The Psychobiology of Sensory Coding

William R. Uttal

• Publisher: Taylor & Francis
• Year: 2014
• Print ISBN: 9781848724297
• eBook ISBN: 9781317669005
• Edition: 1
• Topic: Psychology
• Subtopic: Cognitive Psychology & Cognition
• Index: Psychology

Originally published in 1973, this book deals with what were, even at that time, the well-known neural coding processes of the sensory transmission processes. The book was written to demonstrate the common features of the various senses. It concentrates on the most peripheral neural aspects of the senses starting with the physical transduction process and culminating in the arrival of signals at the brain.

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CHAPTER 1: AN ORIENTATION

I. INTRODUCTION

A. Psychology, Physiology, and Psychobiology

This book is about the acquisition and transmission of information patterns from the external environment to the central nervous system, where presumably these patterns become the substance of our mental life. It deals with a subject matter that thus transcends the conventional borders of neurophysiology and psychology as well as those of related disciplines such as anatomy and physics. From this diversity of substance has come both the richness of content and the major difficulty in the preparation of a satisfactory text of this sort. It is impossible to conceive of a single volume that would be capable of comprehensively covering the broad spectrum of theory and data that now make up such a vast literature. This present book, therefore, is selective and incomplete in a number of important ways. It has not been possible to comprehensively cover either the phenomena of sensory psychology or the abundant detail of sensory physiology to the depth desired. On the contrary, this book is built on a single axiom, and only material specifically relevant to that axiom has been included. That axiom states that the external world is represented within the organism by patterns of neural activity that, while the patterns themselves differ greatly from the external world in terms of the physical energies involved, do maintain some sort of an equivalence of pattern or organization, if not an isomorphism, of neural activity. The persistence of patterns of information, independent of the physical medium or specific neural dimension in which they exist, is the essential idea of the notion of neural coding.

To concretize this idea, let us consider a specific example of what it is that we mean by equivalence and isomorphism. Consider the problem of auditory pitch. Physical signals, varying primarily in the temporal characteristics of waves of compression and rarefaction of air pressure, are interpreted as sounds of different pitches by a listener. The original signal information is, in this example, in the form of the temporal pattern of the air wave. But as we shall see, these temporal information patterns are encoded largely by spatial distributions of activity in the inner ear. Thus, while the spatial neural patterns convey the same information and are hence equivalent, they are in no sense isomorphic (of the same shape), nor do they even possess the same dimensionality as the stimulus. Rather, they are encoded representations of the original pattern.

The concept of coding has in recent years come to be, for sensory studies in particular, the contemporary expression of a continuing intellectual perplexity: how does the body and its parts relate to, represent, or in some way provide a receptacle for the self-awareness that every man has of his own existence? We shall detail in later parts of this chapter the specific nature of this new outlook, but for the moment let us consider the problem of mind-body relationships in their more classical sense. The problem has been considered by some thoughtful philosophers (see, for example, Feigl, 1958) to be a question inadequately stated and inappropriately asked. Certainly, when one considers the fact that the intrapersonal self-awareness that all men seem to possess is not subject to interpersonal examination and comparison, the perplexities of how one can handle the problem of the construction of an internally consistent science of psychology appear substantial. But before one should be discouraged from attempting the task, it should be remembered that in fact most of our sciences—physical, biological, as well as behavioral—share this same difficulty. The physics of nuclear structure, for example, is a science in which the components under investigation can never be seen directly. We can only infer their existence (and transient indeed they may be) from the tracks of a bubble or spark chamber or even more abstractly from the necessity to balance an energy equation.

Astronomy shares this same dilemma. It appears that the facts surrounding the speed of light as an upper limit for the movement of either physical objects or energetic waves through space make travel to distant galaxies just as unlikely as a direct observation of one man’s awareness by another. If the theory that there exists an upper limit of the speed of light is sustained in the future, we shall always be forced to limit our observations of the nature of distant universes to those measurements that can be made on the ancient light signals that have been traveling toward us in some cases for millions of years. It should be remembered that what we may be observing with telescopes now may long ago have ceased to exist. This temporal dilemma, at least, is a problem not faced by psychology. Psychology, a science whose proper content is the set of the inner awarenesses which Feigl (1958) refers to as Raw Feels, is therefore in no way unique from the point of view of invisible subject matter.

