Category: Hearing Loss

Scientists at Georgia State University have found that the ability to hear is lessened when, as a result of injury, a region of the brain responsible for processing sounds receives both visual and auditory inputs.

Yu-Ting Mao, a former graduate student under Sarah L. Pallas, professor of neuroscience, explored how the brain's ability to change, or neuroplasticity, affected the brain's ability to process sounds when both visual and auditory information is sent to the auditory thalamus.

The study was published in the Journal of Neuroscience.

The auditory thalamus is the region of the brain responsible for carrying sound information to the auditory cortex, where sound is processed in detail.

When a person or animal loses input from one of the senses, such as hearing, the region of the brain that processes that information does not become inactive, but instead gets rewired with input from other sensory systems.

In the case of this study, early brain injury resulted in visual inputs into the auditory thalamus, which altered how the auditory cortex processes sounds.

The cortical "map" for discriminating different sound frequencies was significantly disrupted, she explained.

"One of the possible reasons the sound frequency map is so disrupted is that visual responsive neurons are sprinkled here and there, and we also have a lot of single neurons that respond to both light and sound," Pallas said. "So those strange neurons sprinkled there probably keeps the map from forming properly."

Mao also discovered reduced sensitivity and slower responses of neurons in the auditory cortex to sound.

Finally, the neurons in the auditory cortex were less sharply tuned to different frequencies of sound.

"Generally, individual neurons will be pretty sensitive to one sound frequency that we call their 'best frequency,'" Pallas said. "We found that they would respond to a broader range of frequencies after the rewiring with visual inputs."

While Pallas' research seeks to create a basic understanding of brain development, knowledge gained from her lab's studies may help to give persons who are deaf, blind, or have suffered brain injuries ways to keep visual and auditory functions from being compromised.

"Usually we think of plasticity as a good thing, but in this case, it's a bad thing," she said. "We would like to limit the plasticity so that we can keep the function that's supposed to be there."

The research was supported by National Science Foundation grant IBN-0451018, National Institutes of Health Grant EY/MH 12696, the Georgia State University Research Foundation, and a scholarship from the GSU Brains and Behavior program.

The study is "Compromise of Auditory Cortical Tuning and Topography after Cross-Modal Invasion by Visual Inputs," Mao, Y. and Pallas, S. L., Journal of Neuroscience, 32(30):10338-10351.

The world needs new antibiotics to overcome the ever increasing resistance of disease-causing bacteria — but it doesn't need the side effect that comes with some of the most powerful ones now available: hearing loss. Today, researchers report they have developed a new approach to designing antibiotics that kill even "superbugs" but spare the delicate sensory cells of the inner ear.

Surprisingly, they have found that apramycin, an antibiotic already used in veterinary medicine, fits this bill — setting the stage for testing in humans.

In a paper published online in the Proceedings of the National Academy of Sciences, a team from Switzerland, England and the University of Michigan show apramycin's high efficacy against bacteria, and low potential for causing hearing loss, through a broad range of tests in animals. That testing platform is now being used to evaluate other potential antibiotics that could tackle infections such as multidrug-resistant tuberculosis.

The research aims to overcome a serious limitation of aminoglycoside antibiotics, a class of drugs which includes the widely used kanamycin, gentamicin and amikacin.

While great at stopping bacterial infections, these drugs also cause permanent partial hearing loss in 20 percent of people who take them for a short course, and up to 100 percent of people who take them over months or years, for example to treat tuberculosis or lung infections in cystic fibrosis.

U-M researcher Jochen Schacht, Ph.D., a professor of biological chemistry and otolaryngology and director of the Kresge Hearing Research Institute at the U-M Medical School, has spent decades studying why these drugs cause this "ototoxicity" — a side effect that makes doctors hesitant to prescribe them. Hearing damage has also caused patients to discontinue treatment before their antibiotic prescription is over, potentially allowing drug-resistant strains of bacteria to flourish.

Schacht has found that the drugs produce damaging free radicals inside the hair cells of the inner ear. Hair cells, named for the tiny sound-sensing hairs on their surface, are the linchpin of hearing — and once destroyed, cannot be regrown.

