RxPG News : Hearing Imapirment

Anti-epileptics can prevent permanent hearing loss, study reports

Wed, 14 Mar 2007 08:31:05 PST

( from http://www.rxpgnews.com ) On the battlefield, a soldier's hearing can be permanently damaged in an instant by the boom of an explosion, and thousands of soldiers returning from Iraq have some permanent hearing loss. But what if soldiers could take a pill before going on duty that would prevent damage to hearing?

Research at Washington University School of Medicine in St. Louis suggests a medicinal form of hearing protection may someday be a possibility. A group headed by Jianxin Bao, Ph.D., research associate professor of otolaryngology and head of the Central Institute for the Deaf's Presbycusis and Aging Laboratory, has found that two anti-epileptic drugs can prevent permanent hearing loss to a significant degree in mice exposed to loud noises.

"The military has a tremendous need for preventing noise-induced hearing loss," Bao says. "But others would also benefit. For example, many hunters have hearing loss on the side where they hold their gun, and pilots are especially prone to hearing loss because of the noise in airplane cabins. Protective equipment or earplugs aren't always appropriate, and right now no drug on the market can prevent or treat noise-induced hearing loss."

Bao's laboratory is dedicated to the study of both age-related and noise-induced hearing loss. About 28 million Americans have a hearing impairment, and excessive noise is the predominant cause of permanent hearing loss. At least 30 million people in the United States encounter hazardous levels of noise at work, particularly in jobs such as construction, mining, agriculture, manufacturing, transportation and the military.

Bao and colleagues found that if they exposed mice to loud sounds and then gave them trimethadione (Tridione®) or ethosuximide (Zarontin®) anticonvulsive medications used to treat epilepsy they could prevent a significant amount of permanent hearing loss. When mice got the medications before noise exposure, only trimethadione, not ethosuximide, significantly reduced subsequent hearing loss. The results are reported in Hearing Research and are now available through advanced online publication.

Bao notes that other researchers are investigating agents such as antioxidants for their potential in preventing hearing loss, but the two anticonvulsive drugs his lab studied have had FDA approval and so could be used much sooner in clinical trials that study hearing loss.

The experiments in mice showed that the drugs could reduce by about five decibels the permanent threshold shift that can occur after noise exposure. For example, if the softest sound the mice could hear before the noise was 30 decibels, after the noise it might take a louder, 50-decibel sound for the untreated mice to hear but only 45 decibels for the treated mice. A decibel is a standard unit of sound, and normal conversation is around 60 decibels.

"In people, a five decibel difference in hearing ability can be important for everyday speech," Bao says. "We will continue our investigations of these kinds of drugs to see if we can improve the results. One possibility is to combine an anticonvulsant with an antioxidant to increase the protective effect."

Both drugs tested are T-type calcium channel blockers, which inhibit the movement of calcium ions into nerve cells. In the ear, calcium may play a role in causing damage to hair cells (specialized cells that sense sound vibrations) and the nerve cells that connect the hair cells to the hearing centers of the brain.

These anti-epileptic drugs can have unwanted side effects such as dizziness and sleepiness. "The drugs' side effects would be detrimental in certain situations," Bao says. "But lowering the dosage and combining them with other drugs may be effective. Newer versions of anti-epilepsy drugs have fewer side effects, and it may be possible to modify the structure of the drugs so that they don't cross into the brain, which could avert some side effects."

Call centre staff could face hearing damage risk

Sun, 19 Nov 2006 17:36:50 PST

( from http://www.rxpgnews.com ) London, Nov 19 (IANS) People working at call centres could suffer hearing damage from acoustic shock, say health experts.

Acoustic shocks are temporary or permanent disturbances of the functioning of the ear or of the nervous system, which may be caused to the user of a telephone or earphone by a sudden sharp rise in the acoustic pressure produced by it.

The sound could be a whistle, a bleep or any unexpected noise. Two thirds of call centres in Britain fail to protect their workers against hearing damage from noise, reported the online edition of BBC News.

A group of experts from The National Health Service (NHS) in Britain have formed an Acoustic Safety Programme, aimed at making call centre managers aware of the problem.

There are many people who have experienced acoustic shock but do not realise it, according to the experts. They warn that while some organisations are acting to safeguard the hearing of their staff, the vast majority are not.

