THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

236 10. Physiology of Hearing and PsychoacousticsGene Therapy: Formation of New Cochlear Hair CellsIn 1998 the geneticist Huda Y. Zoghbi of the Baylor College of Medicine discoveredthe key to generation of new hair cells in a gene called Atohl, first discovered infruit flies. Variations of this gene have since been discovered in almost all speciesof animals. During fetal development, the gene converts some cells in the ear intohair cells. In other ear cells, called supporting cells, its activity is suppressed.Researchers have demonstrated that working in laboratory dishes, the gene couldconvert supporting cells into hair cells.In 2003, Yehoash Raphael and Kobei Kawamoto at the University of MichiganMedical School reported that inserting the gene into live guinea pigs producedthousands of new hair cells (Kawamoto et al., 2003; University of Michigan HealthSystem, 2003). But in those experiments, the researchers did not deafen the animalsfirst. Later they did, using toxic chemicals to kill the hair cells in both ears of tenguinea pigs. Microscopic images taken 3 days later confirmed that all the hair cellswere destroyed.On the fourth day, they used gene therapy with a viral vector to insert the Atohlgene into the guinea pigs’ left ears. Within 2 months, new hair cells appearedin treated ears, but not in the untreated right ears. In order to determine whetherthe new hair cells were functional, the team applied tests of auditory brainstemresponse to measure the guinea pigs’ ability to hear sounds. In effect, they observedincreases in brain activity when they exposed the animals to noises, which indicatesthat their ability to hear was at least partially restored.Raphael and his colleagues are presently trying to determine how good therestored hearing is. To indicate whether the guinea pigs can hear and how well,they are working with a psychologist who is an expert at training animals todisplay various behaviors. They are trying to determine, for example, whetherthe animals can differentiate between loud and soft sounds and between differentfrequencies. They are also studying animals that have been deafened by othermeans, older animals and animals that have been deaf for longer periods of timebefore treatment begins. If these experiments turn out to be successful, the studiesnecessary to ensure safety and efficacy must be conducted before the techniquecan be tried on humans, and this phase may take the better part of a decade.Direct Drive Hearing System (DDHS)A conventional hearing aid operates by amplifying sound and delivering the soundto the eardrum. From there, the amplified sound passes through the three hearingbones on its way to the cochlea. The direct drive hearing system (DDHS) undergoingtesting at the Department of Otolaryngology and Virginia Merrill BioedelHearing Research Center at the University of Washington replaces this acousticpath with an electromagnetic one (Von Ilberg et al., 1999). Instead of producinghigh-volume sound, this hearing aid is attached to an electromagnetic coil that fitsinside the earmold. The coil is used to drive a magnet that is attached to the thirdhearing bone. This system effectively bypasses the ear canal, the eardrum, and