Under the Scope: In the Absence of Sound
BY JAMIE MORRISON, ASSOC. EDITOR
Deep thinkers continue to ponder the philosophical conundrum: "If a tree falls in the forest and no one is there to hear it, does it make a sound?" Adrin Rodrguez-Contreras, Ph.D., ponders just the opposite. He wants to know why we can hear a tree fall even if one isn't falling. Or more accurately: How is it that the auditory portions of the brain can be active even in the absence of sound?
An assistant professor at the City College of New York, Rodrguez-Contreras studies newborn rodents, which naturally are deaf the first 10 days after birth, to seek a greater understanding of how hearing develops in humans in utero.
"We know that all auditory connections are already there before animals can hear," says Rodrguez-Contreras. "You would think the brain must be silent during this period: The animal is deaf. But if you measure the electrical activity in the brain regions involved in auditory function during this pre-hearing stage, you find that the auditory neurons exhibit activity. It turns out that the nervous system has intrinsic ways of generating electrical activity in the absence of any sensory stimulation."
Rodrguez-Contreras' particular interest is in the olivo-cochlear cells in the brainstem. After sound proceeds through the mechanisms of the middle ear, it excites the hair cells of the inner ear, creating electrical impulses that get transmitted through the auditory nerve into the brainstem. The olivo-cochlear cells are just one of many groups of brainstem cells that are recipients of the electrical impulses originating in the inner ear.
The cochlea, located in the inner ear, contains both inner and outer hair cells. While the process is not yet fully understood, Rodriguez-Contreras and other researchers do know that the inner hair cells in the cochlea initiate electrical activity towards the brain, while the outer cochlear hair cells receive electrical activity from the brain, enabling two-way communication between the ear and the brain. One type of message the olivo-cochlear cells can convey from the brain to the hair cells is an instruction to focus on a particular sound. This is vital to our ability to attend to a particular voice in a roomful of talking people.
An increase over the last decade in the incidence of neurological diseases in humans is at the center of Rodrguez- Contreras' interest in the developmental aspects of the olivocochlear. Many researchers believe that these diseases originate in problems that occur in the late stages of prenatal development, when the fetus is about six months along. But the onset of the disorder may not be evident until several years after the child is born. While not directly related to hearing loss, autism is one of these. Another is auditory processing disorder, where a person has perfect hearing as far as hearing tests can determine, yet they still struggle to understand what is being said. The mechanisms of the ear work well but the problem lies somewhere in the brain where sound is processed. (Read "Auditory Processing Disorders in Children" from the spring 2010 archives at www.hearinghealthmag.com.)
"As we study the normal development of hearing," says Rodrguez-Contreras, "we can begin to identify developmental variables that can be monitored in individuals that could help us predict who is going to have a neurological deficit in several years. Newborns are currently screened for hearing, but we could extend a battery of testing to find if there is an electrical signature in the brain of a child that would suggest that we should keep a watchful eye on them. The more we understand how hearing develops, the better we will be able to know ahead of time what is coming."
Rodrguez-Contreras also sees potential future benefi ts from his research for people with tinnitus. "If you understand how the auditory system is activated in the absence of sound," he says, "you can begin to understand what happens in conditions like tinnitus, when people perceive a sound when there is no sound the ringing in the ears, or 'phantom noise' experienced by those with tinnitus."
Rodrguez-Contreras' research has been significantly enabled by the Deafness Research Foundation (DRF). "My laboratory started in 2008," he says, "so we are a very junior group. It is difficult for junior groups to obtain funding. So having entities such as DRF is of tremendous help for groups like us who are in need of funding to get things started." Born and raised in Mexico City, Rodrguez-Contreras studied biology at the National University of Mexico, where he met his wife, Jennifer, a Californian studying neuroscience. He completed his Ph.D. in biophysics at the University of Cincinnati and then conducted post-doctoral research at the University of California, Davis, and at the Erasmus University in the Netherlands.
With two boys, ages nine and four, Rodrguez-Contreras and his wife keep plenty busy, but he likes to swim whenever he gets a chance and also loves living in New York, where he can hear some of his favorite music, like jazz and alternative. He even plays a bit of guitar himself. And as he plays, if you ask him about the sound made by that tree falling or not falling in the forest, he'll be able to give you an astonishing answer.
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