When the noise stopped, the hearing system of the adult rats
continued to develop
Merit Sloin
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It has been known for many years about the need of the visual system for external stimuli for its development; Light stimuli stimulate the development of the brain areas associated with it. It is less clear how sound stimuli affect the development of the auditory system. Many researchers believe that understanding the relationship between sound stimuli and the development of the brain areas that respond to them will help explain the development of certain hearing problems.
A group of researchers from the University of California in San Francisco led by Michael Marznich decided to check if the auditory system, similar to the visual system, is affected by exposure to external stimuli. For the purpose of the study, the members of the group used rats. Although rats are not the perfect model for studying the development of the auditory system in humans, they make it possible to study the fundamental role of sensory experience in the mammalian auditory system.
The beginning of organizing the brain to receive sound stimuli from the environment begins in the rats around the 12th day of their life. After that, their cerebral cortex is noticed to be intensely active in a large area that responds to sound stimuli, especially to high sound frequencies. This activity is accompanied by the appearance of nerve cells, each of which responds to a wide range of frequencies. After that, during a critical period of about three weeks, the auditory area in the cerebral cortex undergoes extensive organization, following which its final structure takes shape, responding to a wide spectrum of sounds. The area of the "primary auditory area of the cerebral cortex", called A1 for short, is reduced and every part of it responds to a narrow range of frequencies, including low frequencies.
The research group from the University of California decided to test how external acoustic conditions affect the development of the auditory cortex.
To do this, they raised rat pups with a weak and continuous noise. The noise did not harm the hearing, but it muffled the natural sounds heard in the environment and prevented the puppies from clearly hearing the voices coming to them from their mother, siblings and other objects in the environment. The researchers mapped the organization of the auditory cortex in the rats exposed to the continuous noise and compared it to the organization of this cortex in rats raised under normal conditions. The mapping method was based on recording the responses of the nerve cells in the auditory cortex to a variety of sounds played to anesthetized rats.
The researchers, who published their findings in the journal "Science" found out that in the rats that were exposed to noise, the A1 area did not develop and remained the same as in 16-day-old puppies. Further tests found that in the rat pups that grew up in noisy conditions but were removed at some point from the noisy environment and moved to normal conditions, the auditory cortex maintained its flexibility and continued to develop. In the next step, rat pups were exposed to continuous noise until they reached adulthood (at the age of 50 to 90 days), and then they were removed from the noisy environment. Even in these rats, the auditory system, whose development was arrested at an early stage due to the noise conditions, continued to develop as soon as the noise was stopped. The organization of the auditory cortex in these rats was completed a long time after the end of the organization in the rats in the control group, but in both cases the result was the same.
According to the results of the study, it seems that the time window in which the auditory cortex is able to organize is quite wide. "It is as if the brain is 'waiting' for meaningful sounds to continue its development, and when it is finally exposed to them - it reacts. The process also happens when the animal is older," said Edward Cheng, one of the researchers.
The research findings demonstrate for the first time at the cellular level the harmful effect of noise on the development of the auditory cortex. In addition to the congenital hearing impairments, there are babies who develop hearing impairments as a result of fluids in the middle ear, which weaken the volume of sounds reaching the inner ear. Other babies develop hearing impairments as a result of growing up in a very noisy environment. Doctors treating babies with hearing impairments must identify the impairments as early as possible, so that they can begin treatment in the window of time when the auditory cortex develops. The new study raises the possibility that this time window is longer than previously thought.
The brain savant
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