Startle modulation during the exhibition of spontaneous emotional facial expressions in 5-month-old infants

Background and Aims: The startle response is an innate defensive reflex, characterised by an instantaneous motor reaction following an unexpected and intense stimulus (Yeomans, Li, Scott, & Frankland, 2002). Evidence from studies of animals and human adults demonstrates that startle response is systematically modulated by organism’s affective states. It has been proven that startle is potentiated during the processing of unpleasant foreground stimuli and inhibited during the processing of pleasant foreground stimuli. The affective modulation of startle response has been interpreted in terms of the mediating effects of motivational systems in the brain (Lang, 1995). Yet, despite the importance of the study of this reflex in various research fields (pshycophysiology, neuropsychology, etc), the effects of motivational brain systems on startle response are still relatively unexplored in early infancy. The aim of the present study is to investigate whether at 5 months the startle response is affectively modulated during the spontaneous exhibition of positive (smiling) and negative (distress) facial expressions. Methods: Eleven 5-month-old infants were involved in the study. The experimenter presented a brief acoustic noise burst to elicit the reflex when the infant spontaneously exhibited, during the interaction with his or her mother, a smiling expression, a distress expression or a neutral facial expression. The facial expressions were coded offline using Baby FACS (Oster, 2008). Infants’ whole body startle responses were measured by a new non invasive measurement methodology: Automated Infant Motor Movement Startle Seat (AIMMSS). Moreover the Eye-Blink Strength (EBS) video coding system was adopted to describe the strength of the eyeblink to the startle probes (Essex, Goldsmith, Smider, Dolsky, Sutton, & Davison, 2003). Results: There was no startle modulation during the exhibition of smiling and distress expressions. Instead both the AIMMSS and the EBS coding system data showed an unexpected result: a startle modification during the neutral conditions following the two facial expressions. Specifically, the response was enhanced in the neutral condition subsequent to a distress expression and inhibited in the neutral condition subsequent to a smiling expression [AIMMSS magnitude: F (1, 41) = 4.367, p = .043; EBS: F(1, 32) = 4.680, p = .038]. Conclusion: The data obtained induce to hypothesize that the facial expressions act like a sort of motivational priming able to diversify, depending on their negative (distress) or positive (smile) nature, the intensity of the startle response elicited immediately after their exhibition.