- Focuses on thalamocortical interrelationships
- Discusses important problems concerning the function and structure of the thalamus
- Concludes each chapter with thought-provoking questions requiring future research
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Table of ContentsAbbreviations Used.
The Nerve Cells of the Thalamus.
The Afferent Axons to the Thalamus.
Intrinsic Cell Properties.
Function of Burst and Tonic Response Modes in the Thalamocortical Relay.
Maps in the Brain.
Two Types of Thalamic Relay.
Drivers and Modulators.
In a preface, authors are expected to say for whom the book has been written. We hope that this book will serve to introduce graduate students, postdoctoral fellows, and investigators who need to learn about the thalamus to some of the interesting aspects of the subject. Also, since many view the thalamus as an uninteresting, mechanical relay of peripheral messages to cortex that is already well understood, we have tried to explain that this view is far too simplistic and that there are many important problems about the function and the structure of thalamus that remain unrecognized and resolved. One of our aims has been to persuade colleagues that these problems are of interest and worth significant research investment. We have tried to make each chapter more or less independent, so that perhaps one or another can be assigned as course reading for graduate students. This entails some repetition from one chapter to another; we trust that it is not excessive for those who are motivated to read the whole thing.
Our thoughts about who would read the book were not our main focus when we first started discussions and rough drafts almost 10 years ago. Rather, we undertook the task initially because we found that the thoughts, the discussions, and the arguments that have accompanied the job of writing were sufficient stimulus in themselves. As we wrote, exchanged drafts, sent each chapter back and forth many times to be annotated, corrected, reannotated, and recorrected, we gradually learned a great deal about our subject (and about each other). The real truth is that we wrote the book for ourselves, and once we had that aspect of the writing fairly in hand, we worked hard to make it accessible to others.
We have not attempted to present a complete and coherent view of everything that is known about the thalamus. We have instead followed arguments and lines of inquiry that can lead to new questions, interesting thoughts, or new experimental approaches. Knowledge of the thalamus is extraordinarily patchy. The thalamus is divided into many different "nuclei." There are some thalamic nuclei that have been studied in considerable detail, and others about which we know almost nothing. Our plan in writing the book has been to assume that there is a basic ground plan for the thalamus. Although there are often important differences between one thalamic nucleus and another, in one species or between species, there is yet a common pattern of organization seen over and again in essentially all thalamic nuclei. We have tried to explore the nature of this common pattern and to ask questions about its functional significance.
We have both spent the greater part of our careers studying the visual pathways, and it won't take a very subtle reading of the book to recognize this. We turn to the visual replay in the thalamus repeatedly not only because this is the part we know best but also because in our readings and in our discussions with colleagues we find that the visual relay has, time and again, received more detailed experimental study than other thalamic relays. The visual relay may well have some special characteristics that distinguish it from other relays. In some instances this is clear, and we recognize it. However, in many instances it is reasonable to treat the visual relay as an exemplar of thalamic relays in general, and in many parts of the book, that is how we have approached the analysis of thalamic functions. This approach raises important questions about non-visual parts of the thalamus, and our expectation is that the comparisons will stimulate further study of these questions.
We have stressed that this book is not a complete inventory of all that is known about the thalamus. There are many important references we have not cited, and there are several lines of inquiry that we have not included. We say virtually nothing about the development or the comparative anatomy of the thalamus even though each is an extremely interesting subject in its own right. They should perhaps form the nucleus of another book. Nor do we cover the clinical aspects of thalamic dysfunction, another potentially interesting area, although it seems likely to us that this will become of greater interest once we know more about some of the basic ground rules of thalamic function and connectivity that are still missing from our current knowledge. For instance, the thalamus has long been implicated in epilepsy and certain sleep disorders, it is related to the production of pathological pain, and there is new interest in the thalamus as a particularly interesting site of pathology in schizophrenia and other cognitive problems. The complexity of the two-way links between thalamus and cortex and the limited nature of our knowledge about these links, especially in the human brain, make interpretations of clinical conditions extremely difficult and often rather tenuous, and we have not addressed them in this book.
We have tried to achieve two major aims in the book. The first is to look at many of the outstanding puzzles and unanswered questions that arise as one studies the structural and functional organization of the thalamus. The second aim, growing out of a small proportion of these questions, is to move toward an understanding of the possible role(s) of the thalamus in cortical functions, so that some coherent suggestions about this role could be presented as the book proceeds. The first aim is summarized to a limited extend by a short list of "Some Unresolved Questions" that appears at the end of each chapter. These are not questions to which a student can find answers in the text. They are, rather, designed to focus on some of the issues that need to be resolved if we are to advance our understanding of the thalamus. They do not represent an exhaustive list, and the interested reader is likely to find a number of other questions that are currently unanswered and often unasked. The listed questions should be seen as representing a state of mind, and they are an important part of the book as a whole. They should lead to more questions, and they should point to paths that have perhaps never been explored or along which our predecessors have been lost in the past. We hope that by stimulating a questioning attitude to thalamic organization we will encourage a view of the thalamus as far more mysterious than is commonly taught. This clearly implies that our second aim, to understand the role of the thalamus, which we present in detail in the later chapters, can at best be only partially achieved. We present a view of the thalamus that is based on the classical view of it as a relay of ascending messages to cortex. However, we see it as a continually active relay, serving sometimes as a "lookout" for significant new inputs and at other times as an accurate relay that allows detailed analysis of input content in the cortex. This is based on the recognition of two distinct types of input to the thalamus, the "drivers" that carry the message and the "modulators" that determine how the message is transmitted to cortex. The former can carry ascending messages from the periphery as well as descending messages from cortex itself. These messages are generally mapped, giving them a definite locus in the environment or in some other part of the brain. In contrast, the latter either can be mapped and thus act locally like the drivers or can lack a mapped organization and then act globally. Recognizing that drivers can take origin in the cortex leads to an interesting new view of corticocortical communication because it stresses that messages that pass from one cortical area to another may be under the same set of modulatory controls in the thalamus as are the inputs that are passed to the cortex from the peripheral senses.
It is probable that many of the ideas we present in this book will prove wrong. Whether they are right or wrong, we have tried to make them stimulating. To quote Kuhn (1963) quoting Francis Bacon, "Truth emerges more readily from error than from confusion," which provides our best justification for writing this book about a subject that in terms of the currently available literature is often extremely confusing.
Finally, both authors owe thanks and the book itself owes its existence to many people and organizations. A number of colleagues read an early draft of the book, and the comments and critical points that they raised have helped us to reorganize and correct a great deal of the book in terms of style, order of presentation, and content. We thank Paul Adams, Joe Fetcho, Sherry Feig, Lew Laberly, Carsten Hohnke, Jon Levitt, John Mitrofanis, and Phil Smith for their helpful comments. We recognize the amount of time and effort that they have contributed; we are most grateful for it and for the significant improvements that their careful readings have produced. The final version of this book is, of course, entirely our responsibility. All of our colleagues will likely find many places where they can write further instructive comments in the margins, and perhaps some of these will lead to useful explorations of the thalamus in the future. Marjorie Sherman helped with the proof reading. Sherry Feig helped one of us (R.W.G.) learn how to draw on a computer. Both authors received support from the NIH while this book was being written (Grants EY03038, EY11409, and EY11494), and at the early stages, R.W.G., while in the Department of Human Anatomy at Oxford, was supported by the Wellcome Trust. The initial stimulus for planning the book came from the year S.M.S. spent as a Newton-Abraham Visiting Professor at Oxford in 1985-1986.
S. Murray Sherman and R.W. Guillery