The emotional modulation of the startle reflex in 9-month-old infants
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The purpose of this research project was to investigate whether the startle reflex of 9-month-old infants can be modulated by emotional stimuli, as well as to examine the specific characteristics of infants' startle reactions in an emotion-modulated paradigm. Two studies were conducted to address these questions. In Study 1, 32 9-month-old infants viewed photographs of happy, neutral, and angry facial expressions. Infants' startle responses to acoustic probes during the presentation of the facial stimuli were recorded and compared across the three affective conditions. Autonomic and looking time data were also gathered in order to evaluate the contribution of other factors, such as attention, to the modulation of the startle reflex. The results of this study indicated a pattern of startle modulation opposite to that documented in adults. Infants demonstrated a potentiated startle reflex during the viewing of happy faces and an inhibited response during the viewing of angry faces. Differences in heart period and looking time between the affective conditions suggested that these findings were driven, at least in part, by greater allocation of attentional resources to angry expressions.
To further examine the role of emotion in infants' startle modulation, an independent group of 25 9-month-old infants was tested in a second, modified emotion-modulated startle paradigm that involved the presentation of acoustic startle probes while infants were engaged in a pleasant game of peek-a-boo, an affectively neutral presentation of a spinning bingo wheel, and a mildly frustrating arm restraint episode. Autonomic and behavioral data were also gathered. As expected, the results revealed startle potentiation during the unpleasant condition and startle inhibition during the pleasant condition, indicating the existence of the emotion-modulated startle reflex in 9-month-old infants.
Results of both studies are discussed in terms of the role of emotion and attention in startle modulation, the maturation of the appetitive and defensive brain systems in infants, and the importance of establishing a rigorous and age-appropriate startle paradigm to foster the study of infants' emotionality. Suggestions for further studies utilizing such a paradigm to investigate different aspects of emotional reactivity in infancy are also proposed.