Joseph J. Campos
University of Denver
Psychophysiological responses seem to be especially well suited to the study of the human infant. First, they are extremely sensitive to changes in psychological state, often more so than behavioral measures. The electroencephalogram (EEG), for example, can differentiate states of sleep that are not readily discriminable from behavioral observations alone ( Rechtschaffen & Kales, 1968). Second, physiological responses are nonverbal and so are of particular benefit in studies of nonverbal subjects, such as infants. Third, they have an impressive history in studies of learning, habituation, psychophysics, and emotion. As a result, the parameters affecting these responses are often better known than the parameters affecting less studied responses. Fourth, they can be used in longitudinal studies in ways in which overt, coordinated behavioral responses cannot, either because the physiological response is already functional at birth or because the emergence of the response reflects important biological milestones [for example, the development of the EEG K complex ( Emde, Gaensbauer, & Harmon, 1976)].
Many physiological responses have been used in infant studies; of all of these, heart rate (HR) has proved the most sensitive and reliable. In this chapter, some of the major research areas in which HR has been fruit