In the new paper, Schacht and his research group joined teams led by University of Zurich microbiologist Erik Böttger, and structural biologist and Nobel Prize winner Venkatraman Ramakrishnan of England's Medical Research Council Laboratory of Molecular Biology, as well as scientists from ETH Zurich. Each team brought its particular expertise to the issue, and after four years of work they developed and tested this new approach to designing antibiotics.

"Aminoglycosides are some of the most valuable broad-spectrum antibiotics and indispensable drugs today, but we need new options to combat drug-resistant bacteria. Importantly, we must find ways to overcome their ototoxicity," Schacht says. "Instead of the trial-and-error approach of the past, this new hypothesis-driven tactic allows us to design drugs with simultaneous attention toward both antibacterial action and impact on hair cells."

According to the World Health Organization, about 440,000 new cases of multidrug-resistant tuberculosis emerge annually, causing at least 150,000 deaths worldwide. Aminoglycoside antibiotics, while carefully controlled in the U.S., Europe, and other developed countries are available over the counter in many developing nations, leading to overuse that makes it even easier for drug-resistant strains of many kinds of bacteria to emerge and spread.

The new paper outlines a rational approach to designing drugs to combat this threat without ototoxicity, based on a theoretical framework that emerged from the work of the three laboratories and centers around the role of ribosomes, the structures inside the cell that "read" DNA and translate the genetic message into proteins. Böttger's lab, at the Institut für Medizinische Mikrobiologie which he directs, studies aminoglycoside effects on mitochondrial ribosomes and antibacterial activity with an eye toward designing new ones. Ramakrishnan's lab studies ribosomes, and partners from ETH Zurich also collaborated.

Aminoglycosides bind to the ribosomes inside bacterial cells, preventing the ability to produce proteins. But while the drugs spare most human ribosomes, they can attach to ribosomes in the mitochondria of cells, which are similar to bacterial ribosomes.

Consistent with U-M-generated theories about ototoxicity, the drugs then cause the production of free radicals in such quantities that they overwhelm the hair cells' defense mechanisms — destroying the cells and causing hearing loss.

The team's approach is to design drugs that more specifically target bacterial ribosomes over mitochondrial ribosomes, simultaneously testing the impact on hair cells as well as the ability to kill bacteria. In this way, the researchers try to avoid creating antibiotics that harm hearing.

They are already using the platform employed for this study — which involves cells from mouse ears, and tests of hearing and hair cell damage in guinea pigs — to test other promising novel drugs synthesized based on the theoretical framework that was driving the current research.

Meanwhile, the team hopes to launch a clinical trial of apramycin, an antibiotic that could prove immediately useful because multidrug-resistant TB and lung-infecting bacteria have not shown resistance to the drug yet.

The research also lends more evidence to support the use of antioxidants to protect the hearing of patients taking current aminoglycoside antibiotics. Schacht has already led a clinical trial in China that showed a major reduction in hearing loss if aspirin was given at the same time as aminoglycoside antibiotics. "This kind of protection is important, while we search for the long-term answer to drug resistance without ototoxicity," he says.

Schacht's research is funded by grant DC-003685 from the National Institute on Deafness & other Communication Disorders of the National Institutes of Health. Other funding came from University of Zurich, the European Community, Medical Research Council UK and the Wellcome Trust. In addition to Schacht, former U-M researchers Su-Hua Sha and Jing Xie are co-authors along with members of the two Zurich and the LMB teams.

The brain receives information from the ear in a surprisingly orderly fashion, according to a University at Buffalo study scheduled to appear June 6 in the Journal of Neuroscience.

The research focuses on a section of the brain called the cochlear nucleus, the first way-station in the brain for information coming from the ear. In particular, the study examined tiny biological structures called synapses that transmit signals from the auditory nerve to the cochlear nucleus.

The major finding: The synapses in question are not grouped randomly. Instead, like orchestra musicians sitting in their own sections, the synapses are bundled together by a key trait: plasticity.

Plasticity relates to how quickly a synapse runs down the supply of neurotransmitter it uses to send signals, and plasticity can affect a synapse's sensitivity to different qualities of sound. Synapses that unleash supplies rapidly may provide good information on when a sound began, while synapses that release neurotransmitter at a more frugal pace may provide better clues on traits like timbre that persist over the duration of a sound.

UB Associate Professor Matthew Xu-Friedman, who led the study, said the findings raise new questions about the physiology of hearing. The research shows that synapses in the cochlear nucleus are arranged by plasticity, but doesn't yet explain why this arrangement is beneficial, he said.

