How is sound localization involved in our ability to carry a conversation in a busy restaurant?
Have you ever had trouble following a conversation in a loud and crowded place such as a busy restaurant or cafeteria? And you were not sure why because you can hear just fine in less busy environments? It is possible your inability to follow that conversation had nothing to do with your ability to perceive the sound (which is done by the ears) but rather your ability to isolate the sound from background noise (which is done by the brain).
Hearing deficits at the level of the ears are a well-known phenomenon which affects many people, especially at older ages. In many cases they can be treated with devices such as hearing aids, cochlear implants or similar. By contrast, hearing deficits at the level of the brain are much less understood and appreciated. Once the sound is received by the ears and converted into electrical activity (termed “action potentials”), this activity enters the brain and is processed in many different, very complex ways. One such processing step involves segregating multiple simultaneous sounds based on their spatial location. In complex acoustic environments, different sounds originate from different points in space, for example different tables in that busy restaurant. A circuit in the brain termed the “sound localization pathway” separates these sounds from each other based on this spatial information. This same circuit is also used for behaviorally relavent sound localization by many animal species, for example bats and owls when they hunt for prey. In the case of modern-day humans, this brain circuit “localizes” the various competing sounds and “sorts” them into their respective spatial channels. This circuit is very precise and normal hearing humans can localize sound sources with an accuracy of better than 5 degrees in space – meaning they can separate two competing sounds from each other even when they are only a few spatial degrees apart from each other. This spatial separation provides a very important basis for our ability to isolate sounds of interest when distracting background noises are present. While the mammalian (and human) brain has additional mechanisms that build upon this initial separation, the sound localization pathway provides a critical first step in differentiating competing auditory signals.
If this circuit does not function as precisely as it should, we have trouble in that busy restaurant. There are a number of conditions in which humans have trouble functioning in acoustically crowded environments, termed “cocktail party situations” by the current literature. One such condition is a form of age-related hearing loss (presbycusis) which affects the neural circuits of the sound localization pathway. We call this condition “central hearing loss” in contrast with “peripheral hearing loss” which is the result of hearing deficits at the ear level rather than the neural level. Central and peripheral hearing loss are independent of each other such that an older listener may have peripheral hearing loss, central hearing loss, neither, or both. Central hearing loss affects a large portion of the aging population, but can emerge as early as the middle ages. Another condition relevant to this localization circuit is autism spectrum disorder (ASD) and a sub-form of ASD, Fragile X syndrome. In both central hearing loss and ASD, the affected listeners’ sound localization pathways have distinct alterations compared to normal hearing listeners. As a result, the affected individual’s ability to divide an acoustically complex situation into narrow spatial channels is compromised. In these patients, the spatial separation is no longer 5 degrees or better, but, for example, 25 degrees, 45 degrees or even greater. This leads to a decreased ability to function in such cocktail party environments. Note that traditional hearing aids or cochlear implants cannot address this issue, because they can only treat hearing difficulties at the level of the ear, not the brain. Unfortunately, at this point there is no effective treatment available for central hearing loss. Our laboratory aims to understand the normal functioning of the healthy sound localization pathway, and moreover understand how exactly this pathway is altered in central hearing loss. The ultimate goal is to help the development of treatments for these conditions.
Over the last few years our laboratory has studied this problem in both human subjects and animal models with nearly identical results. Regarding aging, we identified a key subcellular mechanism that changes with age, causing less precise sound localization and less precise spatial separation in cocktail party situations. We can experimentally re-create this change in young animals – effectively “making young animals old”, and we can reverse this age-related change in old animals – effectively “making old animals young”. Will the same therapy also work in humans? We have high hopes and are currently preparing a clinical trial to answer this exact question.