Call centres can introduce equipment such as headphones that extract any potential cause of acoustic shock to protect the worker's hearing.

There should also be measures that will raise awareness about the problem, the experts added.

'It (acoustic shock) can be a debilitating occurrence for a call centre worker. They can develop permanent damage to their hearing,' said Chris Atwell, operations director for the Acoustic Safety Programme.

Added Mark Downs of the Royal National Institute for the Deaf: 'Acoustic shock is not the same as noise-induced hearing loss and is believed to occur at sound pressure levels below those which present an immediate risk to hearing damage.'

UK researcher identifies brain region responsible for spatial hearing

Fri, 06 Oct 2006 21:09:37 PST

( from http://www.rxpgnews.com ) A major science prize was today awarded to a researcher who is looking for the region of the brain that helps us to hear someone in a noisy place, such as a party or bar, and is responsible for "training" the brain to hear better in these situations.

Not being able to hear a person's voice in a noisy room and follow conversations is one of the most common problems for Britain's nine million people with a hearing impairment.

Deafness Research UK, the leading medical charity, has awarded the 2007 Pauline Ashley Prize to Sam Irving, a young researcher at the MRC Institute for Hearing Research in Nottingham.

The Pauline Ashley Prize, established in memory of the charity's founder, Lady Ashley of Stoke, is awarded annually to a talented young scientist near the beginning of their career and undertaking research into deafness, or a related condition such as tinnitus.

Most people with a hearing impairment have trouble picking out what someone is saying when they're in a noisy room. Parties or bars are some of the worst places because the level of background noise is high, and so scientists call this the "cocktail party effect".

To see what this was like, Irving wore an earplug in one ear for a week which gave him a one-sided hearing loss. He said: "It was hell - especially when I was in the pub with friends. The background hubbub of the bar seemed to be the same level as the people I was talking to so I could barely hear what they were saying and it took a huge effort of concentration to follow any conversation. During the week, I gave up and spent a lot of time at home on my own because it was so distressing and tiring to be with lots of people or in a noisy place."

Our ability to detect a particular sound in the middle of lots of noise relies on the fact that we have two ears, and each detects an individual sound at a slightly different time (a sound coming from the left will reach the left ear slightly faster than it reaches the right ear). This is known as binaural or "spatial" hearing because it helps us identify where a sound is coming from and to concentrate or focus our hearing on that particular sound.

But, if you have some form of hearing problem in at least one ear, this ability is disrupted and the brain struggles to tell one sound from another.

The key to understanding this ability lies in the brain. Scientists are currently trying to work out exactly what part of the brain is responsible and how it allows us to distinguish one sound from lots of noise. Early research has had some remarkable results.

Most mammals also have this ability and in 2006, scientists working in the Oxford Auditory Neuroscience Group found that spatial hearing in ferrets has the ability to bounce-back or adapt to a hearing loss over time. Their brains are being "trained" to cope with the hearing loss and distinguish sounds much better.

The Oxford study placed healthy ferrets in a "ring of sound" where a sound is played from one of 12 speakers placed in a circle around the ferret and their response is monitored to see if they can detect which speaker the sound is coming from. Ferrets with normal hearing are very good at this and have excellent spatial hearing.

The team then fitted each of the ferrets with a small earplug in one ear which blocks some of the sound and so mimics a hearing loss. They then got the ferrets to perform the same task twice a day for two weeks and made a startling discovery. At first, the ferrets' ability to identify where the sound was coming from was dramatically reduced (because their spatial hearing had been disrupted by the earplug) but after two weeks they regained their ability and by the end of the period were as good at detecting the location of the sounds as they were before being fitted with an earplug.

Something in their brain was changing or adapting to the new situation and they were learning to compensate for the hearing loss.

Irving said: "When we switch on a bright light our eyes detect the increase in light levels and the brain sends a message to the eye to tell it to contract the pupil and let in less light. This is a feedback system where the brain is getting information from the eye about its surroundings, processing that information, and sending messages back to the eye to help it cope with different situations. We think something very similar is happening with the ear in spatial hearing."

"The brain is constantly monitoring the sounds around us and so knows what normal sound levels it would expect. When we introduce an earplug, it can detect the reduction in sound being received and we think it is actively sending messages back to the ear telling it how to cope with the new hearing loss, perhaps by stimulating or increasing the signal which is being blocked. It's compensating for the problem in a really clever way."

