Brain, Vol. 127, No. 5, 1211-1212, 2004
© 2004 Guarantors of Brain
doi: 10.1093/brain/awh056
Book Review |
THE PARAHIPPOCAMPAL REGION: ORGANIZATION AND ROLE IN COGNITIVE FUNCTION
THE PARAHIPPOCAMPAL REGION: ORGANIZATION AND ROLE IN COGNITIVE FUNCTIONBy Menno Witter and Floris Wouterlood
2002. Oxford: Oxford University Press
Price £69.50. ISBN 0–19–850917–0.
In 1953 the patient known by his initials as H.M. underwent surgery to have tissue removed from his medial temporal lobes for the purposes of controlling intractable epilepsy. Upon awakening from the procedure he was found to have acquired a profound anterograde (and temporally graded retrograde) amnesia. This event triggered an eruption of research activity aimed at determining which particular temporal lobe structures were critically involved in the memory dysfunction. By the 1970s and early 1980s it began to look as though the hippocampus and perhaps the amygdala were responsible for the acquisition of consciously accessible, declarative memory and that damage to these structures alone was sufficient to produce the amnesic syndrome. As a result, the hippocampus assumed a central position in the memory and brain zeitgeist of the period. With this ‘hippocentric’ mindset in place, models of memory processing were built whereby the hippocampus sat at the peak of a pyramid of ‘lesser’ cortical structures. These latter regions had the subservient role of performing a variety of preprocessing operations and then supplying this information to the hippocampus, which then performed various encoding, storage and recall functions.
The hippocentric model has, however, been reworked in more recent years on the basis of new data that became available during the late 1980s and early 1990s. These studies demonstrated that recognition memory deficits in animals that had received lesions to the cortical regions neighbouring the hippocampus, but that excluded the latter structure itself, were as large as or larger than those that occurred in animals that had received damage to the hippocampus alone. The cortical regions that lie adjacent to the hippocampus have been grouped together under the (hippocentric) term ‘parahippocampal region’ (although some authors use the term ‘retrohippocampal’). The parahippocampal region encompasses the pre- and parasubiculum, the entorhinal cortex, the perirhinal cortex and the post-rhinal cortex. The general importance of this region for memory processing has now been demonstrated in a number of different studies, and as a result of an MRI scan of H.M.’s brain it has been confirmed that large portions of the parahippocampal region, as well as the hippocampus, were removed during his temporal lobectomy. This revision of our conception of memory structure has led to a marked increase in interest in the parahippocampal region. This is illustrated by a comparison of the number of hits obtained in PubMed searches on ‘hippocampus’ conducted separately for the years 1975 and 2000 with those obtained for ‘entorhinal OR perirhinal’ for the same years. For the former structure, the number of articles has increased approximately eight-fold over these 25 years. By comparison, the number of hits for the parahippocampal structures has increased twelve-fold.
This book by Witter and Wouterlood is a timely addition to the shelves of any researcher interested in the parahippocampal region, and to my knowledge it is the first attempt to summarize current research in this area in a broad review of this type. The book is an edited volume of papers written by some of the leaders in this field of research. It comprises 16 chapters that are grouped into four main sections to take the reader systematically from structure to function. As an opener, the first section includes contributions by the editors and describes how the parahippocampal region is separated from, and links to, neighbouring tissue on the basis of cytoarchitecture, connectivity and developmental data. This theme flows over into the first portion of the second section, where comparisons are made between the nomenclature used to describe the region in rats and primates. What is interesting here is how the terminology (and borders) used to describe some of these subregions has changed over the years. The latter part of this section delves into the electrophysiology of the region, from the level of single neurons through to extracellular field potentials and EEG oscillations. The third section focuses on memory and object representation, and includes reviews of some current thinking about how memory is processed within the parahippocampal region. A strength of this section is that it links together the work of researchers who have used rats, non-human primates and humans in their investigations. The final section of the book comprises three chapters that discuss evidence that implicates the parahippocampal region in three different disorders of the brain. The first of these is Alzheimer’s disease. Given the clear evidence linking dysfunction of the parahippocampal region with memory disorders, it is perhaps not too surprising to find that there is some damage to this area in Alzheimer’s. What is striking is how focused the damage is in the initial stage of the disease. For example, it is now known that the entorhinal and perirhinal cortices are the first cortical areas to show signs of NFTs (neurofibrillary tangles) in the early stages of the disease. The second chapter in this section describes the involvement of the parahippocampal region in schizophrenia. This is perhaps a more surprising connection than that made in the previous chapter, but on reflection we should consider that our personalities and experience are very much a product of our memories. The data reported here show that changes in parahippocampal cytoarchitecture and neurotransmission occur in schizophrenia, and there appears to be a growing consensus that the region has some role in the cognitive deficits and behavioural symptoms observed in the disorder. The final chapter investigates the involvement of the entorhinal cortex in temporal lobe epilepsy and describes ongoing research that attempts to distinguish between the roles of the hippocampus and this former region in the disorder.
Where the book is strongest is in the wide-ranging nature of the research that it brings together. It is almost inevitable, however, that this approach might also lead to gaps in its coverage. Areas that immediately come to mind include the work on synaptic plasticity in this region by Zafar Bashir, the single-unit recording studies of Malcolm Brown and the behavioural work of John Aggleton (although I acknowledge that the work of the last two researchers does get some mention). Inclusion of Bashir’s work on synaptic plasticity, however, might have provided a useful link between the neuronal level data described in the early chapters and the whole-animal studies of memory described in the later ones. A further line of research that might usefully have been included is the work on head direction cells by Jeffrey Taube and colleagues. These cells, which appear to function something like an internal compass, are found in several areas of the brain, including the postsubiculum (the dorsal presubiculum). This work is interesting and relevant because it potentially links parahippocampal function with the spatial processing of the hippocampus.
These criticisms are minor, however, for in summary this book provides an indispensable reference volume and compendium of some of the exciting recent work on parahippocampal structure and function. It will be invaluable for any researcher working in this area and is also likely to stimulate interest in readers who want to be updated on current thinking about how the temporal lobe functions outside of the hippocampus.
Department of Psychology, University of Otago, Otago, New Zealand
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