Psychology, as a science, has responded to the limitations on interpersonal comparison by emphasizing behavioral measures. This is often so, it seems to some, to the exclusion of the subject matter itself. But even though it seems clear that the great initiative for studying awareness or consciousness comes from the individual’s self-awareness, nevertheless, in order to provide a reasonable foundation for replicable and organizable observations, it is to the behavior of others that we must turn for our experimental data. This methodological twist often obscures the fact that behavior is not the subject matter of the psychological sciences, but it is rather only an approach to the real content, which is represented by such symbolic terms as consciousness, awareness, thought, perception, or any of the other terms that have gained momentary popularity over the centuries. In the same vein, who would classify astronomy as either the study of telescopes or the study of ancient light? The objects of attention of most astronomers are planets, stars, and galaxies and not the instruments or measurements necessary to acquire information about these objects. Similarly, the objects of attention of psychology are thought, perceptions, sensations, and other aspects of conscious experience.

The other major approach to sensory science is the neurophysiological one. The objects of concern to this science are the processes that are performed by the specialized tissues and cells of the nervous system. From our particular frame of reference, that of the problem of sensory communication, the objects of attention are the physiological processes of the highly specialized nervous tissues responsible for the transduction, transmission, and integration of incoming information patterns. The scientific accomplishments of the last half century have made it quite clear that it is the brain and the other components of the nervous system to which we must look for a reductive explanation of psychological acts. It may be somewhat surprising to realize how recently in human history this idea gained a solid foundation. We still are all too apt, in the vernacular at least, to refer to the “heart” as the seat of emotional awareness.

There is another idea which has also been current in our modern infatuation with electrophysiological techniques. This idea is that a physiological investigation of the activity produced in the nervous system by peripheral stimuli is sufficient in itself to explain the coding processes of sensory systems. Such a notion is probably untrue. A substantial number of misconceptions have been introduced into our understanding of the mind-body problem by this overattentive concern with physiological recording. On the other hand, it is important not to minimize the enormous contributions that have been made using these techniques when they are placed within a psychological framework.

It is also true, however, that the psychological approach is, by itself, incapable of providing a definitive answer to problems of sensory coding. In fact, because of the intrinsic limitations of the two techniques when used separately, a combined approach has become more and more popular in recent years. This approach has gone under many names—biopsychology, neuropsychology, or physiological psychology, but we have chosen the term psychobiology as the one that seems most appropriate to the emphasis we shall place on the physiological substrates of sensory experience. Psychobiology represents a merger of techniques from both the psychological and the physiological laboratories. Indeed, we see today behavioral techniques being used by physiologists, and electrophysiological recording proficiency developed to a high level among people originally trained in psychology. It is often difficult to distinguish the original training of a practitioner of the new combined art.

The adoption of new techniques in either field reflects the increasing tempo of the interdisciplinary trend in many other sciences. But more important, in this particular field, it reflects the emerging awareness of the significant conceptual pressure that an integrated psychobiology of sensory activity exerts on our perspective of the classic mind-brain controversy. My aim in this book is to tie together the work of the psychologist and the neurophysiologist into a coordinated approach to the problems of sensory experience. Some have pointed out that the psychological approach is a necessary precursor to electrophysiological studies so that the neurophysiologist will know what there is to look for. Neurophysiologists have properly pointed out the inability of the psychological techniques to solve the problems of sensory representation within the “black box” of the closed organism. In fact, both points are correct, and just as a sensory psychology text that ignored the neurophysiological contributions would be meaningless in the terms of today’s knowledge, similarly a text of sensory neurophysiology would be incoherent if it ignored the massive contributions of the psychophysical laboratories.

B. The Scope of “Sensory” Sciences

It is appropriate at this point that we attempt to define the field of inquiry with which we shall be concerned. Sensory processes in biological systems are defined, in large part, by an intrinsic directionality of the flow of information. The sensory system is designed, as we have said, to pick up patterns of information from the external world, transduce this information from any of a number of physical forms of energy to the electrochemical forces of neural activity, and transmit that pattern toward the complex central portions of the nervous system. The nature of the subsequent processing of sensory information by the higher nervous centers in the exquisitely complicated ways cataloged by the cognitive descriptors evolved by psychologists will not, by intent, be among the topics we shall consider. It is, of course, not possible to draw a sharp line of demarcation between the content of this book and that of these other areas of subjective activity. Indeed, as our understanding progresses, the line should be pushed further and further upward.