"It's clearly important, because the synapses are sorted based on this. What we don't know is why," said Xu-Friedman, a member of UB's Department of Biological Sciences. "If you look inside a file cabinet and find all these pieces of paper together, you know it's important that they're together, but you may not know why."

In the study, Xu-Friedman and Research Assistant Professor Hua Yang used brain slices from mice to study about 20 cells in the cochlear nucleus called bushy cells, which receive information from synapses attached to auditory nerve fibers.

The experiments revealed that each bushy cell was linked to a network of synapses with similar plasticity. This means that bushy cells themselves may become specialized, developing unique sensitivities to particular characteristics of a sound, Xu-Friedman said.

The study hints that the cochlear nucleus may not be the only part of the brain where synapses are organized by plasticity. The researchers observed the phenomenon in the excitatory synapses of the cerebellum as well.

"One reason this may not have been noticed before is that measuring the plasticity of two different synapses onto one cell is technically quite difficult," Xu-Friedman said.

Too often, communication barriers exist between those who can hear and those who cannot. Sign language has helped bridge such gaps, but many people are still not fluent in its motions and hand shapes.

Thanks to a group of University of Houston students, the hearing impaired may soon have an easier time communicating with those who do not understand sign language. During the past semester, students in UH's engineering technology and industrial design programs teamed up to develop the concept and prototype for MyVoice, a device that reads sign language and translates its motions into audible words. Recently, MyVoice earned first place among student projects at the American Society of Engineering Education (ASEE) — Gulf Southwest Annual Conference.

The development of MyVoice was through a collaborative senior capstone project for engineering technology students (Anthony Tran, Jeffrey Seto, Omar Gonzalez and Alan Tran) and industrial design students (Rick Salinas, Sergio Aleman and Ya-Han Chen). Overseeing the student teams were Farrokh Attarzadeh, associate professor of engineering technology, and EunSook Kwon, director of UH's industrial design program.

MyVoice's concept focuses on a handheld tool with a built-in microphone, speaker, soundboard, video camera and monitor. It would be placed on a hard surface where it reads a user's sign language movements. Once MyVoice processes the motions, it then translates sign language into space through an electronic voice. Likewise, it would capture a person's voice and can translate words into sign language, which is projected on its monitor.

The industrial designers researched the application of MyVoice by reaching out to the deaf community to understand the challenges associated with others not understanding sign language. They then designed MyVoice, while the engineering technology students had the arduous task of programming the device to translate motion into sound.

"The biggest difficulty was sampling together a databases of images of the sign languages. It involved 200-300 images per sign," Seto said. "The team was ecstatic when the prototype came together."

From its conceptual stage, MyVoice evolved into a prototype that could translate a single phrase: "A good job, Cougars."

"This wasn't just a project we did for a grade," said Aleman, who just graduated from UH. "While designing and developing it, it turned into something very personal. When we got to know members of the deaf community and really understood their challenges, it made this MyVoice very important to all of us."

Since MyVoice's creation and first place prize at the ASEE conference, all of the team members have graduated. Still, Aleman said that the project is not history.

"We got it to work, but we hope to work with someone to implement this as a product," Aleman said. "We want to prove to the community that this will work for the hearing impaired."

"We are proud of such a contribution to society through MyVoice, which breaks the barrier between deaf community and common society," added Attarzadeh.

Seventy-two percent of teenagers participating in a study experienced reduced hearing ability following exposure to a pop rock performance by a popular female singer.

M. Jennifer Derebery, MD, House Clinic physician, along with the House Research Institute tested teens’ hearing before and after a concert and presented the study findings at the American Otologic Society meeting on April 21, 2012. The study has been accepted for publication in an upcoming issue of Otology & Neurotology.

The hearing loss that may be experienced after a pop rock concert is not generally believed to be permanent. It is called a temporary threshold shift and usually disappears within 16-48 hours, after which a person’s hearing returns to previous levels.

“Teenagers need to understand a single exposure to loud noise either from a concert or personal listening device can lead to hearing loss,” said M. Jennifer Derebery, MD, lead author and physician at the House Clinic. “With multiple exposures to noise over 85 decibels, the tiny hair cells may stop functioning and the hearing loss may be permanent.”

