Brain, Vol. 124, No. 10, 2120-2121,
October 2001
© 2001 Oxford University Press
Book reviews |
EXPLORING THE THALAMUS.
By S. Murray Sherman and R. W. Guillery. 2000. New York: Academic Press. Price £39.95. Pp. 312. ISBN 0-12-305460-5.
Department of Visual Science, Institute of Ophthalmology, University College, London, UK
Exploring the Thalamus by S. M. Sherman and R. W. Guillery is a substantial work, comprising 10 chapters, which provides an overview of how the authors see the role of the thalamus. The authors, though acknowledged experts in the field, have avoided writing an exhaustive reference book or extensive catalogue, but have produced more of a thesis, developing a theme as the book proceeds. This makes for interesting and therefore relatively easy reading. Nevertheless, the book is well indexed and has an extensive bibliography, thus making it useful also as a reference and pointer to further literature. It is often assumed that the function and circuitry of the thalamus are relatively well understood: this book highlights and demonstrates that this is really not the case. It might have been better to call this book `Exploring the thalamo-cortical circuit', as much of the book is rightly devoted to the intimate relationship between the cortex and the thalamus, and indeed the authors make the point throughout the book that there is a fundamental mutual dependence between the two, culminating in the final statement `. . . we suggest that the cerebral cortex without the thalamus is rather like a great church organ without an organist: fascinating, but useless'.
The book takes little for granted in what is required of the reader in terms of background knowledge (this makes it easily accessible to many different readers, which is the intention of the authors as stated in the preface). For example, the book gives outlines of what are sometimes thought to be complicated concepts, e.g. cable properties or action potentials, or the different types of neurotransmitter actions (e.g. ionotropic versus metabotropic). This might be an irritating oversimplification to the specialist in that particular field, but these sections can easily be skipped by the reader, as the book chapters are well divided into sections and subsections. Indeed, the authors themselves suggest that the reader may wish to do this in places. Nevertheless, this structure allows a reader who is not familiar with some of these basic neurobiological concepts to appreciate some of the issues which underlie thalamic and cortical function; the authors are to be congratulated for taking the time to do this. The book goes on to elaborate how some relatively modern concepts in neurobiology might be relevant to thalamic function. For example, in the consideration of synaptic glomeruli, the authors suggest that it may be more useful to think of these as areas free of astrocytic processes rather than areas which are specifically enveloped by astrocytic sheets, and discuss the possibility that glial processes may be active participants in transmission. In another example, the authors consider how neuronal cable properties can lead to functional differences between relay cells and interneurones, and how interneurones may thus be able to `multiplex' and have two routes for input/output computations. The authors also caution about judging the functional strength of an input on the basis of the numbers of synapses it contributes. Even for someone who is relatively familiar with the literature, it is refreshing to have such concepts described simply and in a functional context.
A commonly held view of the thalamus is that it serves as a mere relay in the transmission of sensory information from the periphery to the cerebral cortex and that cortico-cortical processing performs analysis of the outside world. Thus, it might be thought that the thalamus does nothing interesting. Such a view is espoused in many textbooks and in its extreme is manifested in illustrations which depict direct retinal projections to the visual cortex. This book challenges this view to the core, discussing in detail how thalamic nuclei can be thought of as first or higher order nuclei with different types of afferent, and how `. . . the cortex can, in principle, control the response mode of its own afferent supply'. The authors put forward the concept that one could regard the thalamus as layer 7 of the cortex, and that `the notion of the thalamus simply providing a supply of afferent messages to cortex through a neatly ordered thalamic `radiation' is dead'. While this may sound radical, it is in fact a case that is cogently argued through the book. As part of this, the authors consider the various afferents to the thalamus and divide these into either `drivers' or `modulators', stating that they consider this to be the most important distinction. For obvious sensory nuclei such as the lateral geniculate nucleus, this is relatively straightforward on functional grounds, with drivers being the retinal input and modulators the afferents from the brainstem and layer 6 of the cortex. The authors then look at the other properties of these afferent types, such as anatomy and transmitter receptor type, and extrapolate to other thalamic nuclei (the majority) where the distinction between drivers and modulators is not so obvious on functional grounds. This leads to the concept that some thalamic nuclei in fact receive their `driver' input from layer 5 of the cortex: these nuclei are thus termed higher-order relay nuclei in contrast to the obvious first-order nuclei such as the lateral and medial geniculate nuclei and ventroposterolateral complex. Thus, so-called higher order nuclei would appear to receive as their signal information which has already been processed in the cerebral cortex. The question which naturally follows is: what exactly is this new driver signal, and what is it for?
A related question is asked with regard to the well-known existence of maps in the thalamus and cortex, which are well described in the major sensory systems—the concept of the homunculus. The authors devote a chapter to maps in the cortex and thalamus, providing an overview in normal and abnormal animals. But they state, correctly, that there is a danger in not asking what maps are for. This is especially pertinent in terms of exactly what might be being mapped in the higher order thalamic nuclei and for what purpose? A related question is what the role of the thalamic reticular nucleus is, located as it is between the thalamus and the cortex. The authors state `What is clear is that the thalamic reticular nucleus can serve as a nexus for interactions between the many thalamocortical and corticothalamic pathways concerned with a given modality.'
The authors cannot provide easy answers to some of the questions that they raise, and they make no apologies for this. Indeed one of the strengths of the book is that it highlights things that are still unknown, rather than merely concentrating on what is known. All of the chapters, apart from the initial Introduction and final Overview, have a set of `unresolved questions', which could be used to formulate questions that can be addressed experimentally. Thus, overall, Exploring the Thalamus is not only a book which provides a fascinating, hypothesis-driven review of the thalamus and its relationship to the cortex, but it is also a gateway into future research ideas. The book thus has much to recommend it to a wide readership: the student, the neurologist, the experimental scientist and the philosopher.
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