Early preference for positive over negative prosody in neonates: Evidence based on event-related potentials

Our ability to process emotional prosody, that is the emotional tone of a speaker, is fundamental to human communication and adaptive behaviours. Very early in development, vocal emotional cues are more critical than facial expressions in guiding infants' behavior. However, the processing of emotional prosody in the very early days of life is still far from clearly understood. It is unclear whether the discrimination between prosodies with different emotional categories is present at birth. Furthermore, it is unknown whether there is a preferential orientation (negativity bias versus positivity preference) in neonates. Here, we used event-related potentials (ERPs) to examine the ability of neonates (from 1 to 6 days old) to discriminate different types of emotions conveyed by speech prosody. The experiment was conducted in the neonatal ward of Peking University First Hospital, Beijing, China. Electroencephalogram recording was carried out when the infants were in a state of active sleep. Using an oddball paradigm, the current study investigated the neural correlates underlying automatic processing of emotional voices of happiness, fear and anger in 18 (Experiment 1) and 29 (Experiment 2) sleeping neonates. In Experiment 1, each category of emotional prosody (20%) was separately mixed into emotionally neutral prosody (80%), forming three blocks with different emotions. In Experiment 2, we not only repeated the procedure of Experiment 1, but also reversed the standard and deviation stimuli in the odd-ball task. Event-related potential data showed that the frontal scalp distribution (F3 and F4) of the neonatal brain could discriminate happy voices from both angry and fearful voices; the mismatch response (MMR) was larger in response to the deviant stimuli of happiness, compared with the deviant stimuli of anger and fear. However, the latter two stimuli, i.e., angry and fearful voices could not be differentiated. The MMR amplitudes at the other four electrodes, i.e., C3, C4, P3, and P4 did not show significant differences across emotional conditions. Note: the MMR is a prototype of the mismatch negativity, i.e. a preattentive component of the auditory ERP that shows a positive (MMR) or negative (MMN) displacement in response to deviant sounds compared to standard sounds in the oddball paradigm. The neural responses recorded here indicate very early preference for positive over negative stimuli, which is contrary to the 'negativity bias' phenomenon established in the affective prosody literature of adult and infant studies. It is suggest that the range-frequency hypothesis could help to interpret the transformation from the 'positivity preference' during the first half year of life to the 'negativity bias' later in development. The present finding provides the first neuroelectrophysiological evidence for the hypothesis of positivity preference in neonatal participants. In addition, this special discrimination between positive and negative prosody in early life may provide a foundation for later emotion and social cognition development.