Olfactory alliesthesia in human neonates: prandial state and stimulus familiarity modulate facial and autonomic responses to milk odors.
Level 4 - case-series / case-control
Small observational/experimental within-subject physiological case series
PubMed 10397891 · doi:10.1002/(sici)1098-2302(199907)35:1<3::aid-dev2>3.0.co;2-f
What was done
In Experiment 1, researchers evaluated 14 three-day-old neonates during episodes of irregular sleep approximately 50 minutes before and 50 minutes after bottle feeding (hunger vs. satiety). Infants were exposed to five olfactory stimuli: familiar regular formula, unfamiliar regular formula, protein hydrolysate formula, vanillin, and an odorless control. Investigators recorded respiration rate, heart rate, and videotaped facial movements scored with the Baby-Facial Action Coding System. In Experiment 2, untrained adult observers viewed the videotaped facial responses to determine if they could decode hedonic signals related to the infants' prandial states.
What was found
Respiration rate differentiated odor stimuli from the control stimulus, confirming olfactory detection. Following feeding, neonates exhibited greater heart rate change specifically when exposed to familiar formula odor. Facial movement coding showed disgust and aversive actions were evoked more often by regular formula odors (familiar or unfamiliar) than by other olfactory stimuli in the postprandial condition. Adult observers successfully decoded differential hedonic signals from the infants' faces according to hunger versus satiety states. The abstract reports no numerical values, effect sizes, or p-values.
Why it matters
This study provides evidence that olfactory alliesthesia—the modulation of sensory pleasantness by metabolic state—is functional in human neonates as early as 3 days of life.
Limits
The neonatal sample size was very small (n = 14), and the number of adult observers in Experiment 2 was not reported in the abstract. Testing was conducted exclusively during irregular sleep rather than active waking states. No numerical data, confidence intervals, or exact statistical values were provided in the abstract.