Researchers Alter Basic Sound Processing Rate In Rats, Offering Insights Into Mechanisms Of Dyslexia, Other Disorders

December 03, 1998

Researchers at UC San Francisco report that they have been able to significantly increase the speed with which adult rats process sound, offering important new evidence that the basic rate at which the brain responds to information can be sharply altered by experience.

Their findings, reported in the December issue of Nature Neuroscience, pave the way for future studies aimed at understanding, and perhaps manipulating, the mechanisms that cause the slow sound-processing difficulties associated with language impairments and dyslexia in children, and such neurological disorders as autism and stroke. More broadly, the observations offer some of the first evidence of the mechanisms that control the brain's 'plasticity' in its reception of information in high speed.

Scientists have known for nearly two decades that the adult brain is capable of rearranging itself to adapt to new conditions, as demonstrated in the growth and strengthening, and loss and weakening, of connections that develop between neurons throughout life. But in the current study, the UCSF researchers have demonstrated that the basic speeds with which the individual neurons of the brain process sound can be altered. And the findings indicate, they say, that neurons that respond to stimuli other than sound could be altered as well.

"Understanding the mechanisms involved in processing speeds and processing rates could aid in the remediation not only of language impairments and possibly dyslexia, but also of any neurological disorder that could be treated by improving the efficiency with which the neurons of the cortex repair, or reorganize, themselves, including damage to the central nervous system," said the lead author of the study, Michael P. Kilgard, PhD. Kilgard is a post-doctoral fellow in the laboratory of senior author Michael Merzenich, PhD, the Francis A. Sooy Professor of Otolaryngology at UCSF.

The finding is particularly pertinent for children with language impairments and a common form of dyslexia, the researchers say. That overall disorder, characterized in children by difficulty in accurate speech reception and reading, appears to result in large part from an inability to process rapidly successive sounds with normal 'processing' speed, which can result in the child perceiving blurred speech sounds. Slower processing rates hypothetically result in weaker than normal phonics abilities because they don't allow words to be parsed into sound parts with normal efficiency.

Notably, the UCSF findings in rats parallel observations that Merzenich and colleagues have recently made in dyslexic children who participated in a computer training program designed to improve the accuracy with which they received high-speed speech. The program motivates children to practice making distinctions between rapidly presented sounds. It is based on the idea that intensive training can be used to restore even severely degraded high-speed brain-processing capacities. After a little more than a month of practice using this program, most children in the study improved by one to two grade levels in their language abilities (Science, vol. 271 pp 77-81, 1996).

The study in rats demonstrates why the computer program works, the researchers say. While in the study in children they relied on external stimuli to stimulate the internal change, in the rat study the investigators directly targeted the neurons that signal the importance of sensory stimuli. These neurons are found in a structure of the brain known as the nucleus basalis. When the nucleus basalis detects a sound that it deems pertinent, such as the cry of a child, rather than, say, the rush of the wind, it dispatches acetycholine, a chemical signal, into the cortex, signaling it to 'save' its representation of that sound.

In their study, the researchers electrically stimulated the nucleus basalis and simultaneously exposed the animals to rapidly presented sound. The sound was a train of tones presented at fifteen beats per second. (Before beginning their study, the researchers determined that the rats' auditory neurons could not keep up with a train of tones presented at more than 12 sounds per second.) In rats, researchers have determined, the cortex requires about a tenth of a second to recover from one sound before it is ready to respond again. The human cortex appears to function similarly.

In the rats, the researchers repeated the combination of electrical stimulation of the nucleus basalis and the exposure to the sounds for several hours each day for a month. The responses of individual neurons were then recorded and compared to neurons from unstimulated animals.

Significantly, although the vast majority of neurons in normal animals did not respond to trains of tones presented at 15 beats per second, the neurons in stimulated animals were able to keep up without difficulty.

The researchers went on to show that they could also train the neurons to process sound more slowly than normal, pairing the electrical stimulation with a rate of sound of five beats per second. In many cases, the maximum rate of response was cut in half.

"This study again shows that the maximum cortical processing rate is not set in stone," said Merzenich. "It clearly demonstrates that the cortical processing rate is plastic, and directly subject to manipulation by intensive learning. It is consistent with growing evidence that processing accuracy and processing rate that lead to normal or impaired abilities in children can arise from early-childhood experiences, and can likely be very positively changed by appropriate intensive training in impaired populations."

The study was funded by grants from the National Institutes of Health, Hearing Research Inc. and the National Science Foundation.

University of California - San Francisco

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