Handbook of Language and Literacy Development - a Roadmap from 0 to 60 Months

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Hearing Development from Birth to Three Months - An Overviewclick to print Print
Research Review / Parent

Written by: Lori Leibold, The University of North Carolina at Chapel Hill

The onset of human hearing occurs during the second trimester of gestation, prior to the onset of vision (Gottlieb, 1991). As a result, infants are ready to process and respond to sound when they are born. Newborns are capable of recognizing voices, distinguishing their native language, and discriminating between speech sounds that differ in a single phonetic feature (e.g., Eimas et al., 1971). Despite these impressive early skills, young infants do not encode sound as precisely as adults. This coarseness in auditory processing limits the ability to make fine-grained distinctions between sounds during the first few months of life.

The Onset of Auditory Function - Hearing Begins In Utero

Infants experience sound before they are born. The inner ear starts to function by at least 24 weeks gestation. Electrophysical potentials and behavioral responses to sound have been recorded at 28 weeks gestation (reviewed by Werner, 1998). One remarkable consequence of prenatal exposure to sound is that it can influence later speech perception. For example, within hours of birth newborns can distinguish and visually orient to the voice of their mothers versus the voice of an unfamiliar woman speaking the same passage (DeCasper & Fifer, 1980).

Early Auditory Behavior - Responsiveness and Preferences

Infants display a variety of systematic behaviors in response to sounds. Examples of common behaviors that occur from birth to three months include eye widening, grimaces, starting and cessation of sucking, Moro (startle) reflexes, and arousal from sleep (e.g., Widen & O'Grady, 2002).

Infants respond to some sounds more than others. For example, young infants respond more consistently to speech than to narrowband sounds such as the pure tones used by audiologists for hearing examinations. Infants also prefer to listen to a specific type of speech called "infant-directed speech". Infant-directed speech, also called "motherese" or "baby talk", is characterized by several acoustic features, including a slower speaking rate and an expanded range of frequency and intensity changes. Researchers have suggested that adults might use infant-directed speech or motherese to compensate for infants' immature hearing (Werner, 2007).

Auditory Sensitivity - Rapid Improvements from Birth to Three Months

Auditory sensitivity is typically measured by assessing the absolute auditory threshold - the ability to detect a sound in quiet. Studies that have examined the development of auditory sensitivity have shown that young infants do not respond to sounds at the low intensities to which adults will respond. Threshold sensitivity improves dramatically from birth to three months (reviewed by Werner, 2002). Frequency-specific thresholds for one-month-olds are thirty to forty decibels higher than for adults. Thus, most one-month-olds are not able to hear very soft sounds such as rustling leaves or faint whispers. From one to three months, auditory thresholds improve by approximately ten decibels at lower frequencies and approximately twenty decibels at higher frequencies. Even at three months of age, however, infants' behavioral thresholds remain immature.

The rapid improvement in auditory sensitivity during the first few months of life does not appear to reflect development of the inner ear. Physiological responses to sound indicate mature cochlear function at term birth (e.g., Abdala, 2001).

An important contributor to infants' immature sensitivity is development of the middle ear. The anatomy of the ear canal and middle ear changes significantly during infancy and childhood. These changes include an increase in the size of the ear canal and middle ear spaces as well as changes in the density and orientation of tissues in the surrounding areas. As a result, conduction of sound energy through the middle ear and into the inner ear becomes progressively more efficient throughout infancy and well into childhood. For example, the amount of energy transmitted through the middle ear is about twenty decibels less for a one-month-old infant than for an adult at higher sound frequencies (Keefe et al., 1994). This means that a large amount of the acoustic power that reaches an infant's ear canal does not get transferred to the inner ear.

Determining the Location of Sounds

Infants must learn to locate and identify sounds originating from different locations in their environment. This localization must be mastered even though infants' small heads and ear canals limit the acoustic cues that are available to perform the task. Given these limited cues, it is not surprising that progressive improvements in the ability to detect changes in sound source location take place between birth and sixty months of age (reviewed by Werner, 1998).

It is clear, however, that even the youngest infants can localize sounds to some degree. Newborns will slowly orient to either the left or the right toward a sound source, provided adequate head support is provided. By one month of age, infants can distinguish the location of a sound source to an accuracy of about thirty degrees in angular position, compared with approximately four degrees for adults (Morrongiello et al., 1990).

Leibold, L. (2007). Research Review: Auditory Development 0 - 3 Months. In L.M. Phillips (Ed.), Handbook of language and literacy development: A Roadmap from 0 - 60 Months. [online], pp. 1 - 8. London, ON: Canadian Language and Literacy Research Network. Available at: Handbook of language and literacy development