The Neuropsychiatry of Limbic and Subcortical Disorders

The Neuropsychiatry of Limbic and Subcortical Disorders

Salloway S, Malloy P, Cummings JL, editors

Washington (DC): American Psychiatric Press; 1997. 217 pp with index

ISBN 0-88048-942-1 (cloth)

The limbic system is a topic of considerable interest to both psychiatrists and neurologists. For biologically oriented clinicians, one can barely go a week without encountering some reference to a limbic disorder. The explosion of information about the interconnectedness of various brain regions is forcing both practising clinicians and neuroscientists to grapple with all brain regions, not just a favorite few. Inevitably, one is confronted with the need to understand the function of the limbic system. The definition of the limbic system has steadily broadened over time to include not just the medial circuit of Papez, but structures that are functionally associated with it. In common usage then, the term “limbic system” speaks to a set of cognitive functions; these functions are subserved by a network of cortical and subcortical structures.

In this spirit, The Neuropsychiatry of Limbic and Subcortical Disorders represents a snapshot of the state of knowledge of the limbic system. This book expands on the summer 1997 special issue of The journal of Neuropsychiatry and Clinical Neurosciences on the same topic. Published by the American Psychiatric Press, it is a high-quality book at a reasonable price, with many colour figures and photomicrographs.

This book is not an easy introduction to the anatomy and function of the limbic system. As the editors state in their introduction, “The essays in the volume cover a broad range of basic and clinical material at various levels of difficulty. … Some of the chapters present complex material requiring careful study and perhaps a second reading.” This is not an understatement. The book is divided into 2 sections: Anatomy and Neurochemistry, and Clinical Syndromes. It is the first, more technical section that presents the most difficult material. For neuroscientists familiar with the terminology, these chapters represent an excellent resource. They are good summaries of the anatomical literature with extensive references. Most chapters have 50 to 100 well-selected references, though a few contain up to 300. There is an abundance of photomicrographs, many of which have been previously published and retain an alphabet soup of anatomical abbreviations. This may present a problem, primarily to the student of behaviour wanting to learn more about the neural substrates.

If the anatomy section suffers from over-inclusion of information, then the clinical section suffers from a lack of rigour, and must be taken as hypothetical in many cases. There are interesting ideas here, and the authors have put forth several theories regarding the pathology of syndromes including temporal lobe epilepsy, emotional experience, recovered memory and religious experience.

While many of the chapters focus purely on the anatomical or clinical aspects of the limbic system, a few successfully link anatomy and function. The chapter entitled “Neurobiology of Fear Responses,” by Michael Davis, is a particularly cogent exposition of the role of the amygdala in fear. This chapter, appropriately positioned between the 2 major sections, introduces concepts such as classical conditioning, and outlines the evidence from lesion and excitation studies for the amgydala’s function. It is accessible to both anatomists and behaviourists. The chapter entitled “The Neurobiology of Emotional Experience,” by Kenneth Heilman, lucidly outlines several theories of emotion, ultimately arriving at the modular theory. One version of the modular theory states that emotions are mediated by anatomically distributed modular networks, and it is the relative activation of these modules that gives rise to the variety of human emotion. The location of the modules, of course, overlaps with the limbic system. The chapter entitled “Limbic-Cortical Dysregulation,” by Helen Mayberg, is an excellent exposition of a theory of the functional organization of medial cortical and limbic structures. This theory, based largely on human functional imaging (positron emission tomography and functional magnetic resonance imaging), is quite successful in unifying often contradictory studies regarding cingulate function. Finally, the chapter by Koob and Nestler entitled “The Neurobiology of Drug Addiction” is a good summary of the neural substrates that underlie reward behaviour, and how drugs of abuse affect them.

All of the authors in this book have published extensively in their fields. Consequently, most of the material has appeared in other review articles. Nevertheless, it is convenient to have the information all in one place, together with the colour reproductions.

In a book that juxtaposes both anatomical and syndromic chapters, it becomes painfully obvious that our knowledge of brain wiring is fast outpacing our ability to describe behaviour. A great deal is known about connectivity, neurotransmitters and gene expression, but how can these be related to only a crude description of human experience? Saver and Rabin, in their chapter on religious experience, offer several convincing descriptions that would suggest that the mystical quality of a religious experience is a manifestation of limbic activity, if not outright seizure activity. While quite reductionist, it may even be true, but something is lost in the characterization of the experience. It is no coincidence that virtually every work of fiction is fundamentally concerned with “limbic function.” The conclusions of all classical tragedies are known — it is the human experience that captures our interest. When speaking about the function of the limbic system, one quickly realizes that the putative functions, emotion, memory and motivation, are difficult to describe, let alone quantify — hence, an unlimited supply of literature. Unlike other cognitive functions such as perception, language and motor behaviour, these limbic processes do not lend themselves easily to experimentation.

Are these processes unquantifiable? Perhaps the language is wrong. For example, anxiety is an emotion variously localized to the limbic system. One can go to great lengths using different rating scales to quantify the severity of anxiety, but ultimately one relies upon individual interpretation of crude descriptions. What if an emotion like anxiety were compacted to a measure of probability? For example, “I feel like I’m going to die,” represents the assignment of a non-zero probability to the outcome of death. While the individual may know death is unlikely, it nevertheless creates a situation of uncertainty: “I know I won’t die, but then again, what if I do?” Measures of uncertainty, while not in the usual parlance of emotion, do lend themselves to quantification, and ultimately correlation with neural activity. Perhaps it is time for a shift in the description of limbic behaviour. Only when these phenomena are accurately described will we be able to relate them to brain function and dysfunction.