of lateral septal neurons, theta sequences of CA3 and CA1 ensembles are the ‘unit’ of information transmission out of the hippocampus (Figure 2). Overall, our findings demonstrate that transformation of the hippocampal cognitive map depends on higher order theta sequence structure. Assuming similar transformations in downstream hypothalamic and brainstem circuits, such an ‘action selector’ mechanism may be able to quickly identify the source of information and mediate action from a higher order map.

David Tingley and Gyorgy Buzsáki. Transformation of a Spatial Map across the Hippocampal-Lateral Septal Circuit. Neuron, Published 6/27/2018.

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