Babies Have A Different Way Of Hearing The World By Listening To All Frequencies Simultaneously

"Babies have a different way of listening to the world," contends Werner. "In real life we are confronted with a variety of sounds. Somehow the adult brain takes all sounds we hear and separates them into where they are coming from and then focuses on the one we want to hear. Adults usually hear in a narrow band of sound, while babies seem to use a different approach. They don't have the selective attention of adults and they don't pay attention all of the time. Instead they always seem to be listening broadband or to all frequencies simultaneously."

Researchers have known for some time that babies are born with functioning hearing and that sensitivity to sound improves dramatically during infancy. Improvements continue through age 10 when the average child's hearing is comparable to an adult's.

To further understand how babies hear, Werner tested 73 infants ages 7 to 9 months and 40 adults 18 to 30 years old. All had normal hearing. They were individually exposed to four half-second bursts of a computer-generated 1000-hertz tone and a 1000-hertz broadband noise that sounded like telephone dial tone or static. Sometimes the tone or the noise was played alone and sometimes the sounds were masked with background noise. The sounds were played at different levels of loudness to see if the subjects could detect them. Subjects heard the sounds through foam earphone inserts that were placed in the right ear canal.

The computer randomly generated the four different types sound and the infants were trained to respond when they heard a sound. Infants were seated in their mother's lap in a test booth and an assistant kept the child attentive and entertained by manipulating silent toys on a table in front of the child. The assistant and mother wore headsets and listened to masking sounds to ensure that they could not hear any of the sounds presented to the child. An observer outside the booth watched and scored the child's responses through a window or on a video monitor. Infants were rewarded for responding to a signal by the activation of a mechanical toy. Typically babies turned toward the sound or changed their activity level when they heard something. Infant also responded by pronounced change in their facial expression or by looking at their mother.

Adults were tested in a similar fashion, sitting alone in the booth. They were told to raise a hand when they heard a sound that would activate the mechanical toy.

Werner found that on average babies are relatively better at detecting noise than tones. In the quiet condition the infant-adult difference in detecting noise was 14 decibels versus 7 decibels in the masked trials. A 15-decibel deficit in adults is the equivalent of a minor hearing loss, she said. With the background masking the infant-adult difference in detecting the tone was 10 decibels and 5 decibels for the noise.

In addition, Werner said preliminary examination of 11 infants' psychometric function for detecting broadband noise does not support the idea proposed by some scientists that babies pay more attention to broadband noise than to tone. Like 10 adults also tested, infants produce similar psychometric functions for broadband noise and tone detection.

"If you are a baby it is sensible to listen broadband, and it was valuable for our ancient ancestors for survival in the Serengeti (the Serengeti Plain of Eastern Africa)," says Werner. "But in today's western culture a baby is at a great disadvantage. All the noise we expose people to makes it difficult for babies. The practical lesson from this research is, if you are talking to a baby or reading her a story, background noise can be a problem. Turn off the television or radio."

Co-author of the study is Kumiko Boike, a UW doctoral student who did preliminary research as an undergraduate. The study was funded by National Institute of Deaf and Communicative Disorders, one of the National Institutes of Health.