Olfactory alliesthesia in human neonates: prandial state and stimulus familiarity modulate facial and autonomic responses to milk odors.

Abstract

This study examines the effects of a shift in the motivational state (from hunger to satiety) of human neonates on their behavioral and autonomic responsiveness to artificial and food-related odors as a function of stimulus familiarity. In Experiment 1, videotaped facial movements and autonomic (respiration rate: RR, heart rate: HR) responses to five olfactory stimuli (familiar regular formula, unfamiliar regular formula, protein hydrolysate formula, vanillin, control) are recorded in 3-day-old neonates (n = 14) during episodes of irregular sleep. The infants are tested on average 50 min. before and after bottle feeding. RR discriminates the odor stimuli from the control stimulus, indicating clear olfactory detection. Furthermore, neonates react with higher HR change only when exposed to their familiar formula milk during the postprandial condition. The measurement of facial movements with the Baby-Facial Action Coding System indicates that disgust and aversive actions are more often evoked by the odor of regular formulas (familiar or unfamiliar) than by the other olfactory stimuli during the postprandial condition. In Experiment 2, untrained adult observers, presented with the videotapes of the infants' facial responses to the odors, are able to decode differential hedonic signals from the sender faces as a function of the infants' motivational states. The present findings are in line with the concept of olfactory alliesthesia as defined in adults.