The body is the medium of experience and the instrument of action. Through its actions we shape and organize our experiences and distinguish our perceptions of the outside world from sensations that arise within the body itself. (Miller, 1978, p. 14)
Anatomy, physiology, and psychophysiology are all branches of science organized around bodily systems with the collective aim of elucidating the structure and function of the parts of, and interrelated systems in, the human body in transactions with the environment. Anatomy is the science of body structure and the relationships among structures.
Physiology concerns the study of bodily function or how the parts of the body work. For both of these disciplines, what constitutes a body part varies with the level of bodily organization, going from the molecular to cellular to tissue to organ to body system to the organism. Thus, the anatomy and physiology of the body are intricately interrelated. Neuroscience, in particular, stands at this intersection.
Psychophysiology is intimately related to anatomy and physiology but is also concerned with psychological phenomena - the experience and behavior of organisms in the physical and social environment. The primary distinctions between psychophysiology and behavioral neuroscience are the focus of the former on higher cognitive processes and the interest in relating these higher cognitive processes to the integration of central and peripheral processes. Among the complexity added when moving from physiology to psychophysiology are the capacity by symbolic systems of representation (e.g., language, mathematics) to communicate and to reflect on history and experience; and social and cultural influences on physiological response and behavior. These factors contribute to plasticity, adaptability, and variability in behavior. Psychology and psychophysiology share the goal of explaining human experience and behavior, and physiological constructs and processes are an explicit and integral component of theoretical thinking in psychophysiology. The subject matter of psychophysiology, after all, is an embodied phenomenon.
The technical obstacles confronting early studies, the importance of understanding the physiological systems underlying observations, and the diverse goals and interests of the early investigators in the field fostered a partitioning of the discipline into physiological/measurement areas. The organization of psychophysiology in terms of underlying physiological systems, or what can be called systemic psychophysiology, remains important today for theoretical and pedagogical reasons. Physiological systems provide the foundation for human processes and behavior and are often the target of systematic observation. An understanding of the physiological system(s) under study and the bioelectrical principles underlying the perceptual and output responses being measured contribute to the plausible hypotheses, appropriate operationaliza-tions, laboratory safety, discrimination of signal from artifact, acquisition and analysis of the physiological events, legitimate inferences based on the data, and theoretical advancement.
Like anatomy, physiology, and psychology, however, psy-chophysiology is a broad science organized in terms of a thematic as well as a systemic focus. The organization of psychophysiology in terms of topical areas of research can be called thematic psychophysiology. For instance, cognitive psychophysiology concerns the relationship between elements of human information processing and physiological events. Social psychophysiology concerns the study of the cognitive, emotional, and behavioral effects of human association as related to and revealed by physiological measures, interventions, and consequences including the reciprocal relationship between physiological and social systems. Developmental psychophysiol-ogy deals with ontological changes in psychophysiological relationships as well as the study of psychological development and aging using physiological measures. Clinical psychophysiology concerns the study of disorders in the organismic-environmental transactions and ranges from the assessment of disorders to interventions and treatments. Environmental psychophysiology elucidates the vagaries of organism-place interdependencies as well as the health consequences of design through unobtrusive physiological measurements. And applied psychophysi-
In each of these areas, the focus of study draws on, but goes beyond, the description of the structure or function of cells or organs, to investigate the organism in transactions with the physical or sociocultural environment. Some of these areas, such as developmental psy-chophysiology, have counterparts in anatomy and physiology but refer to complementary empirical domains that focus on human experience and behavior. Others, such as social psychophysiology, have a less direct counterpart in anatomy or physiology because the focus begins beyond that of an organism in isolation; yet the influence of social and cultural factors on physiological structures and functions, and their influence as moderators of the effects of physical stimuli on physiological structures and functions, leaves little doubt as to the relevance of these factors for anatomy and physiology as well as for psychophysiology. Meaney and colleagues (Meaney, Bhatnagar, Larocque, McCormick, Shanks, Smythe, Viau, & Plotsky, 1996), for instance, provide evidence that rat pups who are ignored by their mothers develop a more reactive hypothalamic pituitary adrenocortical (HPA) axis than rat pups who are licked and groomed by their mothers.
Because psychophysiology is intimately related to anatomy and physiology, knowledge of the physiological systems and responses under study contribute to both theoretical and methodological aspects of psychophysio-logical research. However, knowledge of the physiological systems is logically neither necessary nor sufficient to ascribe psychological meaning to physiological responses. The ascription of psychological meaning to physiological responses ultimately resides in factors such as the quality of the experimental design, the psychometric properties of the measures, and the appropriateness of the data analysis and interpretation. For instance, although numerous aspects of the physiological basis of event-related brain potentials remain uncertain, functional relationships within specific paradigms have been established between elementary cognitive operations and components of these potentials by systematically varying one or more of the former and monitoring changes in the latter.