Irving is trying to locate the place in the brain which is channeling these feedback messages back to the ear.

"We already have a likely candidate called the OCB, the Olivocochlear Bundle, which is a part of the brain that we know is a centre of feedback information being transmitted from the brain back to the ear. We're now trying to work out if the OCB is responsible for spatial hearing in ferrets."

The Pauline Ashley Prize will allow Irving to work with a team led by Professor Charles Liberman at the Eaton Peabody Lab at MIT/Harvard, leading experts on the OCB system. His study will compare the performance of ferrets which have had their OCB removed with normal ferrets in the "ring of sound".

At the same time, Irving is conducting a study with human subjects who have volunteered to wear an earplug for five days. These subjects will be tested in a similar ring of sound and their performance measured over time. Early results show that humans also have the same ability to train their brain to cope with the hearing loss and become better at the task the longer they're wearing the earplug.

Irving said: "Understanding how this system works is fairly basic science, but will be vital in the future for engineering new ways of helping people with hearing impairment cope with difficult situations. They could be helped by computer generated training programs which run like regular computer games, but can target weaknesses in listening skills. By incorporating training exercises much like those performed by the ferrets, they can lead to auditory learning and an improved ability to listen."

Beta-actin mutations linked to deafness and dystonia

Mon, 10 Jul 2006 20:25:37 PST

( from http://www.rxpgnews.com ) Findings of a recent genetic study on developmental brain disorders may be the "tip of an iceberg" revealing factors involved with a number of congenital diseases, according to UC Irvine researchers.

The study is the first to find that mutations in the structural proteins in brain cells - beta-actin - are linked to disorders such as deafness and dystonia, a debilitating neural disease, and further suggests that genetic variants of these proteins may play a wider role with inherited human diseases. Study results appeared in the June issue of the American Journal of Human Genetics.

The findings give vital clues to the basis of some developmental disorders and make early diagnosis possible for diseases such as dystonia, allowing for greater treatment opportunities, said Dr. Vincent Procaccio of UCI's Center for Molecular and Mitochondrial Medicine and Genetics and lead author, though the study does not point to potential therapies.

"These types of actin proteins are prevalent throughout the body and play a key role in processes that are an essential part of development," said Procaccio, who is also an assistant professor of pediatrics. "To find that these mutations are involved with brain disorders seems to be the tip of an iceberg. Since beta-actin is involved with many developmental cell functions, it would appear that its genetic variants can be involved with a number of other congenital disorders."

Procaccio and his colleagues studied brain tissue samples from deceased twins who had a number of developmental disabilities including dystonia, a neurological disorder that causes twisting or jerking movements in parts of the body. Genetic analysis revealed mutations in the beta-actin gene. These mutations affected protein conformation, which would not allow beta-actin to bind with ATP - the chemical fuel synthesized by mitochondria that give a cell its energy.

Beta-actin is a structural protein that helps form the cytoskeleton - a cell's skeleton that gives it structure and strength. Unable to receive fuel, the mutated beta-actin proteins break down, ultimately damaging and destroying the cell. In the brain, this leads to the neural tissue damage related to congenital neurological disorders like dystonia.

Taking this information, Procaccio and his fellow researchers are working to demonstrate that beta-actin mutations are a common cause of neurological disorders. They are currently analyzing several DNA samples from patients to identify additional abnormalities. In addition, they are investigating the cellular and biophysical abnormalities resulting from beta-actin mutations, which will serve as a basis to identify other mutations and disease phenotypes arising from genetic abnormalities of beta-actin proteins.

"Ultimately, we hope to prove that the identification of genetic abnormalities of the beta-actin are likely to explain the causes of a spectrum of disease phenotypes, including congenital malformation syndromes and other inherited degenerative diseases, that are presently poorly understood," he said.

Aldosterone linked to good hearing as we age

Sun, 12 Feb 2006 18:37:37 PST

( from http://www.rxpgnews.com ) Researchers have linked a hormone known to adjust levels of key brain chemicals to the quality of our hearing as we age. The more of the hormone that older people have in their bloodstream, the better their hearing is, and the less of the hormone, the worse their hearing is.

