San Francisco (dpa) - Noisy homes could be putting the mental development of babies at risk, scientists warn. New research indicates that constant background noise may swamp an infant's brain, holding back its ability to recognise sounds.

The findings might explain why language learning disorders have increased over the last few decades, it is claimed. Modern homes are often filled with a cacophony of noise from blaring TVs and radios, the rumble of traffic, jets flying overhead, and domestic machinery such as washing machines and fridges.

Being born into such a noisy world may sabotage the hearing centre of a baby's sensitive developing brain, according to a group of researchers in the United States. The scientists from the University of California at San Francisco reared young rats in an environment of moderate continuous noise.

Although not loud enough to harm hearing, the "whooshing white noise" was able to mask normal sounds. Usually, the hearing centre, or auditory cortex, in a growing rat's brain undergoes swift and radical reorganisation. The neurons cluster into a smaller area, and become more tuned into different pitches and patterns of sound.

The scientists found that in rats exposed to white noise, the auditory cortex took three or four times longer than normal to mature. Although the findings involve rats, they are likely to apply to humans since all mammals share similar basic patterns of brain development.

The researchers wrote in the journal "Science": "These findings suggest that environmental noise, which is commonly present in contemporary child-rearing environments, can potentially contribute to auditory and language-related developmental delays. However, further experiments showed that rats whose brain development was held back were able to catch up" once they left the noisy environment.

One of the scientists, Edward Chang said: "On the negative side, these findings suggest that noise can have devastating effects on the rate of development of the brain. They emphasise the importance that children, especially those at risk, be exposed to salient features in speech sounds in order for their auditory development to be normal.

"On the positive side, our findings may mean that the time frame may be longer in which treatment of such children will allow them to catch up."

The team is now to investigate whether people with developmental disorders are unusually vulnerable to noise. "If we knew that a child had a susceptibility to noise, we could intervene to enrich the child's acoustic experience to foster more normal auditory and language development," said Chang.

Copyright 2003 dpa Deutsche Presse-Agentur GmbH

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Last updated: 07/01/2003 - 08:13 PM