Developmental psychologists often use physiological measures to assess variables such as stress, hormones, heart rate, and skin conductance (Berntson & Cacioppo, chap. 12, this volume). Typically, these variables are measured along with observations or self-reports of a similar construct (e.g., a stress index is taken before or while Cortisol levels are measured). In terms of their advantages, physiological and biological methods are extremely useful for measuring psychological processes that individuals are unable to report (e.g., emotional reactivity in young children). They also are relatively free from social desirability biases. Disadvantages of these methods include the cost of instruments and time spent editing and interpreting the data. Additionally, carryover effects may threaten internal validity because of previous stimuli responses affecting subsequent reactions and not accurately reflecting the individual's homeostatic state (Rothbart, Chew, & Gartstein, 2001). Moreover, physiological responses often can be due to a variety of factors, including movement and the temperature in the room. Nevertheless, researchers using psychophysiological methods have shown relations between measures of biological variables and behavior problems, aspects of temperament, and coping strategies (Rothbart & Bates, 1998).
To illustrate the use of physiological measures in developmental psychology, we discuss the measurement of reactivity and regulation and how these variables are typically measured. Major components of reactivity and regulation are the underlying physiological processes involved in arousal and its management, and both the experience of an emotion and its regulation are partly linked to the autonomic nervous system (Porges, Doussard-Roosevelt, & Maiti, 1994). Researchers of autonomic correlates of emotion have focused primarily on two branches of the nervous system: the sympathetic branch, which mobilizes the body to react in an emergency, and the parasympathetic branch, which conserves and maintains bodily resources. The parasympathetic branch works to regulate and decrease emotional arousal and usually counteracts the activation of the sympathetic branch, which is responsible for arousal (Gottman, Katz, & Hooven, 1997). Researchers typically examine sympathetic activation in the following ways: through skin conductance levels (SCL), which indicate how much an individual sweats, and through measures of cardiovascular reactivity (CVR).
Research on parasympathetic activation/regulation has relied primarily on indices of cardiac activity. Respiratory sinus arrhythmia (RSA), heart-rate variability that occurs at the frequency of breathing, is believed to be controlled by the vagus nerve and is thought to provide a good estimate of parasympathetic influence (although some heart rate measures and programs believed to tap parasympathetic functioning, often called vagal tone, have not actually measured respiration). Baseline cardiac vagal tone or RSA has been associated with differences in infants' appropriate emotional reactivity (e.g., Stifter, Fox, & Porges, 1989), as well as with the ability to adapt to a new situation such as preschool (Fox & Field, 1989). Moreover, high RSA has been found to relate to infants' attentional abilities (e.g., Porges, 1991; Stifter et al., 1989).
Another physiological measure of emotion/reactivity involves the assessment of electroencephalo-graphic (EEG) patterns (i.e., brain activity). Researchers using EEG patterns to assess the physiology of reactivity or regulation often examine activation in the frontal lobe and compare activation in the right versus left hemispheres. Individuals with right frontal asymmetry (less activation in the right hemisphere compared to the left) are more likely to exhibit negative affect in response to stress (Fox et al., 1995). In addition, effortful, voluntary regulation has been linked to prefrontal cortical responding (Casey et al., 1997). Further, initial evidence indicates that asymmetries in activation of the frontal cortical lobes are linked to approach versus withdrawal tendencies (Tomarken & Keener, 1998); thus, brain waves (EEG) may be associated with effortful and/or reactive approach or inhibition systems (Harmon-Jones & Sigelman, 2001). In summary, psychophysiological methods represent the future for research of constructs such as emotion regulation and reactivity because they are invaluable tools for linking unobservable internal feeling states and arousal, and processes involved in their modulation, to their behavioral manifestations.
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