Wednesday, November 21, 2007

Observing rat brains as they look to the future

An emerging view is that the hippocampus is essential to imagining the future as well as remembering the past (which makes a lot sense, since we usually base our imagined future on our past experience). Johnson and Redich have now observed ensembles of cells in the CA3 region of the rat hippocampus whose firing transiently encodes paths forward of an animal at decision points in a maze, as if they are reflecting on possible futures and deciding what to do next. The figure, from Heyman's review of the work in Science, illustrates that as a rat looks in one direction, neurons representing that position (inset) fire over a half-second period. Here is the abstract of the work:
Neural ensembles were recorded from the CA3 region of rats running on T-based decision tasks. Examination of neural representations of space at fast time scales revealed a transient but repeatable phenomenon as rats made a decision: the location reconstructed from the neural ensemble swept forward, first down one path and then the other. Estimated representations were coherent and preferentially swept ahead of the animal rather than behind the animal, implying it represented future possibilities rather than recently traveled paths. Similar phenomena occurred at other important decisions (such as in recovery from an error). Local field potentials from these sites contained pronounced theta and gamma frequencies, but no sharp wave frequencies. Forward-shifted spatial representations were influenced by task demands and experience. These data suggest that the hippocampus does not represent space as a passive computation, but rather that hippocampal spatial processing is an active process likely regulated by cognitive mechanisms.

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