Lateralized processes constrain auditory reinforcement in human newborns.

We investigated operant sucking response learning in human newborns. Auditory reinforcers always occurred monaurally to see whether their potency differed between ears. Experiment 1 - we controlled the reinforcers, either intrauterine heartbeat sounds or unfamiliar speech, while infants chose which ear received it. Experiment 2 - we controlled the reinforcers and the receiving ear. Unfamiliar speech reinforced learning only if infants could use their right ear and heartbeats reinforced learning only if infants could use their left ear. Experiment 3 - we controlled the ear while infants chose between their mothers' vs. a stranger's voice and between their mothers' vs. a foreign language. The more familiar speech reinforced learning only if infants could use their left ear. We proposed reinforcers' potencies differed between ears because the newborn's auditory system, just like adult's, optimizes their perceptual clarity by left-lateralized processing of their rapid temporal variations and right-lateralized processing of their longer-lasting spectral characteristics.