& Cairns, 1970). Although Stayton et al. ( 1973) have reported differential crying to the mother and a stranger in the second quarter year of life, the percentage of subjects showing this differentiality is small. More significant results may emerge through the use of HR.
Several other emotions might be fruitfully indexed with HR. Bower 1971), for example, found HR to be sensitive to manipulations that induced surprise in young infants of various ages. The interesting finding in animals of marked bradycardia, cited earlier, may illuminate the psychophysiology of infant depression following maternal loss (such as still occurs in humans, unfortunately, after late adoptions, divorce, or maternal death). Laughter and smiling are still other emotional responses that may yield interesting relationships with HR.
For years, psychologists have postulated physiological activation as one of the defining criteria of emotion. Although none of the research we have discussed suggests that HR change is specific to any given emotion, or to emotions in general, it seems clear that under certain circumstances the sensitivity of the HR response under emotionally stimulating circumstances has been demonstrated. It remains for future research to document the applications and limitations of this response. Nevertheless, it seems a fair conclusion at this time to suggest the addition of cardiac response measurement to the tools of the student of infant emotion.
This research has been supported by grants MH 19053 and MH 23556 from the National Institutes of Mental Health, by Training Grant MH 08297, and by faculty research funds awarded by the Graduate School of Arts and Sciences, University of Denver.
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