The hormone, aldosterone, is known to regulate kidney function and also plays a role in controlling levels of two crucial signaling chemicals in the nervous system, potassium and sodium. For nerves to send signals crisply and work properly, potassium and sodium must be in precise proportion, without any disruption in the molecular channels or gates through which they move. Levels of potassium are particularly crucial in the sensitive inner ear, where fluid rich in potassium plays a central role in converting sounds into signals that the nervous system recognizes.

The team of scientists in Rochester, N.Y., put 47 healthy men and women between the ages of 58 and 84 through a battery of sophisticated hearing tests. Scientists also measured their blood levels of aldosterone, which is known to drop as people age. They found that people with severe hearing loss had on average about half as much aldosterone in their bloodstream as their counterparts with normal hearing. The researchers noted, however, that the levels of aldosterone found in all the participants is considered normal, and that no patients or physicians should consider altering aldosterone levels without more research.

The findings come from researchers at the International Center for Hearing and Speech Research (ICHSR), a group funded by the National Institute on Aging that is recognized as a leader in research on age-related hearing loss. The center includes scientists from the National Technical Institute for the Deaf at Rochester Institute of Technology and neuroscientists from the University of Rochester.

"The inner ear is especially sensitive to any disruption in potassium levels," said Robert D. Frisina, Ph.D., professor of Otolaryngology at the University of Rochester Medical Center and an adjunct professor at Rochester Institute of Technology. "We know that potassium levels in the inner ear seem to decrease as we age and that these falling levels play a role in age-related hearing loss, and we also know that blood levels of aldosterone generally decrease with age.

"We found a direct link between blood levels of aldosterone and the ability of people to hear normally as they age. Depressed hormone levels may hurt hearing both in the inner ear and the part of the brain used for hearing. More research is needed, however, to understand the precise role that aldosterone plays â for instance, whether it's a cause of failed hearing, or whether it's symptomatic. Before we understand the issue more fully, people should not worry about their aldosterone levels or look to boost the amount in their bloodstream."

The team led by Frisina published its results in the November issue of the journal Hearing Research. This week at the annual international meeting of the Association for Research in Otolaryngology in Baltimore, the team presented its latest results showing just how important potassium regulation is to age-related hearing loss.

In Baltimore, Otolaryngology medical resident Jared Spencer, M.D., presented results from "knockout" mice whose genes controlling the potassium channels in the inner ear don't function properly, and confirmed that malfunctioning potassium channels are central to age-related hearing loss, or presbycusis. The channels are highly concentrated in a part of the brain that plays an important role providing feedback from the brain to the ears. Frisina's team previously discovered that the feedback system is one of the first things to go wrong in age-related hearing loss, often declining in people who are in their 40s and 50s, usually before they even realize their hearing is declining.

"We are now working out some of the underlying biology about how the decline occurs," said Frisina. "We have evidence that these potassium channels may play an important role in the failure of the feedback system, which is a big part of age-related hearing loss."

Nearly everyone wrestles with failing hearing at some point. While some people suffer from hearing damage as a result of exposure to loud noise, or from other causes such as the side effects of some medications, for many people hearing problems occur with no known cause. Some people notice problems in their 40s and 50s, but the process becomes very noticeable for most people in their 60s and older.

Frisina said that until the biology of the problem is better understood, the best advice for people concerned about hearing loss is to limit exposure to loud, damaging noise and to medications that are toxic to the ears. He also counsels people to eat healthy and to exercise â "all those things you know you should be doing to stay healthy with age," he said.

Meanwhile, his team is looking at the possibility of using gene therapy to try to correct the problem. It may be possible some day to modify a person's inner ear to correct the potassium imbalance that is central to hearing loss. Such an approach might also address the biggest cause of congenital deafness, which involves a genetic mutation that mucks up the potassium balance in the inner ear.

Salicylate causes tympanic membranes to rupture more easily

Mon, 26 Sep 2005 15:50:38 PST

( from http://www.rxpgnews.com ) It's well known that high doses of aspirin can cause ulcers and temporary deafness, but the biochemical mechanism responsible for these phenomena has never been deciphered. New research from Rice University offers clues, showing for the first time how salicylate -- an active metabolite of aspirin -- weakens lipid membranes. Researchers believe these mechanical changes disrupt the lining of the stomach, which functions to protect underlying tissue from the acidic contents of the gut. By a similar mechanism, the changes may result in aspirin-related deafness by interfering with the proper function of prestin, a transmembrane protein that's critical for mammalian hearing.