The point is not that either the physiological or the psychological perspective is preeminent, but rather that both are fundamental to psychophysiological inquiries; more specifically, that physiological and psychological levels of organization are complementary. Inattention to the logic underlying psychophysiological inferences simply because one is dealing with observable physiological events is likely to lead either to simple and restricted descriptions of empirical relationships or to erroneous interpretations of these relationships. Similarly, "an aphysiological attitude, such as is evident in some psychophysiological research, is likely to lead to misinterpretation of the empirical relationships that are found between psychophysiological measures and psychological processes or states" (Coles et al., 1986, ix-x). Thus, whether organized in terms of a systemic or a thematic focus, psychophysiology can be conceptualized as a natural extension of anatomy and physiology in the scientific pursuit of understanding human processes and behavior. It is the joint consideration of physiological and functional perspectives, however, that is thought to improve operationalization, measurement, and inference and therefore to enrich research and theory on cognition, emotion, and behavior.
Early definitions of the field of psychophysiology were of two types. One emphasized the operational aspects of the field such as research in which the polygraph was used, research published by workers in the field, and research on physiological responses to behavioral manipulations (e.g., Ax, 1964b). Other early definitions were designed to differentiate psychophysiology from the older and more established field of physiological psychology or psychobi-ology. Initially, psychophysiology differed from physiological psychology in the use of human in contrast to animals as participants, the manipulation of psychological or behavioral constructs rather than anatomical structures or physiological processes, and the measurement of physiological rather than behavioral responses (Stern, 1964). Although this heritage can still be found, this distinction is often blurred by the fact that psychophysiologists may modify physiology with drugs or conditioning procedures, and psychobiologists often manipulate psychological or behavioral variables and measure physiological outcomes. Contemporary definitions are more likely to emphasize the mapping of the relationships between and mechanisms underlying psychological and physiological events (e.g., Hudgahl, 1995; Stern, Ray, & Quigley, 2001).
A major problem in reaching a consensus has been the need to give the field direction and identity by distinguishing it from other scientific disciplines while not limiting its potential for growth. Operational definitions are unsatisfactory for they do not provide long-term direction for the field. Definitions of psychophysiology as studies in which psychological factors serve as independent variables and physiological responses serve as dependent variables distinguish it from fields such as psychobiology but have been criticized for being too restrictive (Furedy, 1983). For instance, such definitions exclude studies in which physiological events serve as the independent/blocking variable and human experience or behavior serve as the dependent variable (e.g., the sensorimotor behavior associated with manipulations of the physiology via drugs or operant conditioning, or with endogenous changes in cardiovascular or electroencephalographic activity) as well as studies comparing changes in physiological responses across known groups (e.g., the cardiovascular reactivity of offspring of hypertensive vs. normotensive parents).
Moreover, psychophysiology and psychobiology share goals, assumptions, experimental paradigms, and, in some instances, databases, but differ primarily in terms of the analytic focus. In psychophysiology the emphasis is on integrating data from multiple levels of analysis to illuminate psychological functions and mechanisms rather than physiological structures per se. All of these substantive areas have a great deal to contribute to one another, and ideally this complementarity should not be masked in their definition by the need to distinguish these fields. Indeed, the formulation of structure-function relationships is advanced to the extent that "top down" and "bottom up" information can be integrated. The emergence of areas of research in cognitive neuroscience, psychoneuroendocrinology, and psychoneuroim-munology raise additional questions about the scope of psychophysiology.
Anatomy and physiology encompass the fields of neurology, endocrinology, and immunology due both to their common goals and assumptions, and to the embodiment, in a literal sense, of the nervous, endocrine, and immunologic systems within the organism. Relatedly, psy-chophysiology is based on the assumptions that human perception, thought, emotion, and action are embodied phenomena; and that measures of physical (e.g., neural, hormonal) processes can therefore shed light on the human mind. The level of analysis in psychophysiology is not on isolated components of the body, but rather on organismic-environmental transactions. That is, psy-chophysiology represents a top-down approach within the neurosciences that complements the bottom-up approach of psychobiology. Thus, psychophysiology can be defined as the scientific study of social, psychological, and behavioral phenomena as related to and revealed through physiological principles and events in functional organisms. Thus, psychophysiology is not categorically different from behavioral neuroscience, but rather there is currently a greater emphasis in psychophysiology on higher cognitive processes and on relating these higher cognitive processes to the integration of central and peripheral processes.
In the following section, we review some of the major historical developments that have influenced contemporary thinking and research in psychophysiology. As might be expected from the discussion thus far, many of these early developments have stemmed from studies of human anatomy and physiology.
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