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Cognitive graphs, resistive grids, and the hippocampal representation of space

The Journal of General Physiology
The Rockefeller University Press
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662.tif Commentary Cognitive Graphs, Resistive Grids, and the Hippocampal Representation of Space NEIL BURGESS and JOHN O'KEEFE From the Department of Anatomy, University College London, London WC1E 6BT, United Kingdom The hippocampal formation appears to play an impor- tant role in the ability of rats and other mammals to perform sophisticated spatial tasks, beyond simply fol- lowing a familiar route or approaching a visible cue. Ev- idence for this role came from the observation of selec- tive spatial deficits after hippocampal lesions (O'Keefe et al., 1975; O'Keefe and Nadel, 1978; Morris et al., 1982; Sutherland et al., 1982) and from the spatially se- lective firing of hippocampal complex spike cells (O'Keefe and Dostrovsky, 1971; McNaughton et al., 1983; Muller et al., 1987) ("place cells"), and led to the suggestion that the hippocampus was the locus of a cognitive map in the mammalian brain (O'Keefe and Nadel, 1978). The hip- pocampus is thought to have a broader mnemonic func- tion in higher mammals like monkeys and humans, (Scoville and Milner, 1957), such as episodic memory (Kinsbourne and Wood, 1975; O'Keefe and Nadel, 1978; Tulving, 1983). The extensive electrophysiological data available from studies in freely moving rats raise the possibility that the functional properties of the neural machinery of the hippocampus might be understood in the spatial domain, which could also shed light on other putative hippocampal functions for which there are fewer neuronal data. In this issue of The Journal of General Physiology, Muller et al. (1996) present a model in which the synaptic effi- cacies of the recurrent collaterals in the CA3 region of the hippocampus form a "cognitive graph." They ini- tially demonstrate that CA3 has properties adequate to support a directed connected graph, and they go on to outline their model. In this model, the net resistance of the synaptic connection between two place cells in- creases monotonically with the

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