PREFACE

Imagine what life would be like if you couldn’t move. Unable to speak, activate your arms, walk, or even glance around, it would be impossible for you to gesture, transport yourself from one location to another, reach for objects, communicate with others, or see. The prospect of being paralyzed is terrifying. But by considering paralysis, even briefly, one recognizes how vital movement is for all that we do.

Movement is just one of the functions of the motor system. The other is stability. Maintaining balance and resisting mechanical disturbances are prime examples of stabilizing behaviors. When the motor system is impaired, stabilization becomes difficult. In the worst case, behaviors that we take for granted—the ability to ride a subway, walk through a thickly weeded forest, or hold an upset baby—are all but impossible.

This book is an introduction to research on human motor control. The volume is mainly written for students still gaining an introduction to behavioral science. In teaching about motor control to a wide range of students, I have been struck by the fact that many important and interesting topics have so far been presented only in highly technical terms. The sight of frustrated students struggling to read this material has convinced me that they need an approachable, friendly introduction to the field.

To help you feel comfortable with the material at hand, I have organized the book in a modular fashion. Chapter 1, “Introduction,” sets the stage for the field at large, stressing the main questions to be addressed, the types of answers to be sought, and the methods used to pursue them. Chapter 2, “Physiological Foundations,” describes the physiological approach to motor control, emphasizing neural substrates. Chapter 3, “Psychological Foundations,” is concerned with a more abstract level of analysis—one not explicitly concerned with the physical basis of performance. In my teaching, I have found that students unfamiliar with scientific psychology somtimes have difficulty seeing that a psychological model can be a bona fide scientific explanation. Chapter 3 shows how serious research can be done, and important advances can be made, using psychological techniques.

The next seven chapters contain the bulk of the book’s material. Each chapter is devoted to a distinct set of activities in which motion and stability are key. The chapters are titled “Walking,” “Looking,” “Reaching and Grasping,” “Writing and Drawing,” “Keyboarding,” “Speaking and Singing,” and “Smiling.” These titles are meant to exemplify rather than encompass the behaviors they suggest. Thus, the chapter on walking includes a discussion of running, the chapter on smiling includes a discussion of scowling, and so forth. I have organized the chapters by activities because much of the work in human motor control has been compartmentalized this way. Devoting separate chapters to distinct activities allows attention to be given to the unique methods and concerns that have arisen in each area. Treating each activity separately also makes it easy to look for similarities among subsystems while scouting for differences.

The last chapter, “Conclusions,” wraps things up. It also describes two integrative activities (hand-eye coordination and hitting approaching balls), the nature of individual differences, and several new approaches that are likely to play an increasingly important role in the years ahead.

Although this book is primarily meant for students, it should also be useful for nonstudents whose work or recreation touches on motor control or its ramifications. Motor control impinges on a large number disciplines. Some are defined almost entirely with respect to movement and stability: Physical education, physical therapy, occupational therapy, prosthetics, dance, kinesiology, and biomechanics. Other disciplines have vested interests in understanding motor performance, even if movement and stability are not their sole province: Medicine, sports, musical performance, the fine arts, psychology, physiology, human factors, ergonomics, robotics, engineering. Because I want this book to be an approachable introduction to the entire field of motor control, I hope it will be useful to people in all these areas.

I am a psychologist, and as such want to comment briefly on the special importance that this book may have for my colleagues. Psychology is the science of mental life and behavior. Yet psychologists have paid remarkably little attention to the physical substrates of behavior (by which I mean the control of bodily motion and stability). The reasons for this neglect are not entirely obvious. I have heard it said that motor control can only be studied with highly technical appratus (not true), that one should start studying stimulus-response connections by studying stimulus processing (a view that assumes that we react to the world but do not act on it), and that when one studies motor performance, one is at the whim of the experimental participant (not if one is a skillful experimenter). In my view, the neglect of motor control by psychologists ultimately derives from a belief that motor control takes very little intelligence, even viewing intelligence in broad computational terms. Moving and stabilizing the body are, after all, done as well by animals as by people. If one wants to understand how humans differ from animals (an aim bequeathed by Descartes and embraced by many psychologists), then one might as well focus on those aspects of behavior (such as language) which clearly demarcate our species from others.

One message I want to communicate to psychologists is that motor control requires highly sophisticated computational and cognitive machinery. Were this not true, modern robots might now be able to pick berries, put band-aids on scraped knees, climb trees, or amble over rocky terrains. These tasks, which most 5-year-olds can do reasonably well, cannot be achieved by robots today. This outcome is ironic when one recalls that contemporary computer systems can play chess at nearly grand-master level, can give credible legal advice, and can diagnose illnesses as well as skilled physicians (within particular domains of medical practice). So it is not the case that motor performance, or perceptual-motor integration, is computationally undemanding. On the contrary, it is so demanding that we are only beginning to develop the tools to understand it.

I might add that it is incorrect to assume that motor performance provides no lines of demarcation between human and nonhuman species. Besides speech, movements with purely symbolic functions like pointing or drawing appear to be exclusively human. Likewise, physical behaviors which are defined relatively arbitrarily by human culture and which usually take enormous amounts of time to learn well—such as playing the piano, performing pirouettes, or weaving—appear to have no counterparts in the animal kingdom.

There is an excellent chance that the computational machinery underlying perception and action predated the evolution of human language and “high-level” intellectual competence. If this assumption is correct, then understanding the computational basis of perception and action should help us understand the higher mental capacities as well as perception and action themselves. Psychology should therefore benefit from the study of motor control and in turn contribute to the explication of motor function.

Like any book, this one has limitations. First, it contains relatively little information about anatomy or biomechanics. Historically, motor control has been pursued without much attention to these topics, though this situation is now changing. To reflect the way the field has been pursued, and to present what I know best, I have concentrated on the psychological and neural aspects of motor control.

Second, though the book is mainly organized by activity, some activities are not covered. Digestion, copulation, and excretion are omitted, for example, not because they are unsavory, but because their analysis has been offstage. Some other activities are omitted or discussed only in passing because they probably are assembled out of more basic motor elements. Bowling, somersaulting, pole-vaulting, and flipping pizzas are all wonderful maneuvers to watch and try, but until their constituent actions are understood, their holistic control will likely remain unappreciated.

Third, this book is not meant to be an exhaustive treatment of motor control. Exhaustive treatments are exhausting for readers and (especially) for authors. I have omitted a number of studies that make points that I feel are too detailed, too uncertain, or too technical for this kind of text. (Some of the studies on this list are my own). The currency of some of the material may also be less than what one would ideally like. In preparing a volume of this kind, one finds oneself between a rock and hard place with respect to the inclusion of new material. On the one hand, one wants to present the latest word on everything. On the other hand, doing so keeps the book from ever being completed. One of my most frustrating experiences has been to discover in the library or in a package mailed from a helpful colleague some exciting new theory or experimental result which bears on a topic I have just written about and sent to the publisher. In such circumstances, I have often had to...