In the study, twenty-nine teenagers were given free tickets to a rock concert. To ensure a similar level of noise exposure for the teens, there were two blocks of seats within close range of each other. The seats were located in front of the stage at the far end of the venue approximately 15-18 rows up from the floor.

Parental consent was obtained for all of the underage study participants. The importance of using hearing protection was explained to the teenagers. Researchers then offered hearing protection to the subjects and encouraged them to use the foam ear plugs. However, only three teenagers chose to do so.

Three adult researchers sat with the teenagers. Using a calibrated sound pressure meter, 1,645 measurements of sound decibel (dBA) levels were recorded during the 26 songs played during the three hour concert. The sound levels ranged from 82-110 dBA, with an average of 98.5 dBA. The mean level was greater than 100 dBA for 10 of the 26 songs.

The decibel levels experienced at the concert exceeded what is allowable in the workplace, according to Occupational Safety and Health Administration (OSHA). OSHA safe listening guidelines set time limits for exposures to sound levels of 85 dB and greater in the workplace. The volumes recorded during the concert would have violated OSHA standards in less than 30 minutes. In fact, one third of the teen listeners showed a temporary threshold shift that would not be acceptable in adult workplace environments.

Following the concert, the majority of the study participants also were found to have a significant reduction in the Distortion Product Otoacoustic Emissions (OAE) test. This test checks the function of the tiny outer hair cells in the inner ear that are believed to be the most vulnerable to damage from prolonged noise exposure, and are crucial to normal hearing, the ability to hear soft (or low level sounds), and the ability to understand speech, especially in noisy environments. With exposure to loud noise, the outer hair cells show a reduction in their ability to function, which may later recover. However, it is known that with repeated exposure to loud noise, the tiny hair cells may become permanently damaged. Recent animal research suggests that a single exposure to loud noise may result in permanent damage to the hearing nerve connections themselves that are necessary to hear sound.

Following the concert, 53.6 percent of the teens said they did not think they were hearing as well after the concert. Twenty-five percent reported they were experiencing tinnitus or ringing in their ears, which they did not have before the concert.

Researchers are especially concerned, because in the most recent government survey on health in the United States National Health and Nutrition Examination Survey (NHANES) 2005-2006, 20% of adolescents were found to have at least slight hearing loss, a 31% increase from a similar survey done from 1988-1994.

The findings of the study clearly indicate more research is necessary to determine if the guidelines for noise exposure need to be revised for teenagers. More research is also needed to determine if teenager’s ears are more sensitive to noise than adults.

“It also means we definitely need to be doing more to ensure the sound levels at concerts are not so loud as to cause hearing loss and neurological damage in teenagers, as well as adults,” said Derebery. “Only 3 of our 29 teens chose to use ear protection, even when it was given to them and they were encouraged to do so. We have to assume this is typical behavior for most teen listeners, so we have the responsibility to get the sound levels down to safer levels.”

Researchers recommend teenagers and young adults take an active role in protecting their hearing by utilizing a variety of sound meter ‘apps’ available for smart phones. The sound meters will give a rough estimate of the noise level allowing someone to take the necessary steps to protect their hearing such as wearing ear plugs at a concert.

In addition, Derebery and the study co-authors would like to see concert promoters and the musicians themselves take steps to lower sound levels as well as encourage young concert goers to use hearing protection.

The study was funded through the House Research Institute’s national teen hearing loss prevention initiative, It’s How You Listen that Counts®, as part of its broader Sound Partners hearing conservation education program.

Can blindness or other forms of visual deprivation really enhance our other senses such as hearing or touch? While this theory is widely regarded as being true, there are still many questions about the science behind it.

New findings from a Canadian research team investigating this link suggest that not only is there a real connection between vision and other senses, but that connection is important to better understand the underlying mechanisms that can quickly trigger sensory changes. This may demystify the true potential of human adaptation and, ultimately, help develop innovative and effective methods for rehabilitation following sensory loss or injury.

François Champoux, director of the University of Montreal's Laboratory of Auditory Neuroscience Research, will present his team's research and findings at the Acoustics 2012 meeting in Hong Kong, May 13-18, a joint meeting of the Acoustical Society of America (ASA), Acoustical Society of China, Western Pacific Acoustics Conference, and the Hong Kong Institute of Acoustics.

