The focus of my research is the ability of humans to function in their environment using sound. Most environments in which we spend time have complicated acoustics, with echoes coming from many directions, and with multiple sounds occurring simultaneously. We are therefore faced with the challenge of interpreting sounds as they reach our ears, learning to ignore echoes and other irrelevant, distracting signals. Some common examples are classrooms, restaurants, playgrounds and "cocktail parties". In order to understand how the brain determines the location and the content of important sounds I study hearing in adults and in children with normal hearing, as well as individuals with impaired hearing.
Research Questions:
(1) How does the brain handle echoes?
The brain has a special mechanism for suppressing echoes called the Precedence Effect, so that we can attend to the original sound source and avoid confusion in rooms that have hard surfaces, such as walls, furniture, etc. Having two ears (binaural hearing) is important since the auditory system compares sounds arriving at the two ears and uses that information to determine
(2) How can we understand speech in noisy environments?
People often complain of not being able to hear well in noisy rooms, which can be quite debilitating in schools, work environments and social settings. Our studies focus on understanding which scenarios are most difficult, whether having binaural hearing is helpful, and the effect of room acoustics. We simulate "complex" environments in our quiet, sound-proof room, by measuring speech perception and sound localization in the presence of multiple sounds arriving from varying directions. Some of the important variables are the number and types of sounds, their distance from the "target" speech, and whether the listener is using one or two ears.
(3) What happens with young children?
Children spend many hours a day in enclosed spaces, where they attempt to understand their teachers and peers, and to do so they must be able to ignore distracting sounds around them. My work has been aimed at understanding how children are able to do so, and what scenarios allow them to do so without straining to hear in a noisy environment. We developed a new test we call the "hearing game" whereby a child identifies words presented from loudspeakers, either in quiet or in the presence of other sounds. Their task is to ignore the other sounds and hear the words that matched the pictures. Our findings show that all children found the noise difficult to ignore, and that most children did much better when the noisy was on the side rather than in front, near the words they were listening for. This improvement comes about from having two ears and being able to use the ear that is farther away from the noise to listen to the words in front. We call this the "better ear effect". Where in the brain does this happen? Do children who have difficulty with attention do worse on this test? These are questions that we are currently working on along with many others.
(4) What are the clinical implications of these effects?
Recently, we have begun to apply our paradigms to clinical populations. The ultimate goal is to have these tests utilized in clinics in order to assess children's hearing in realistic complex environments, which might help in the fitting process and programming of hearing aids and cochlear implants. We have recently studied a group of young children who were born deaf and had their hearing restored through cochlear implants. We found that for each child there exists an "ideal" listening environment and strategy, which differs in quite and in noisy situation.
Through collaborations at the University of Oxford in England we are now also testing children who have experienced prolonged periods of otitis media with effusion (ear infections with "glue ear") and are investigating the extent to which having suffered conductive hearing loss affects the children's ability to benefit from spatial separation of target speech and competing sounds.
Ultimately, we would like to investigate the extent to which our hearing tests can also help us to better understand developmental disabilities such as attention deficit disorders, central auditory processing, autism and others.
Litovsky, R.Y., Johnstone, P.M., Godar, S., Agrawal, S., Parkinson, A., Peters,
R. and Lake, J. (2006).
Bilateral cochlear implants in children: localization
acuity measured with minimum audible angle. Ear and Hearing. 27(1):43-59.
Litovsky, R.Y., Johnstone, P. and Godar, S. (2006).
Benefits of bilateral
cochlear implants and/or hearing aids in children. Int. J. Audiology. Jul;45
Suppl:78-91.
Long, C.J., Carlyon, R.P., Litovsky, R.Y. and Downs, D.H. (2006).
Binaural
Unmasking with Bilateral Cochlear Implants. J Assoc Res Otolaryngol.
7(4):352-60. Epub 2006 Aug 29.
Litovsky, R.Y., Parkinson, A., Arcaroli, J. and Sammath, C. (2006). Clinical
Study of Simultaneous Bilateral Cochlear Implantation in Adults: A Multicenter
Study. Ear and Hearing. 27(6):714-31.
Litovsky, R.Y. (2005).
Speech intelligibility and spatial release from masking
in young children. J. Acoust. Soc. Amer. 117:3091-9.
Litovsky, R.Y. and Shinn-Cunningham, B.G. (2001).
Investigation of the
relationship between three common measures of precedence: fusion, localization
dominance and discrimination suppression. Journal of the Acoustical Society of
America, 109, 346-358.
Litovsky, R.Y., Colburn, H.S., Yost, W.A., and Guzman, S. (1999).
"The
precedence effect." Review & Tutorial paper, Journal of the Acoustical Society
of America, 106, 1633-1654.