A cogent argument can be made that the “simpler” sensory processes, with which we shall concern ourselves, are in reality no simpler than the more complex processes. They may represent only a subset of cognitive functions which, for a number of reasons, are more susceptible to experimental control and probing. Furthermore, we must recognize that in defining the field to be covered by this book, we are specifically faced with the knotty problem of distinguishing between those subject matters that are usually classified as sensory and those that are usually classified as perceptual. It seems most appropriate to ignore any real difference between the two. Rather, for the purposes of this volume, we shall make a very artificial distinction. We shall talk about the sensory phenomena that are closely related to well-defined stimulus patterns and known or plausible neural mechanisms. We shall simply exclude from our discussion those more complex perceptions that are apparently based upon such complex neural interactions and weakly defined stimulus conditions that the reductive aspects of our science have not yet come to grips with them. For example, we shall not deal with the well-known fact that familiarity enhances word recognition. There is no physiological theory that can even begin to account for such a phenomenon.

The conceptual and anatomical straightforwardness of the sensory functions has been their great asset in establishing their preeminence in the reductionism of modern psychobiology. We understand information processing better in sensory systems in large part because they are much simpler in both anatomical and functional organization than the systems underlying, for example, such processes as problem solving. They are in some cases so simple that they can be considered characterizations or abstractions of the operation of more complex and general neural nets, just as simple invertebrate “model” nervous systems often help us to understand mammalian neural mechanisms. For this reason, through the study of sensory processes, we are often able to advance our knowledge not only of the immediate problems surrounding sensory coding, but also of the possible modes of action of the more complicated mechanisms.

We shall be looking at the sensory systems mainly in terms of their function of communicating information from the periphery toward the central nervous structures. This has been called, by some, communication in the ascending direction (the pun being associated with the notion of the “higher” centers of the brain), even though in some cases the “ascending” nerves may actually travel downward during at least part of their route. Others have used the term afferent emphasizing the inward-going direction of these communication links as distinguished from the efferent lines of the effector or motor systems. The word sensory is always used in this book as a synonym for either input, ascending, or afferent and not—as some would choose—as a synomym for simple perceptions. Thus, it is entirely consistent to speak of sensory neurophysiology or sensory coding without referring to the psychological connotations. However, this simple categorization is inadequate to handle some recent data. We shall be concerned with information processing between and among units of the afferent pathways in ways that require awareness of such concepts as feedback and reverberation. In fact, we shall also be concerned with some signal pathways that are intimately tied to the sensory systems but are patently efferent or descending. We refer here to observations which indicate that there is centrifugal (from the center) activity which operates to powerfully modulate the sensory input pattern.

From a more psychological point of view, the material with which we shall be most concerned will deal with those relatively simple awarenesses of relatively simple stimulus patterns. Psychophysical techniques are in a rapid stage of metamorphosis as the traditional techniques of measuring the responses to brief and transient unitary stimuli give way to more interestingly designed experiments with more elaborately patterned stimulus situations. But in spite of this, there is a clearly defined group of experimental paradigms, which will be seen to fit the problem area we have chosen for this book more closely than others involving other more complex, although equally interesting, stimuli. The key test of relevance will be: is a given sensory or perceptual phenomenon an input function with a plausible neural explanation?

As we have said, the main goal of this book is to integrate the two sets of data from psychology and neurophysiology into a unified discussion. In addition to selecting a circumscribed topic area—sensory processes—for consideration, we shall also be making a number of implicit assumptions in the development of this book, which should be overtly stated as fundamental premises. One of these premises is our point of view concerning the nature of the interaction between the psychological and the neurophysiological. Our approach is one that would certainly be classified within the general rubric of monisms. It is, explicitly, an expression of the belief that sensory activities, and for that matter all psychological activity, are ultimately reducible to the terms, linguistic and conceptual, of patterns of neural activity.

In addition to the premise of monism, which underlies our attempt to integrate the two approaches to the study of sensory processes—the psychological and the neurophysiological—we shall also attempt a synthesis at another level of discussion. Traditionally (Troland, 1930; Geldard, 1953; Wyburn, Pickford, and Hirst, 1964), sensory textbooks have been organized in chapters, which discuss one sensory modality at a time. Although Osgood (1953) and Corso (1967) do partially deviate from this tradition, we usually see conventional chapters on vision separated from those on audition, which are likewise separate from those on the chemical or somatic sensations. It is a further premise of the present work that there are general principles of sensory organization that can and should be emphasized independent of a specific modality. The processes of transduction or of transmission, for example, have features common to all the senses, and these commonalities become self-ev...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Original Title Page
• Original Copyright Page
• Dedication
• Table of Contents
• Preface
• Chapter 1. An Orientation
• Section One—Fundamental Materials
• Section Two—Sensory Coding
• Suggestions for Further Reading
• Bibliography
• Index