Hippocampus & Behavior
Just as for the cerebellum, we decided to expose the postnatal hippocampus to X-rays to see what type of anatomical disruptions would result. To our surprise, the hippocampus did not show any anatomical disruption (see photo at right). The only effect was a moderate to profound reduction of cells in the dentate granular layer that depended on the number of X-ray exposures; the remaining granule cells were still in a recognizable granular layer. We decided to put the hippocampal X-irradiated animals into behavioral tests that had been used on animals with large lesions of the hippocampus (see results from Bayer et al., 1973). Here we made the amazing discovery that when approximately 85% of the dentate granule cells were depleted by the X-ray schedule, the animals behaved exactly as if the entire hippocampus had been lesioned. Thus, whatever is the function of the hippocampus in the brain, the dentate granule cell population plays a critical role.