Facial and autonomic responses to biological and artificial olfactory stimuli in human neonates: re-examining early hedonic discrimination of odors.

Responses of awake and asleep 3-day-old human neonates were recorded to the presentation of artificial (vanillin, butyric acid, formula milks) and biological (breast milk, amniotic fluid) odorants matched on subjective intensity and trigeminal dimensions. The responses included behavioral (facial and oral movements) and autonomic (respiration, differential skin temperature) measures. The neonates reacted with significant facial and respiratory changes to low concentrations of olfactory stimuli during the various behavioral states. The analysis of olfacto-facial configurations revealed that behavioral markers of disgust (nose wrinkling, upper lip raising) discriminated between some odors judged as being pleasant and unpleasant by adult raters (vanillin vs. butyric acid). However, although some early predisposition to process the affective significance of stimuli may be suggested, no convincing evidence was obtained that neonates discriminated the hedonic valence of odors within the same perceptual space as adults. Finally, neonates evinced a differential pattern of respiratory responding to the presentation of milk odors according to the mode of feeding (breast vs. bottle feeding), suggesting that early olfactory discrimination may be mediated by stimuli with high ecological salience.