Affective influences on the startle response in three-and-five-month-old infants

The modifying effects on startle parameters of affective dimensions, such as positive and negative valence, have been revealed repeatedly in studies that employed a variety of experimental procedures. These studies demonstrated that the magnitude as well as the onset latency of the startle response varies as a function of motivational drive and emotional state. One major difficulty for developmental studies of affective modulation of the startle reflex is to find stimuli that are developmentally appropriate and ethically acceptable for inducing emotions in infants. A second major difficulty is to reliably measure the startle reflex. EMG of Orbicularis Oculi activity represents a preferred methodology for startle measurement in adults but it is considered intrusive and moderately invasive especially during the very early months. The main goal of a series of studies we are running is to examine viable solutions to both these problems: (1) investigating whether smiling and distress expressions (coded by using Baby FACS) spontaneously exhibited by three- and five-month-old infants during mother-infant interaction might be employed to index the activation of the motivational systems in the brain that are supposed to affectively modulate the reflex in adults; (2) exploring a new and non-invasive procedure for whole-body startle measurement in infancy. In two independent studies, we found that at five months the infants who displayed a prevalent negative emotional activation exhibited a greater startle magnitude than the infants characterized by a prevalent positive emotional activation. This result is consistent with adult pattern of startle affective modulation as well as with EMG data reported by Balaban (1995) in five-month-old infants. Unexpectedly, at the age of 3 months we found an opposite data trend.