Book Review

The Psychobiology of the Hand

N Engl J Med 2001; 345:1284October 25, 2001

Article

The Psychobiology of the Hand
(Clinics in Developmental Medicine. No. 147.) Edited by Kevin J. Connolly. 276 pp., illustrated. London, Mac Keith Press, 2000. $69.95. ISBN: 1-898-683-14X

“Our hands are central to our psychology as they continually switch between executive, exploratory and expressive activity.” This statement from the editor's preface gives an idea of the content of this fascinating book by well-known experts on hand function. The editor has assembled a coherent account of research on the human hand, which was rekindled in the mid-1980s and is now a hot topic.

The main focus of the book is on normal manipulatory functions, with an emphasis on development. To be sure, the capacity of nonhuman primates for fine control of hand grip is hardly less than that of humans; chimpanzees can even use pounding tools to open nuts. Arguably, however, human hands can perform tasks that are not possible for other primates. Perhaps the ultimate manual skill is a virtuoso performance of music. These higher human faculties are typically represented asymmetrically in the hemispheres of the brain, a topic of considerable clinical relevance.

This is one of the few books that synthesize progress in our understanding of the physiological and behavioral principles underlying manipulations of the hand. It does not include many new discoveries, nor does it address current research on the representations of the hand in widely distributed cortical and subcortical networks. The intended audience for the book consists primarily of neurologists, pediatricians, persons involved in rehabilitation, psychologists, and neurophysiologists.

Forssberg provides a useful overview of grasping during development. Changes may be observed until the age of six years or later, when the features of adult grasping are established. For example, the force for lifting an object (load force) and the grip force are less coordinated and the force traces not as smooth in young children as they are in adults. This lag has to be taken into account when one evaluates brain-damaged children. The rich supply of cutaneous and deep receptors is not only important for conscious perception, but is also essential for proper grasping. The configuration of the grip and the degree of its force depend critically on the form, weight, and surface texture of the grasped object. If the force is not strong enough, the object will slip, whereas too much force may destroy it. The actual force ratio (i.e., the ratio of grip force to load force) is carefully controlled and is normally only slightly higher than the ratio at which the object would slip out of the hand (i.e., a force ratio equal to the friction between hand and object).

We learn from experience and program the forces on the basis of an internal model (motor memory and representations), and we rely on vision for shaping the configuration of the hand when we are about to grasp an object. But the hand receptors are also essential and come into play to update the memorized representations. Besides the (unconscious) use of the hands in adaptive corrections of the grip, the hands are, of course, used for conscious perception of an object's properties (reaching to sense the immediate environment, a particularly important activity for normal development).

Because of the complexity of the hand, the functional changes that occur during development must often rely on some key features that can be assessed by videotape clips or simple behavioral measures. Newell and Cesari explore changes in the configuration of the grip for various objects as a function of body scale during development. De Manoel and Connolly illustrate nicely how goal-directed grasping tasks are solved by children of various ages. The younger child who has not yet acquired fine motor skills may nevertheless succeed with less effective strategies. Many manipulatory skills of everyday life require both hands. Fagard summarizes her pioneering work on the development of interlimb coordination during infancy.

The last four chapters of the book deal with problems in children who have particularly difficult disorders, such as the developmental coordination disorder, learning difficulties, and dyspraxia and dysgraphia. It is suggested that, particularly in these complex cases, evaluation of distinct functions such as the kinetics of grasping, the temporal coordination in reach-to-grasp actions, or interlimb coordination (the study of modular components) may help identify and quantify disordered functions.

Overall, I highly recommend this book on the hand and on its use in daily life. It is clearly not a comprehensive work, but it will help readers interested in pursuing research on the human hand. Much remains to be done on this fascinating topic.

Mario Wiesendanger, M.D.
University of Bern, CH-3010 Bern, Switzerland
mario.wiesendanger@insel.ch