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

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Hearing from Four to Six Months - An Overviewclick to print Print
Research Review / Parent

Written by: Lori Leibold, University of North Carolina

Between four and six months of age, babies start to code sound in a precise way. As newborns just a few months ago, speech and other sounds were a little "fuzzy". As six-month-olds, the basic properties of sound are coded almost as well as for adults. This fast development means that these babies are now much better prepared to learn about sounds. Despite this improved sound coding, many aspects of hearing are not fully developed.

Auditory Behavior - Consistent and Reliable Responses to Sound

It is much easier to test hearing in babies that are five- to six-months of age than in younger babies. Audiologists are health-care professionals who are trained to assess hearing difficulties. Audiologists routinely measure hearing in babies over about five months of age using a procedure called Visual Reinforcement Audiometry (VRA). VRA takes advantage of babies' natural instinct to turn their heads towards sounds. When the head turn responses to sound are rewarded by an entertaining mechanical toy or short video, babies will respond long enough for audiologists to measure the softest sounds they can hear at different frequencies or pitches. For example, Audrey's parents were asked to take her to the local audiology clinic for a hearing examination when she was six months old. They were surprised at how quickly she learned to turn her head towards the dancing bear when she heard sounds and were delighted that she had so much fun during the test.

Researchers use similar procedures to study hearing in babies. Most babies seem to really enjoy coming to the laboratory for the experiments and will continue to respond to sounds for twenty to thirty minutes. This is plenty of time for researchers to learn something valuable about how hearing changes during infancy.

Primary Processing

In order for babies to hear and understand complex sounds like speech, the ear must be able to provide the brain with a precise code of the basic properties of sound. These basic properties are intensity, frequency and temporal resolution. The ability to code the basic properties of sound matures early in life. Researchers have shown that six-month-olds encode sound almost as accurately as adults. The basic properties of sound are presented next.

Intensity Coding - Auditory Sensitivity

From four- to six-months, babies' become more sensitive to soft sounds. Just three months ago, Nico did not respond when his father whispered to him when he was out of sight. Now six months old, Nico's father has to be quiet when he enters his room to check on him as he is trying to fall asleep. However, six-month-olds still do not hear the softest sounds that adults and older children can hear. The reason that very soft sounds remain difficult for babies to hear is that their ear canal and middle ear are still growing.

Frequency Coding

Another basic property of sound coding is the ability to tell the difference between sounds that differ in frequency or pitch. Researchers call this ability frequency resolution. Precise frequency resolution is very important for understanding speech and music. For example, Audrey would not be able to separate her father's voice from background sounds like noise from the dishwasher if she had poor frequency resolution. Frequency resolution appears to be developed by the time babies are about six months old.

Temporal Coding

Temporal resolution is the final basic property of sound coding and refers to our amazing ability to follow very brief changes in sound over time. Fast and accurate temporal resolution is critical for perceiving the intonation and rhythm of speech (prosody). Researchers have shown that temporal resolution develops early in life, likely by at least three months of age.

Sound Localization

In the period between four and six months, babies become much better at localizing sounds in their environment. However, they continue to have difficulty figuring out the location of sounds that are close together in space.

Six-month-olds can also judge the distance of some sounds. In one clever study, researchers had babies reach for objects that made sound, such as a rattle. After they played with the objects for a while, the researchers turned out the lights. When the objects could be heard but not seen, the babies could tell the difference between objects that could be reached compared to objects that were not within reaching distance.

Leibold, L. (2007). Parent/Cargeiver Narrative: Auditory Development 4 - 6 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