Studies have shown, in terms of hearing, that blind people are better at localizing sound. One study even suggested that blindness might improve the ability to differentiate between sound frequencies. "The supposed enhanced tactile abilities have been studied at a greater degree and can be seen as early as days or even minutes following blindness," says Champoux. "This rapid change in auditory ability hasn't yet been clearly demonstrated."

Two big questions about blindness and enhanced abilities remain unanswered: Can blindness improve more complex auditory abilities and, if so, can these changes be triggered after only a few minutes of visual deprivation, similar to those seen with tactile abilities?

"When we speak or play a musical instrument, the sounds have specific harmonic relations. In other words, if we play a certain note on a piano, that note has many related 'layers.' However, we don't hear all of these layers because our brain simply associates them all together and we only hear the lowest one," Champoux explains.

It's through this complex computation based on specific components of the sound that the brain can interpret and distinguish auditory signals coming from different people or instruments. The ability to identify harmonicity — the harmonic relation between sounds — is one of the most powerful factors involved in interpreting our auditory surroundings.

"Harmonicity can easily be evaluated using a simple task in which similar harmonic layers are set up and one of them is gradually modified until the individual notices two layers instead of one," says Champoux. "In our study, healthy individuals completed such a task while blindfolded. This task was administered twice, separated by a 90-minute interval during which the participants conversed with the experimenter in a quiet room. Half of the participants kept the blindfold on during the interval period, depriving them of all visual input, while the other half removed their blindfolds."

They found no significant differences between the two groups in their ability to differentiate harmonicity prior to visual deprivation. However, the results of the testing session following visual deprivation revealed that visually deprived individuals performed significantly better than the group that took their blindfolds off.

"Regardless of the neural basis for such an enhancement, our results suggest that the potential for change in auditory perception is much greater than previously assumed," Champoux notes.

Cochlear implants have restored basic hearing to some 220,000 deaf people, yet a microphone and related electronics must be worn outside the head, raising reliability issues, preventing patients from swimming and creating social stigma.

Now, a University of Utah engineer and colleagues in Ohio have developed a tiny prototype microphone that can be implanted in the middle ear to avoid such problems.

The proof-of-concept device has been successfully tested in the ear canals of four cadavers, the researchers report in a study just published online in the Institute of Electrical and Electronics Engineers journal Transactions on Biomedical Engineering.

The prototype — about the size of an eraser on a pencil — must be reduced in size and improved in its ability to detect quieter, low-pitched sounds, so tests in people are about three years away, says the study's senior author, Darrin J. Young, an associate professor of electrical and computer engineering at the University of Utah and USTAR, the Utah Science Technology and Research initiative.

The study showed incoming sound is transmitted most efficiently to the microphone if surgeons first remove the incus or anvil — one of three, small, middle-ear bones. U.S. Food and Drug Administration approval would be needed for an implant requiring such surgery.

The current prototype of the packaged, middle-ear microphone measures 2.5-by-6.2 millimeters (roughly one-tenth by one-quarter inch) and weighs 25 milligrams, or less than a thousandth of an ounce. Young wants to reduce the package to 2-by-2 millimeters.

Young, who moved the Utah in 2009, conducted the study with Mark Zurcher and Wen Ko, who are his former electrical engineering colleagues at Case Western Reserve University in Cleveland, and with ear-nose-throat physicians Maroun Semaan and Cliff Megerian of University Hospitals Case Medical Center.

The study was funded by the National Institutes of Health (NIH-DC-006850).

Problems with External Parts on Cochlear Implants

The National Institutes of Health says almost 220,000 people worldwide with profound deafness or severe hearing impairment have received cochlear implants, about one-third of them in the United States, where two-fifths of the recipients are children.

In conventional cochlear implant, there are three main parts that are worn externally on the head behind the ear: a microphone to pick up sound, a speech processor and a radio transmitter coil. Implanted under the skin behind the ear are a receiver and stimulator to convert the sound signals into electric impulses, which then go through a cable to between four and 16 electrodes that wind through the cochlea of the inner ear and stimulate auditory nerves so the patient can hear.

"It's a disadvantage having all these things attached to the outside" of the head, Young says. "Imagine a child wearing a microphone behind the ear. It causes problems for a lot of activities. Swimming is the main issue. And it's not convenient to wear these things if they have to wear a helmet."

