IMAGING THE SCHEMA: CHARACTERISTICS OF OVERLAP BETWEEN IEG-EXPRESSING NEURAL POPULATIONS IN SIMILAR BUT COGNITIVELY DISTINCT NAVIGATION TASKS

N. Insel*; F.P. Houston; A. Vazdarjanova; M.R. Bower;
M.K. Chawla; B.L. McNaughton; C.A. Barnes

NSMA, Univ Arizona, Tucson, AZ, USA

The cerebral cortex is organized hierarchically, such that information is represented more concretely in primary sensory and motor cortex and more abstractly in higher areas. If an animal is exposed to two separate episodes with similar sensory and motor stimulation but in which the schema/strategy used by the animal are different, a greater difference in active cell populations would be predicted in higher than in lower cortical areas. Moreover, activation patterns in cortical layers receiving top-down input might resemble higher areas more than those receiving bottom-up input. These predictions were tested using double in situ hybridization of genes Arc and Homer 1a (each indicating neural activity during a different time epoch) in brains of four rats that had performed two navigation tasks: one task required them to follow a cue and the other to follow memory for a well-learned sequence. Consistent with the prediction, the CA1 region of the hippocampus had less overlap of active populations between the tasks (i.e., fewer cells stained for both Arc and Homer 1a) than did primary visual and motor cortices. Interestingly, deep layers of all cortical regions had greater population overlap than CA1, while superficial layers of primary visual cortex, prelimbic cortex, and entorhinal cortex (but not primary motor cortex) were not significantly different from CA1 (see also Burke et al., this session). Together, these data show that sets of active neurons in CA1 differ more between tasks than sets of neurons in primary sensory and motor areas. Moreover, given the direct outputs from CA1 to the deep layers of entorhinal and prelimbic cortices, the orthogonalization of two events that takes place in the hippocampus may not be reflected directly in the regions to which it projects.

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