Infant Cortical Auditory Evoked Potentials to Lateralized Noise Shifts Produced by Changes in Interaural Time Difference

Objectives: Newborns reliably orient to sound location soon after birth; by age 1 month this orienting disappears until after age 4 months. It has been suggested that orienting by the newborn reflects subcortical-mediated reflexes, which are suppressed by age 1 month; reappearance of orienting then occurs after age 4 months with maturation of cortical mechanisms of sound localization. In the present study, we assess auditory lateralization in young infants (and adults) by recording slow cortical auditory evoked potentials to lateralization shifts in dichotic noise produced by changes in interaural time difference (ITD). Design: Fifteen normal infants aged under 4 months (mean = 10.7 weeks) had cortical auditory evoked potentials assessed in response to (1) diotic "onset" noise bursts (0 msec ITD) and (2) shifts in continuous lateralized noise (75 dB SPL) produced by ITD shifts of 0.5, 0.8, 1, 2, 4, and 8 msec. Shifts alternated between ears occurred every 2 sec. Stimuli were presented using insert earphones; infants slept during recordings. For comparison, similar recordings were obtained in 11 normal-hearing, awake, adults. Additionally, "control" recordings to the ITD-shift stimuli presented to only one ear were obtained in the adults. Results: Similar to previous research, adults showed clear N1-P2 responses to the lateralization shifts (ITD 0.5 to 2.0 msec). Responses decreased for longer ITD shifts, with no adult responses to the 8-msec ITD shift. N1 latencies to ITD-shift stimuli were 28 to 34 msec longer than to the onset stimuli. No responses were seen in the control conditions when ITD-shift stimuli were presented to only one ear (confirming the binaural nature of the ITD-shift responses). All infants showed P2 responses to one or more of the ITD-shift stimuli up to ±1 msec; compared with adults, infants showed larger amplitude decreases and fewer responses to longer ITD-shift stimuli. As was seen with the adult responses, infant response (P2) latencies to ITD shifts were longer compared with their responses to the onset stimuli; however, these increases, 32 to 78 msec, were significantly longer than those seen in the adults. Conclusions: Young infants (even as young as 5 weeks) show clear evidence of auditory cortical responsivity to lateralization shifts produced by changes in the ITD of continuous noise, indicating that they have the capacity to process binaural ITD timing cues well before the age of 4 months. Further research is required to determine whether the larger latency increase in infants for ITD-shift stimuli (relative to the onset stimuli) and the greater effect of longer ITD shifts on response presence and amplitude in infants reflects immaturity of lateralization processing and/or reduced responses recorded during sleep. Slow cortical auditory evoked potentials elicited to lateralization shifts in dichotic noise provide a method to investigate binaural hearing processes in young children with normal or impaired hearing.

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