Neural discrimination of emotional prosody in natural sentences in 2- to 3-month-old infants using OPM-MEG
From birth, infants are sensitive to the emotional prosody of speech, yet it remains unclear when the developing brain discriminates emotions conveyed by natural spoken sentences. Here, we recorded brain activity with optically pumped magnetometer magnetoencephalography (OPM-MEG) in 15 infants aged 2-3 months while they listened to semantically neutral sentences spoken with happy or angry prosody. Natural sentences elicited robust auditory evoked responses, with prominent components at 88, 216, 380 and 1,356 ms after sentence onset and sources centred on bilateral superior and middle temporal cortex. Time-resolved multivariate pattern analysis revealed that distributed neural activity discriminated recordings spoken with happy from angry prosody 346-454 ms after the onset of the prosodically prominent word (the first word carrying a pitch peak), despite no difference in global field power between conditions. This discrimination was transient, emerged before the end of the sentence, and was carried primarily by phase-locked delta-band activity, followed by later differences in induced alpha power. Angry prosody also delayed the late auditory response by approximately 15 ms with no detectable changes in amplitude. Our findings show that cortical responses to happy and angry prosody in natural speech differ by 2-3 months of age, and demonstrate that OPM-MEG combined with time-resolved decoding can reveal the temporal, spatial and spectral organisation of early emotional-prosody processing.