How is sound localization involved in our ability to carry a conversation in a busy restaurant?
Did you ever feel you have trouble following a conversation in a loud and crowded place such as a busy restaurant or cafeteria? And you were not sure why that is because in more quiet environments you can hear just fine? It is possible your inability to follow that conversation had nothing to do with your ability to perceivethe sound (which is done by the ears) but rather your ability to isolatethe sound from the background noise (which is done in 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 well understood and appreciated. Once the sound is received by the ears an converted into electrical activity (termed “action potentials”), this activity enters the brain and is processed in many different and very complex ways. One such processing step is the separation of multiple simultaneous sounds from each other 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 spatial information. This is the same circuit which is also used for actual 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 also separate two competing sounds from each other when they are at least 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 separation step.
There are a number of medical conditions in which humans have trouble functioning in acoustically crowded environments, often termed “cocktail party situations”. One such condition is a form of age-related hearing loss (presbycusis) which affects the brain circuits of the sound localization pathway. We call this condition “central hearing loss”. Central hearing loss affects a large portion of the aging population, and some people even starting at the middle ages. Another condition is autism spectrum disorder (ASD) and a sub-form of ASD, Fragile X syndrome. In both of central hearing loss and ASD, the affected patients’ sound localization pathways have distinct alterations compared to normal hearing patients, with the result that the affected individual’s ability to divide an acoustically complex situation into narrow spatial channels is compromised. In these patients, the spatial separation is not 5 degrees or better, but, for example, 25 degrees. This leads to a decreased ability to function in such cocktail party environments. Note that this mechanism is completely independent of a person’s ability to perceive sounds at the level of the ear and thus, traditional hearing aids or cochlear implants cannot address this issue. Unfortunately, at this point there is no effective treatment available. 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 and autism. The ultimate goal is to help the development of treatments for these conditions.