“What's that, say again?” Study takes aim at cocktail party problem

Toronto , CANADA – Researchers in Toronto are a step closer to figuring out why aging adults with little or no hearing loss may still have a hard time holding a conversation at a loud restaurant or party.

A new study from Rotman Research Institute at Baycrest has found that age-related changes in the brain's auditory cortex may help explain one of life's most difficult challenges in sound perception, known as the “cocktail party problem”. It is the ability to understand speech in the presence of other speakers or background noise.

For the past 50 years, psychologists and neuroscientists have been studying the problem and much has been written on how aging may impair a listener's ability to identify speech in a noisy environment. Hearing sensitivity, memory, attention and speed of processing have all been proposed to account for the difficulties, but there is no consensus about the brain mechanisms underlying age-related speech perception problems.

The auditory cortex is located in the top part of the temporal lobe, which is a part of the brain that is behind the ears. While other parts of the brain receive input from the ear, the auditory cortex is thought to be the main hub in the cerebral cortex that performs more complicated types of processing. Short sounds, long sounds, frequency, pitch, loudness and complex patterns such as speech and music are all processed here.

“Our study found that a particular low level function of the auditory cortex – the ability to automatically segregate two vowel sounds – was degraded in older adults,” says Rotman research fellow Dr. Joel Snyder who led the study with senior scientist Dr. Claude Alain. “Low level” function refers to an automatic response in the brain that does not require active listening or concentration.

In the study, 16 young adults (average age 24) and 16 older adults (average age 67) with normal hearing and English as their first language donned headsets and listened to a mixture of two phonetically different vowels with the same or different frequency. The stimuli were five American English vowels: AH, EE, AE (as in apple), OO and ER. All participants underwent an initial practice phase that tested their ability to identify each vowel sound separately; all did very well in this task.

In the main part of the experiment, young and older adults were asked to listen to two vowel sounds played at exactly the same time as if spoken synchronously by two different people and identify them by pressing appropriate computer keys. During the task, researchers measured participants' brain activity using event-related potential (ERP) recording which involves a swimming cap placed on the head that contains tiny electrode sensors. ERP recording is a powerful tool for measuring the precise timing of specific stages of processing which occur in the brain in response to external stimuli, such as vowel sounds. It is also a powerful technique for examining the effects of age on the neural correlates underlying speech separation and identification.

Overall findings
While older adults performed as well as younger adults in identifying the first or more dominant vowel, older adults had significantly more difficulty in identifying the second or less dominant vowel sound. Another important difference noted was that older adults required more processing time on the stimuli for the second non-dominant vowel and especially when there was a small separation in pitch level for the two vowels.

In addition to these age-related behavioural changes, an ERP response that was previously shown by Dr. Alain to indicate successful segregation of sounds (in an experiment with younger adults) was greatly reduced in amplitude in older participants.

In previous studies this brain response, related to segregation of sounds, occurred whether or not participants were paying attention to the sounds. This suggests that brain mechanisms in the auditory cortex that automatically segregate sounds are impaired in older adults.

The findings may one day contribute to the development of a “training paradigm” or auditory exercise regimen to rehabilitate this low level, sound-processing function in older adults, says Dr. Snyder. “Such rehabilitation would be particularly helpful to older adults who are feeling socially isolated and longing to enjoy an outing at a restaurant, sports stadium or party with more confidence in their ability to listen to and partake in conversations,” he points out.

The study was funded by grants from the Canadian Institutes of Health Research, the Natural Sciences and Engineering Research Council of Canada, and the Premier's Research Excellence Award. Baycrest is an internationally renowned academic health sciences centre, affiliated with the University of Toronto.

The study, “Age-related changes in neural activity associated with concurrent vowel segregation”, is in the August 31, 2005 issue of Cognitive Brain Research (Volume 24, Issue 3).