Neural Ensembles in CA3 Transiently Encode Paths Forward of the Animal at a Decision Point

A terrific paper by Adam Johnson and A. David Redish in The Journal of Neuroscience from November 2007 shows that neurons in the hippocampus transiently represent paths forward of an animal when the animal is deciding between more than one path.

We've known since the late 1970s that some hippocampal cells are spatially selective. The cells came to be called 'place cells' and the spaces to which they responded their 'place fields'.

We're still finding out what exactly they're up to, and how they support the computational demands of different sorts of problem. Johnson and Redish trained rats to navigate two kinds of maze for reward, in order to ask some specific questions about what part of the hippocampus was up to at short time scales. The kind I'll focus on here is illustrated here: a 'multiple-T maze', with a sequence of choices. (The picture is figure 1 from the paper.)

Each rat also had multiple tetrode recording devices implanted into the CA3 region of his hippocampus. Using Bayesian decoding, movement of the rats through the maze was related to measured activity in the hippocampus, to identify place fields and corresponding patterns of neuronal firing. (The analysis associated places with ensembles of spikes, rather than individual cells.)

The upshot of this analysis was, partly, a representation of hippocampus activity associated with the physical space of the maze.

The really exciting part of the work was relating those two (physical space and hippocampus activity) with a third stream of data relating to the changing physical position of the rat, and the specific contingencies of the task faced by the rat. Rats are known sometimes to pause and look around when they're at decision points in mazes. In a properly designed maze where observed behaviour tells us that they're pretty good at taking the turn that gives them better reward, we have a decent idea what information they're drawing on when they eventually start moving.

Johnson and Redish observed that at the decision points the firing patterns in the hippocampus showed that place fields forward of the animal, and down the available paths, were active. This very clearly suggests that the hippocampus is involved in mediating memory functions relating to decision making.

There's a telling graphical representation of the key finding, that conceals all the analysis that went into this. (My second figure is one panel of figure 7 of the paper.) The white circle shows the physical position of the rat as it moves through the maze, and the blue the place fields arrayed to correspond to the physical space of the maze. The coloured pixels on the place field show activity as recorded by the tetrodes at the time - mostly corresponding to the space where the rat is, but clearly sweeping ahead (especially at 120-280 milliseconds) while the rat pauses at the decision point.

This is even more vivid when you see an animation, and there is one in the supplemental materials to the paper - you can also view it (this is slowed down, not actual speed) here:



This animation is a really terrific example of how very complex data can be made totally compelling when presented in a coherent way, and especially one that makes effective use of the number of dimensions in the graphic. Physical space, the position of the rat, and the activity corresponding to place fields are co-ordinated, and then updated over time so that the the fact that some cells "Transiently Encode Paths Forward of the Animal at a Decision Point" is totally clear. Bravo.



Johnson, A., Redish, A.D. (2007). Neural Ensembles in CA3 Transiently Encode Paths Forward of the Animal at a Decision Point. Journal of Neuroscience, 27(45), 12176-12189. DOI: 10.1523/JNEUROSCI.3761-07.2007