OBJECTIVE: To determine stimulus level effects on speech-evoked cortical auditory evoked potentials (CAEPs) in infants for a low (/m/) and high (/t/) frequency speech sound.METHODS: CAEPs were recorded for two natural speech tokens, /m/ and /t/. Participants were 16 infants aged 3-8months with no risk factors for hearing impairment, no parental concern regarding hearing or development, and normal tympanograms and otoacoustic emissions. Infants were either tested at levels of 30, 50, and 70dB SPL or at 40, 60, and 80dB SPL, in counterbalanced order.RESULTS: Input-output functions show different effects of increasing sound level between stimuli. There were minimal changes in latency with increase in level for /t/. For /m/, there were approximately 50-60ms latency increases at soft compared to loud levels. Amplitudes saturated at moderate-high levels (60-80dB SPL) for both stimuli.CONCLUSIONS: Infants' CAEP input-output functions differ for /t/ versus /m/ and differ from those previously reported for adults for other stimuli. Effects of stimulus and level on CAEPs should be considered when using CAEPs for hearing aid or cochlear implant evaluation in infants.SIGNIFICANCE: Speech-evoked CAEPs provide an objective measure of central auditory processing. Possible differences in CAEP growth between infants and adults suggest developmental effects on intensity coding by the auditory cortex.
Acoustic Stimulation Auditory Cortex