Psychophysical Studies with Adults:

           The "cocktail party effect" in normal-hearing adults: We are investigating people's ability to understand speech and to ignore other competing or distracting sounds, such as noise or other voices. We are specifically interested in the extent to which speech intelligibility improves when the target sound and competing sounds are spatially separated, the auditory cues that contribute to this benefit, and the extent to which echoes interfere with this improvement.

           The "precedence effect" and echo suppression. In a reverberant environment, sounds reach the ears through several paths. Although the direct sound is followed by multiple reflections, which would be audible in isolation, the first-arriving sound is treated with greater “weight” by the auditory system, and therefore dominates many aspects of perception. The ‘‘precedence effect’’ refers to perceptual phenomena that are thought to be involved in resolving competition for perception and localization between a direct sound and a reflection. We are studying these phenomena in normal-hearing people in the hopes of better understanding how the brain suppresses echoes and enables listeners to function in reverberant environments.

           Bilateral cochlear implants: In recent years there has been tremendous success with the restoration of hearing to deaf individuals through the implantation of cochlear implants. However, most implant users complain of having difficulty hearing in noisy environments. We are investigating the extent to which having a second implant, hence bilateral hearing, will provide the necessary benefit to enable implant users to overcome trouble in realistic auditory environments. We are studying how bilateral implants affect sound localization and speech understanding in noise. In addition, in collaboration with Dr. Richard Van Hoesel from the CRC in Melbourne, we are using direct stimulation of electrode pairs in the two ears we are investigating the ability of the listeners to process binaural stimuli. The effect of age at which adults acquired deafness turns out to be an important predictor of their ability to use binaural stimuli. In addition, we are investigating the importance of the rate of stimulation and electrode interactions. (see our recent conference presentations).

           Cochlear implant simulations of speech signals: We are simulating listening through cochlear implants to study how normal hearing people cope with reduced number of auditory channels and various speech processing strategies, especially in the presence of noise. This work is being conducted in collaboration with Dr. Fan-Gang Zeng at U.C. Irvine.
 

Psychophysical Studies with Children:

           Speech intelligibility in noisy and reverberant environments: Young children spend a majority of their time in noisy environments, where they must cope with constant competition between the target voice (such as a teacher in a classroom) and noise or other children talking. Some children cope with this challenge better than others. We are attempting to understand how the auditory system of young children processes multiple signals and which auditory cues are most effective in providing improved speech understanding. CRISP (children’s realistic index of speech perception) is our new assessment tool for obtaining these measures. It enables the presentation of target words in the presence of various types of competing sounds whose locations can be varied relative to the child’s head. This test comes in both computer-game and manual versions that can be purchased from the lab (contact Litovsky@waisman.wisc.edu).

           Directional hearing: We are assessing how well children can localize sounds in the environment. We know from our previous work that they are worse than adults at suppressing echoes. Now, we are interested in knowing more about how they integrate information about the location and content of sounds. How do they solve the problem of "what" is the signal and "where" is it coming from?

           Bilateral cochlear implants: The majority of children with implants world-wide have a single device. While there is a growing number of children with bilateral implants, the benefits of 2 vs. 1 must be documented.  In our lab we are studying the effects of bilateral implants on the ability of children to hear speech in noise (or two-talker babble), and to localize sounds. To date, we have studied over 20 children, the majority are recipients of Nucleus-24 devices, and smaller numbers have either the MedEl or Clarion devices. We focus on tracking their performance during the first 2 years after receiving the second CI. To date, several of the children have reached the 24-month milestone. Nearly all children have been showing improved performance on speech understanding in noise under bilateral conditions compared with unilateral listening modes. The improvement is measurable within a few months of bilateral hearing. Sound localization abilities, which are rather poor at 3- and 9-months, begin to improve at 15-months or longer following the onset of bilateral hearing.  Children with bilateral hearing aids, and with CI+HA have also been participating in this research. From this work we hope to better understand the relative benefits of unilateral and bilateral amplification and implantation, and the impact of each approach on children's ability understand speech and to localize sounds in their environments.  The first two publications from this work are  in the journal Ear and Hearing and International Journal of Audiology, and can be found at http://www.waisman.wisc.edu/bhl/publications.htm. For further information please contact Ruth Litovsky.