Page 32 - Spring2022

FEATURED ARTICLE
Listening to Mom: How the Early Auditory Experience Sculpts the Auditory Cortex of the Brain
Patrick O. Kanold
Introduction
Our ability to understand a language is shaped by how we experience speech as a child. However, when auditory experience is important and how auditory experience acts on the different parts of the brain have been unre- solved. In particular, our experience with sounds starts before we are born, and many expecting parents wonder if early exposure to music or other stimuli can influence their developing child. Underlying our ability to hear is the precise wiring or circuitry between neurons in the brain. Auditory processing involves many interconnected structures, including the most complex auditory part of the brain, the auditory cortex. This is the region of the brain that is essential for the processing of complex sounds such as speech and music (Wang, 2018). Results from animal studies have started to reveal the influence of early sound exposure on circuits in the auditory cortex (Meng et al., 2021). These studies indicate that early sound experience, which in humans occurs in the womb starting around midgestation, already has the potential to shape auditory cortical circuits.
Thus, early sound experience or lack of sound experience, for example, in deafness, can potentially impact the brain before birth. Moreover, early insults to the developing brain (e.g., due to injury or exposure to drugs) might interfere with the early wiring processes, resulting in altered development. Moreover, these considerations are relevant for the care of prematurely born infants who are suddenly exposed to a different auditory environment in the neonatal intensive care unit (NICU).
Effects of the Auditory Experience
To show how early experience can shape hearing, I trace the steps occurring in the early development of the
auditory system and the influences of an early sensory experience on circuits in the auditory cortex. Hearing, or audition, is central to our ability to communicate. Under- lying the ability to identify and distinguish sounds, such as phonemes in languages or the identity of speakers, is the precise wiring between neurons in the auditory system. Hearing is shaped by the experience with sounds, and the effect of this experience is the largest in early childhood. This is illustrated by the ease with which a second language can be learned in childhood versus in adulthood as those of us who learned a second or third language have experienced. Therefore, early exposure to the sounds of a particular language is crucial for perceiv- ing subtle differences between words in that language. From this, it seems to follow that an enhanced audi- tory experience might be beneficial to the brain. Indeed, playing music or speaking during pregnancy has been popular; however, benefits of such enrichment are unknown. The critical questions to ask are when does the effect of sound experience start and which neurons and which brain circuits are influenced by the experi- ence of sound?
Auditory processing in humans starts in utero, but the effects of fetal sound experience has long been debated. Many parents wonder if playing music or singing to their unborn child will enhance brain function. A vari- ety of studies suggest that a sound experience shapes the fetal brain because fetuses or premature infants can distinguish speech sounds from nonspeech sounds and respond to maternal voices before term (40 gestational weeks [GWs]). For example, 35-GW-old fetuses have been shown to discriminate language (Minai et al., 2017) and newborns have a preference for the voice of their mother (DeCasper and Fifer, 1980) but not the father
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32 Acoustics Today • Spring 2022 | Volume 18, issue 1
https://doi.org/10.1121/AT.2022.18.1.32