Hearing and Behavioral Responses to Sound in Full-Term Newborns

Abstract

NEWBORN HEARING Behaviors Indicating Hearing in the Newborǹ`Can the baby hear this sound?''``Can he appreciate the difference between this sound and that one?''``Does she recognize my voice?''`I s that noise keeping him awake?'' These and similar questions are of interest to those caring for newborn infants and also to researchers. The answers to them depend in part on the baby behaviors that adults attend to because human newborns, despite their large brains and fairly well-developed hearing, are limited in what they can do to indicate they have heard something. Researchers have, therefore, devoted considerable effort to defining newborns' behavioral responses precisely and to developing measurements that all but eliminate unintentional bias from behavioral observations. Humans at birth have reasonably good control over the muscles that govern eye movement. With careful support they also have control over the muscles that turn the head. These abilities coupled with an apparently innate tendency to coordinate vision with hearing lead newborns to orient toward (turn eyes or head or both toward) new or preferred sounds if the sounds last 1 second or longer. 1Term newborns also have a reliable blink reflex that can be used to test hearing for fairly loud sounds. Newborn infants have good control over the muscles involved in sucking and tend to suck faster or harder in response to a new or interesting sound Ð or sight. Additionally, they can indicate their attention to a sound with a package of body movements and facial expressions that a parent or trained observer can learn to identify reliably. Researchers have catalogued some of the details of newborn hearing by making careful measurements of these naturally occurring movement responses to sound. The observant parent or clinician can also use these behaviors to try to understand an individual newborn's responses to the sounds in his or her environment.Unfortunately, however, newborns as individuals and as a group are very variable. Their periods of alertness can be brief, they pay attention differently at different times, and they are easily distracted. Their 4-to 8-second lag before responding and slow or uncoordinated head-turn movements can lend the impression of not responding, particularly if the sound signal is shorter than 1 to 2 seconds. 2After several repetitions of the same, moderately loud sound they may habituate to it (tune it out) and stop responding altogether. Infants at birth may show less sensitive hearing than a few days afterward if there is amniotic fluid in the middle ear although this drains away or is absorbed within 2 or 3 days. Of course, newborns' responses will be atypical if they are not well or are affected by medications or street drugs that the mother has transmitted before birth. Moreover, animal studies indicate that some aspects of hearing may develop very rapidly in the immediate period just after birth so that responses to a particular sound at time A may be quite different from responses to the same sound at time B only a sho...

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A combined sonic crystal rod of Bragg type with ability of vibration reduction in broad frequency band

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