2007/01/18

Goal-related activity in Hippocampal Place Cells

Place Cells in rat hippocampus fire selectively at certain locations. These cells may therefore be said to represent locations in space. If a rat has learned a certain maze, then every location in the maze is associated with some neurons, and the rat's location can be reconstructed by the population firing pattern.

Anecdotally, place cell RFs (called "firing fields") are clustered near the goals of the maze (the platform in the water maze, the food in an arm maze). But data has been unclear. This paper provides clear data to show that this is true.

The rats are in a cylinder maze, and the goal is "hidden." That just means that a computer drops a pellet when the rat crosses a certain unmarked location. Once the rat learns the maze, many place cells acquire a secondary, weaker, response field at the goal location.

The authors suggest that these signals indicate the hippocampal cells represent monitoring of the rat's performance as well as providing a map of space. They do not speculate on how these unrelated signals are decoded, which would seem to be a major problem.

But I don't have any better explanations.

4 comments:

Paul said...

On the "place cells as cognitive map theory", is it not possible that place cells 'simply' form associations between various stimuli, which in rodents happen to be mainly spatial. Thus the clustering of place cells around goal states in an environment would, in a way, represent the relevance of these states to the animal. These views are based on a paper by Eichenbaum et al. (1999) - full ref to paper http://paul-baxter.blogspot.com/2006/11/hippocampus-memory-and-place-cells.html
Just a thought :-)

JohnnyB said...

Eichenbaum talked about this at SfN this year. Many who are married to the "place field as random past places" theory do not agree. One of the foremost researchers in decoding place fields in retrospective and prospective representations is Matthew Shapiro, which may yield insight into how place fields are encoded in the current paper.

Ryan Hunsaker said...

Well, it must be mentioned here (and as a proviso I concur with Eichenbaum's & Shapiro's assertions that place cells are much more than an allocentric map without a purpose relating to learnign and memory) that many place field recorders simply statistically exclude fields at reward locations as they represent more than just space per se.

One way to look at it is as Shapiro posits, that cells that do not show a mnemonic purpose (e.g. encoding retrospective or prospective memory) are the place cells as reported by McNaughton and colleagues. A more thorough explanation can be found in the work of Sheri Mizumori who suggests that place cells encode context as a whole; both spatial and nonspatial contexts are represented (e.g. a reward zone will be represented and nonrewarded zones as well, but obviously a rewarded location moreso). This means that info from the nucleus accumbens, amygdala, parietal cortex, and entorhinal cortex will be integrated into a "context" that is represented as place cell firing throughout the brain, most notably in the hippocampus.

Her theory will be better codified in a book she has coming out soon from Oxford University Press. A stripped down version of her model can be read in the book "Neurobiology of Learning and memory" edited by Ray kesner and Joe Martinez from Elsevier publishing.

reyt said...
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