Young adds that "for adults, it's social perception. Wearing this thing indicates you are somewhat handicapped and that actually prevents quite a percentage of candidates from getting the implant. They worry about the negative image."

As for reliability, "if you have wires connected from the microphone to the coil, those wires can break," he says.

How Sound Moves in Normal Ears, Cochlear Implants and the New Device

Sound normally moves into the ear canal and makes the eardrum vibrate. At what is known as the umbo, the eardrum connects to a chain of three tiny bones: the malleus, incus and stapes, also known as the hammer, anvil and stirrup. The bones vibrate. The stapes or stirrup touches the cochlea, the inner ear's fluid-filled chamber. Hair cells (not really hair) on the cochlea's inner membrane move, triggering the release of a neurotransmitter chemical that carries the sound signals to the brain.

In profoundly deaf people who are candidates for cochlear implants, the hair cells don't work for a variety of reasons, including birth defects, side effects of drugs, exposure to excessively loud sounds or infection by certain viruses.

In a cochlear implant, the microphone, signal processor and transmitter coil worn outside the head send signals to the internal receiver-stimulator, which is implanted in bone under the skin and sends the signals to the electrodes implanted in the cochlea to stimulate auditory nerves. The ear canal, eardrum and hearing bones are bypassed.

The system developed by Young implants all the external components. Sound moves through the ear canal to the eardrum, which vibrates as it does normally. But at the umbo, a sensor known as an accelerometer is attached to detect the vibration. The sensor also is attached to a chip, and together they serve as a microphone that picks up the sound vibrations and converts them into electrical signals sent to the electrodes in the cochlea.

The device still would require patients to wear a charger behind the ear while sleeping at night to recharge an implanted battery. Young says he expects the battery would last one to several days between charging.

Young says the microphone also might be part of an implanted hearing aid that could replace conventional hearing aids for a certain class of patients who have degraded hearing bones unable to adequately convey sounds from conventional hearing aids.

Testing the Microphone in Cadavers

Conventional microphones include a membrane or diaphragm that moves and generates an electrical signal change in response to sound. But they require a hole through which sound must enter — a hole that would get clogged by growing tissue if implanted. So Young's middle-ear microphone instead uses an accelerometer — a 2.5-microgram mass attached to a spring — that would be placed in a sealed package with a low-power silicon chip to convert sound vibrations to outgoing electrical signals.

The package is glued to the umbo so the accelerometer vibrates in response to eardrum vibrations. The moving mass generates an electrical signal that is amplified by the chip, which then connects to the conventional parts of a cochlear implant: a speech processor and stimulator wired to the electrodes in the cochlea.

"Everything is the same as a conventional cochlear implant, except we use an implantable microphone that uses the vibration of the bone," Young says.

To test the new microphone, the researchers used the temporal bones — bones at the side of the skull — and related ear canal, eardrum and hearing bones from four cadaver donors.

The researchers inserted tubing with a small loudspeaker into the ear canal and generated tones of various frequencies and loudness. As the sounds were picked up by the implanted microphone, the researchers used a laser device to measure the vibrations of the tiny ear bones. They found the umbo — where the eardrum connects to the hammer or malleus — produced the greatest sound vibration, particularly if the incus or anvil bone first was removed surgically.

The experiments showed that when the prototype microphone unit was attached to the umbo, it could pick up medium pitches at conversational volumes, but had trouble detecting quieter, low-frequency sounds. Young plans to improve the microphone to pick up quieter, deeper, very low pitches.

In the tests, the output of the microphone went to speakers; in a real person, it would send sound to the implanted speech processor. To demonstrate the microphone, Young also used it to record the start of Beethoven's Ninth Symphony while implanted in a cadaver ear. It is easily recognizable, even if somewhat fuzzy and muffled.

"The muffling can be filtered out," says Young.

To hear a recording of the start of Beethoven's Ninth Symphony through the new microphone implanted in a cadaver's middle-ear, go to: http://unews.utah.edu/news_releases/a-middle-ear-microphone/

New research confirms an association between smoking and a reduced risk for a rare benign tumor near the brain, but the addition of smokeless tobacco to the analysis suggests nicotine is not the protective substance.

The study using Swedish data suggests that men who currently smoke are almost 60 percent less likely than people who have never smoked to develop this tumor, called an acoustic neuroma. But men in the study who used snuff, which produces roughly the same amount of nicotine in the blood as smoking, had no reduced risk of tumor development.

