Auditory processing of complex tones in newborn infants
Pitch extraction from complex acoustic stimuli during neonatal period in infants was examined in this study. It is one of the most fundamental auditory processes. Pitch processing is crucial for speech intonation and musical melody perception. Preferences of newborns for “motherese” (or infant directed speech), which has different pitch characteristics compared to adult directed speech, suggest an early pitch information processing capability in newborns. Although auditory functional development in the auditory system at early ages draws much attention of researchers from different disciplines, the number of studies during the neonatal period is limited. We first asked how pitch information processing develops. It was hypothesized that newborns already possess ability to process pitch information early during perinatal period. Second, we hypothesized that the extraction of pitch information in acoustic stimuli depended on the integrity of the auditory pathway. Brain insult in the perinatal period has been shown highly likely to affect subcortical structures, including the auditory brainstem and midbrain, through different mechanisms. We reasoned that if there were delays in or problems with pitch processing in brain-injured or premature neonates when compared to healthy or premature infants of equal gestation, then, in the absence of peripheral disturbances, evidence for CNS mediation of the development of pitch processing could be argued.
Two types of auditory brainstem responses were studied: (1) auditory brainstem evoked responses (ABRs) to click stimuli, and (2) the frequency following response (FFR) to complex tones were utilized to study the above questions in N=128 premature and term infants assigned to the Neonatal Intensive Care Unit (NICU) at birth, all at varying risk for a CNS injury (mean gestational age at birth=34±3.6 weeks).
In summary, we confirmed that auditory pitch information processing of complex sound was present in newborn infants as early as 32 weeks gestation. Neonatal FFR studies provided positive evidence of responses to the pitch-related information both in the envelope-related frequency as well as in the difference tone and stimulus component-related frequencies. Similar responses at younger and older ages at test indicated that this capability remains stable across age during the first month of life. Moreover, NICU infants with evidence of a structural perinatal brain injury showed impairment in this type of auditory processing.