In Chapter 11 (Box 11-2) of Principles of Neurobiology, we learned how the hippocampus and entorhinal cortex form a network that integrates information from place cells, grid cells, border cells, and head direction cells—all to robustly provide animals with an internal representation of space. Although traditional approaches to studying spatial navigation, as well as cognition in general, focus on how neural activity represents what is happening in the moment, this is only one piece of the puzzle. Life itself is a dynamic process, and we constantly need to link our past (memory), present, and future together, often at high speed, as we progress through time
In a fascinating paper by Kay et al., the authors explore the fundamental question of how brain activity occurring in the present is able to represent what might happen in different hypothetical futures. This encoding of “imagination” is relatively poorly understood, but is clearly important to our ability to make good decisions in life. It is much better to have an idea of what might happen, whether it is good or bad, by playing out different scenarios in our minds first, before actually doing something! It turns out that this is exactly what happens in the firing patterns of neurons in the hippocampus as rats are finding their way around a bifurcating M-shaped maze (Figure A).
In this study, the authors recorded from place cells, and found that as rats approached a fork in the maze, cell firing that corresponded to two different possible futures (going into the left vs. right arms) came online in different subsets of cells, but activity in each of these subsets would switch back and forth with each other at the fast rate of ~125 milliseconds per representation (Figure B). It is as if the cells are saying: "Here I am, should I go left, here I am, should I go right?" In addition to the representation of location, the authors found that the representation of travel direction also followed this same basic rhythm of cycling between different alternatives, which also occurred at the same rate. Thus, it seems that neural activity in the hippocampal network has a basic design where consistent and quickly fluctuating time slots lasting ~125 milliseconds at a time, are available to represent different competing and important scenarios as they unfold and become relevant in an animal’s life. This type of mechanism is undoubtedly important for our ability to make quick decisions on-the-fly, and may even be broadly related to how we use our imagination in more abstract contexts.
Reference
Kay, K., Chung, J.E., Sosa, M., Schor, J.S., Karlsson, M.P., Larkin, M.C., Liu, D.F., and Frank, L.M., Constant Sub-second Cycling between Representations of Possible Futures in the Hippocampus Cell 180, 552-567.e525. Link