"We see this effect with current smokers but don't see it with current snuff users, so we think that maybe the protective effect has something to do with the combustion process or one of the other chemicals in cigarettes that are not in snuff," said Sadie Palmisano, a doctoral student in epidemiology at Ohio State University and lead author of the study. "We learned something from exclusion."

Acoustic neuroma is a tumor that grows on the vestibular cochlear nerve connecting the ear to the brain. It is not cancer, but it can cause nerve damage as well as symptoms that include vertigo, ringing in the ears and hearing loss. The only treatment for these slow-growing tumors is surgical removal or high-powered radiation that reduces their size. About one in 100,000 people per year develops these growths, which account for approximately 8 percent of all primary tumors inside the skull in the United States.

A few previous studies have found a similar link between smoking and lowered risk for development of these tumors, but did not take snuff use into account. Though the research is aimed at prevention of acoustic neuromas, the researchers emphasized that they do not endorse smoking as a way to avoid developing a tumor.

The findings suggested to the scientists that a lack of oxygen associated with smoking might help prevent the tumors by starving the cells whose overgrowth leads to the formation of an acoustic neuroma. These are called Schwann cells, and they produce the myelin coating on nerve cells in the peripheral nervous system outside the brain and spinal cord.

The research is published online and scheduled for future print publication in the American Journal of Epidemiology.

The scientists conducted a nationwide study of acoustic neuroma between 2002 and 2007, compiling data on Swedish patients between the ages of 20 and 69 years at the time of diagnosis with the tumors. These patients, as well as healthy Swedish control participants, also completed questionnaires about environmental exposures and lifestyle choices.

Palmisano and colleagues applied statistical analysis to these data to determine associations between smoking and snuff use and risk for acoustic neuroma. The analysis included data on 423 patients with tumors and 645 controls matched for age, sex and home location.

"We got practically all of the diagnosed cases in Sweden — there was an 84 percent participation rate. On top of that, a population-based registry served as the basis for the control sample. This made it incredibly representative of the population, and with the sample being this large, we make the case that the link between smoking and reduced risk for acoustic neuroma is there," Palmisano said.

The link was especially strong in men who were current smokers — a 59-percent reduction in risk for acoustic neuroma compared to people who had never smoked. In women current smokers, the association was smaller — a 30 percent reduced risk compared to never-smokers, with more statistical room for this link to be attributed to chance.

Current smokers were those who smoked at least one cigarette per day for six months or longer. For people who had smoked and then quit, including even longtime smokers, "we didn't find as much of an effect. It's like a puzzle," Palmisano said.

The researchers evaluated smokeless tobacco use among only men because too few women reported using snuff. The scientists found no difference in the risk for acoustic neuroma between current or past snuff users and people who had never used snuff.

These findings about snuff imply that nicotine is not providing the protection because habitual snuff users and smokers have similar levels of nicotine in their blood. By determining that snuff users reap no acoustic neuroma preventive benefits from the smokeless tobacco, the researchers determined that nicotine should probably be ruled out as a potentially protective compound in this context.

The Swedish and American forms of snuff differ substantially, so these findings do not translate to users of American smokeless tobacco, which is fermented and contains more chemicals than does Swedish snuff.

Many studies have also linked smoking with a reduced risk for Parkinson's disease, leading scientists to continue looking for clues about how tobacco can have this effect on nervous-system processes. Palmisano also will use the Swedish data to explore other potential causes of acoustic neuromas, including exposure to loud noises. The only known cause of the tumors is a genetic condition called neurofibromatosis. Two different sources of radiation also have been linked to increased risk for the tumors.

This study was funded by the Swedish Council for Working Life and Social Research.

Palmisano conducted the research with Judith Schwartzbaum, associate professor of epidemiology at Ohio State, as well as Swedish co-authors Michaela Prochazka, David Peterson and Maria Feychting of the Karolinska Institutet; Tommy Bergenheim of Umeå University Hospital; Rut Florentzson of Sahlgrenska University Hospital; Henrik Harder of Linköping University Hospital; Tiit Mathiesen of Karolinska Hospital; Gunnar Nyberg of Uppsala University Hospital; and Peter Siesjö of Lund University Hospital.