The study appears in the September issue of Biophysical Journal.

"Our studies found that membranes exposed to physiological concentrations of salicylate were thinner, more permeable, easier to bend and more likely to rupture," said study co-author Robert Raphael, the T.N. Law Assistant Professor of Bioengineering.

All cells are surrounded by membranes, ultrathin barriers of fatty acids that are just a few nanometers thick. Membranes act like a skin, sealing off the inner machinery of the cell from the outside world. About 40 percent of human proteins are "transmembrane" proteins, molecules that stick through the membrane like a needle through a cloth.

First identified five years ago, prestin is a transmembrane protein found in the inner ear. A motor protein, prestin is thought to act like a piezocrystal, converting electrical signals into mechanical motion. In the outer hair cells of the cochlea, prestin acts as a molecular motor, causing the cells to move rhythmically and amplify the sounds we hear.

"If you change the mechanical properties of the membrane, you will likely affect the biophysical processes that take place there, including those that are mediated by membrane proteins like prestin," Raphael said.

Raphael's findings also provide a mechanistic basis for the observations of Texas Medical Center researchers, who have found that the debilitating and dangerous gastrointestinal side-effects of anti-inflammatory drugs like aspirin and ibuprofen are independent of biochemical signaling cascades mediated by cyclo-oxygenase (COX). Raphael's research was co-sponored by the Texas Technology Development and Transfer Program and PLX Pharma, a Houston-based startup that began the final phase of clinical trials for its reformulated version of ibuprofen last December.

"Effectively, our results proved that salicylate can stabilize holes that spontaneously form in lipid membranes, thus increasing membrane permeability", Raphael said. "Our study highlights the pivotal role played by the mechanical properties of membranes in biological processes."

In their experiments, Raphael and graduate student Yong Zhou, the first author of the study, used a technique called micropipette aspiration. Working with needle-like glass capillary tubes, Zhou measured the mechanical properties of phospholipid membranes, which are very similar to those of live cells.

Raphael credited Zhou's initiative in applying new technology to the problem.

"Yong was the driving force for introducing the new technique of dynamic tension spectroscopy into my laboratory," Raphael said. "This enabled us to really get insight into the subtle details associated with the mechanism by which salicyalte affects membrane stability."

Zhou was recently awarded a Student Research Fellowship award from the American Gastroenterological Association to conduct studies on another salicylate-like molecule.

Amount of hearing in an ear prior to surgery is unrelated to a patient's ability to interpret speech using an implant

Sun, 04 Sep 2005 08:07:38 PST

( from http://www.rxpgnews.com ) Hearing-impaired individuals with severe to profound hearing loss and poor speech understanding who possess some residual hearing in one ear may experience significant communication benefit from a cochlear implant even if it is placed in the worse-hearing ear, a Johns Hopkins study suggests.

There is growing evidence that the amount of hearing in an ear prior to surgery is unrelated to a patient's ability to interpret speech using an implant, says Howard W. Francis, M.D., lead author of the study and an associate professor of otolaryngology-head and neck surgery. Therefore, the better-hearing ear could be saved for the continued use of a hearing aid or future technology to complement a cochlear implant, Francis says.

Reporting in the August issue of the journal Ear and Hearing, Francis and colleagues compared patients with no residual hearing, patients with some residual hearing in one ear and patients with some residual hearing in both ears. The patients' ability to interpret sounds and speech was measured before and after cochlear implant surgery.

Patients with residual hearing in one or both ears prior to surgery scored significantly higher on the speech perception tests following surgery, even when the implanted ear was profoundly deaf prior to surgery. The researchers also noted that patients' ability to interpret speech in a noisy environment increased dramatically over time in proportion with the amount of residual hearing in the non-implanted ear.

"In cases where even a small amount of hearing ability remains in one ear, the central nervous system is better able to integrate auditory information with a cochlear implant, and equally so from either ear," Francis says. "This speaks to the brain's circuitry and its ability to interpret electrical signals generated by the implant even in the presumably more degenerated ear."

Spatial Hearing Aid Can Provide Direction of Sound

Thu, 28 Apr 2005 18:10:38 PST

( from http://www.rxpgnews.com ) Over three million Australians suffer from hearing loss but fewer than 20 per cent of them use hearing aids. Part of the problem is that technology just isn't good enough for them. Researchers from Sydney are changing that.

