Interaural time coincidence detectors are present at birth: evidence from binaural interaction

Binaural processing of sounds in mammals is presumably initiated within the auditory nuclei of the caudal pons. The binaural difference waveform (BD) can be derived from the sum of the waveforms evoked by right monaural clicks plus left monaural clicks minus the waveform evoked by binaural clicks. In adults, the BD's first positive peak (beta) is large only for stimuli with interaural time differences (ITDs) that produce a fused acoustic percept. Humans at birth can localize and discriminate sound sources, but their head circumference is about two-thirds of an adult head. In order to test whether beta is related to head circumference, we recorded beta in human neonates as a function of ITD. Binaural clicks with ITDs ranging between 0 and 1000 micros were used to derive BD waveforms in 34 neonates. For ITD=0, beta was detectable in 56% of newborns. The incidence of beta detection then decreased as ITD increased. Only 9% of the babies had detectable beta for all ITDs. No correlation was found between the existence of beta and other properties of the monaural or binaural auditory brainstem response. The finding that for some infants beta was present for all ITDs up to 1.0 ms suggests that there is no recalibration of brainstem delay lines with head growth. Our data suggest that the brainstem auditory pathway for detecting interaural time differences in the adult is probably present at birth. Maturational factors such as increased myelination and greater firing synchrony probably improve the detectability of beta with age. The second peak in the BD waveform (delta) was highly correlated with the existence of wave VI in the binaural and monaural waveforms.