Behavioural Brain Research 174 (2006) 265–271 Research report Constraints on hippocampal processing imposed by the connectivity between CA1, subiculum and subicular targets John Gigg * Faculty of Life Sciences, Jackson Mill, University of Manchester, Manchester M60 1QD, UK Received 24 May 2006; accepted 12 June 2006 Available online 21 July 2006 Abstract A long-held fundamental function of the rat hippocampus is to provide a spatial map of the environment. The principal hippocampal output region for spatial information is area CA1 and the major target of CA1 is the subiculum. Thus, one possible role of the subiculum is to receive, process and transmit information regarding location to areas outside the hippocampus. Anatomical experiments in the rat have shown that the projections from CA1 to subiculum exist in a series of ‘nested loops’. Thus, each part of CA1 does not connect with each part of the subiculum. In turn, the majority of subicular principal neurons at the end of these loops have only one axonal projection target. The identity of these target areas depends on the location of the projecting cell within the subiculum, that is, neurons in different loops (and different septo-temporal levels of the subiculum) project to different sites. Principal neurons in subiculum and CA1 are not highly interconnected, suggesting that spatial information within any one of the nested loops is not shared amongst the others. This anatomical arrangement suggests that spatial information from any one part of CA1 is sent only to the sub-population of subicular principal cells within the same loop. In turn, this sub-population of subicular cells projects information to only a subset of the total range of subicular target areas. Thus, these subicular targets receive spatial information from only a subset of those in CA1. One conclusion from this is that for subicular targets to receive spatial information from the whole environment, area CA1 must have multiple spatial maps, one for each major projection (nested loop) to the subiculum. © 2006 Elsevier B.V. All rights reserved. Keywords: Hippocampus; CA1; Subiculum; Spatial map 1. Introduction The ‘spatial map’ hypothesis of the hippocampal formation is one of the most popular explanations for hippocampal func- tion [42]. Place cells and their associated place fields constitute the neural basis for these cognitive maps. Place cells were first recorded more than 30 years ago and continue to form the basis of a large research effort into how the hippocampus can guide spatial behaviour [32,40,41,43,45]. Place cells have now been recorded in all hippocampal fields (dentate gyrus (DG), CA3 and CA1) as well as the entorhinal cortex (EC) and the subiculum (SUB). However, whilst the presence of place cells in these areas is ubiquitous, little effort has been given to analysing how the environment is represented within hippocampal areas. This is * Tel.: +44 161 306 3876; fax: +44 161 306 3887. E-mail address: j.gigg@manchester.ac.uk. largely because there is no obvious topography for the represen- tation of space in the hippocampus (e.g., [46]; but see [21,27]). Thus, adjacent place cells do not necessarily fire in topographical register. Nevertheless, the fact that there is no obvious topogra- phy from electrophysiological recordings does not mean there is no anatomical constraint on the mapping of space within the hippocampus. Recent anatomical findings reviewed here have shown that hippocampal areas, particularly those that provide hippocampal output, are connected in a complex fashion. For example, not every part of CA1 is connected to every part of the subiculum. A major role of the hippocampus is to provide a map of the environment. However, for every hippocampal target to receive a complete map, a full map must be present in each of these hippocampal loops. Such a spatial re-representation may help to explain the remarkably high chances of finding hippocampal place cells for any given environment at the end of randomly placed hippocampal electrodes. Portions of the current manuscript have been published elsewhere in abstract form [16]. 0166-4328/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2006.06.014