They've developed a more effective way of solving the biggest problem of the hearing impaired-how to carry on a conversation with more than one person or in a noisy environment. The result is a spatial hearing aid which provides the listener with direction as well as sound. It is now undergoing clinical trials in Sydney. If it passes the trials it could be on the market in three years.

The Spatial Hearing Aid aims to enable normal segregation of speech, and to provide a significant increase in speech intelligibility. The device should allow users to regain their ability to participate fully in their family, social and business lives.

"We humans naturally use our brains to sort out what sounds we want to pay attention to. The Spatial Hearing Aid provides spatial cues to help the hearing-impaired do this, without arbitrarily deciding which sounds are important," says Fresh innovator Dr Craig Jin, Senior Lecturer in the School of Electrical and Information Engineering at the University of Sydney and lead researcher on the project. "This is markedly different to the current industry trend which focuses on allowing the technology built into the hearing-aid to decide which sounds are important."

Up to 22 per cent of Australians suffer some sort of hearing impairment and, in people over 70, this rises to almost three out of four.

The costs in lost productivity, special education and medical care from untreated hearing impairment in the US are estimated at US$56 billion a year and growing. The Hearing Aid industry is worth about US$6 billion, with about 5.5 million units sold each year.

"Our research used a unique approach," Jin says. "We have simulated hearing-impaired listening in ourselves so that we really understand the issues confronting our end users. We aren't simply developing another hearing aid. Through our research we are examining how best to solve some of the most pressing problems facing hearing aid wearers in a new way."

"In addition, we are testing the Spatial Hearing Aid in a range of potential users to demonstrate the real world benefit for the hearing-impaired. And the University has established a spin-off company, VAST Audio Pty Ltd, to commercialise our efforts," says Jin.

Jin's innovation has won him a place at Fresh Innovators-a national initiative to bring the work of 16 early-career inventers to public attention. After training in Sydney, the Innovators are talking to the media, schools and business about their ideas. One of the 16 will win a study tour to the UK courtesy of the British Council Australia.

Stem Cells to Help Hearing Impaired

Tue, 29 Mar 2005 15:28:38 PST

( from http://www.rxpgnews.com ) Researchers at Indiana University School of Medicine are several steps closer to the day when a profoundly deaf patient's own bone marrow cells could be used to let him or her hear the world.

The IU group, led by Eri Hashino, Ph.D., was able to transform, in the laboratory, stem cells taken from adult bone marrow into cells with many of the characteristics of sensory nerve cells -- neurons -- found in the ear. The results suggest that these adult stem cells could be used to treat deaf patients in the future, said Dr. Hashino, an associate professor and Ruth C. Holton Scholar in the Department of Otolaryngology -- Head and Neck Surgery.

The cells used in the research are called marrow stromal cells -- a type of stem cell from which fat, bone and cartilage normally develop.

"We were interested in marrow stromal cells because of their potential for use in autologous cell-based therapy," said Dr. Hashino, referring to cell transplantation in which a patient's own cells are used in treatment. The cells can be collected easily and kept alive in the laboratory until needed, she said.

Other researchers had previously shown that the marrow stromal cells could be induced to transform into neuronal cells, but it wasn't clear whether, or how, the cells could be further transformed into useful specialized neurons.

In a two-step process, Dr. Hashino and her colleagues first cultivated mouse marrow stromal cells with chemicals known to encourage stems cells to change into primitive neurons. The bone marrow cells took the shape and other characteristics of neurons. Next, they exposed the cells to two molecules that are secreted from nearby tissues of the ear during embryonic development. The two molecules -- known as Sonic hedgehog and retinoic acid -- together caused the marrow stromal cells to further develop into cells with many of the characteristics of auditory neurons, such as the presence of specific genes and proteins.

Dr. Hashino said she and her colleagues are beginning new experiments to test the feasibility of marrow stromal cell transplantation to stimulate the growth of the nerve cells that are often missing from the inner ears of patients with profound hearing loss.

"Sonic hedgehog and retinoic acid are molecules found in embryonic tissues, but not in adult tissues," said Dr. Hashino. "This suggests that treating marrow-derived stem cells with these molecules before transplantation might greatly enhance the possibility that the process would result in development of specific sensory neurons."