Newborn acoustic reflexes to noise and pure-tone signals.

A variable-frequency probe-tone acoustic-impedance bridge has been developed to enable an artifact-free pure-tone acoustic reflex study to be carried out on neonates. Contralateral reflex thresholds for pure tones, broadband noise, and filtered noise were measured in 28 newborns aged 4-8 days. The mean reflex threshold for the broadband noise was 73 dB SPL, 4 dB lower than the reflex threshold for the 2600-Hz low- and high-pass noise bands. Reflex activation at 500, 1000, 2000, and 4000 Hz gave responses that closely followed the normal adult pattern although reflex thresholds were approximately 10 dB higher. Two infants failed to demonstrate reflexes. One of these failed a behavioral-response test using the Auditory Response Cradle and was found to have mild jaundice. The second infant passed the behavioral test and had measurable reflexes 5 weeks later. A third baby with elevated reflex thresholds also was jaundiced but had normal behavioral responses and was discharged. Discussion of these results emphasizes their value to those engaged in neonatal auditory assessment.