The auditory sensitivity of preterm infants toward their atypical auditory environment in the NICU and their attraction to human voices

Introduction: Vulnerable preterm infants (PIs) hospitalized in neonatal intensive care units (NICUs) are exposed to environmental stimuli that differ from the stimuli fetuses encounter in utero. Their auditory environment, in particular, is atypical. This new sensory “milieu” may interfere with their neurodevelopment. Aims: This review briefly summarizes the anatomical and functional development of the auditory system. We describe the abilities of preterm infants to perceive sounds emerging from background noise, based on studies appropriately designed to assess their auditory sensitivity to the NICU sound environment. Conclusions: Neurophysiological and neuroimaging studies, as well as specific auditory tests, demonstrated the anatomical and functional development of the auditory system of PIs. These infants showed fine auditory capabilities before term-equivalent age. PIs in the NICU react physiologically and behaviorally to sounds exceeding 70 dBA and are able to detect and discriminate among sounds emerging from background noise at a minimum signal-to-noise ratio of 5-10 dBA. PI responses to sound depend on sound source and frequency, as well as on sound pressure levels. Loud, high-frequency, and artificial NICU sounds may alter their well-being and disrupt their sleep. Vocal sounds seem to elicit a different pattern of responses. PIs seem particularly sensitive to the biologically meaningful and attractive sounds of their mothers’ voices. The ability of PIs’ perceptual experience of NICU sound ecology to shape their long-term neurodevelopment has not yet been determined, although associations between their early auditory experience and later ability to communicate have been observed.