Independent Codes for Spatial and Episodic Memory in Hippocampal Neuronal Ensembles

S. Leutgeb¹ *; J.K. Leutgeb¹; L. Colgin¹ ; K. Jezek¹; C.A. Barnes^1,2; E.I. Moser¹; B.L. McNaughton^1,2; M.-B. Moser¹

1. Centre for the Biology of Memory, Norwegian University of Science and Technology, 7489 Trondheim, Norway
2. NSMA , Univ Arizona , Tucson AZ , USA

Hippocampal neuronal ensemble activity appears to play an important role in the establishment of both spatial and non-spatial episodic memories, but there has long been controversy as to which of these processes best characterizes the role of the hippocampal formation in mnemonic processes. Hippocampal neurons (330 CA3 cells, 487 CA1 cells) were recorded under conditions in which the recording chamber was varied but its location remained unchanged (variable cue-constant place), versus conditions in which an identical chamber was encountered in different places (variable place-constant cue). Two forms of neuronal pattern separation occurred in CA3. In the variable cue-constant place condition, the firing rates within the active cell population varied, often over more than an order of magnitude for individual cells, while their location of firing remained constant (rate remapping). In the variable place-constant cue condition, both location and rates changed, so that population vectors for a given location in the chamber spanned statistically independent subspaces corresponding to a switch from one active cell population to a second, independent one (global remapping). Similar trends were found in CA1 but with much smaller rate changes and a greater degree of spatial remapping even in the variable cue-constant place condition. These independent encoding schemes may enable simultaneous representation of spatial and episodic memory information in the hippocampus. We are currently testing whether global remapping in CA3 may be necessary for the bistability of hippocampal representations in morphed environments as observed under some conditions (Wills et al., 2005) but not others (J. K. Leutgeb et al., Soc. Neurosci. Abstr. 2004).