Sound Localization: What the Study is Really About, How it is Done
By Eric Sandgren, Director UW-Madison Research Animal Resource Center
Watch a fox stalking its prey, hidden under two feet of snow. He tips his head this way and that, listening for movement, then leaps into the air and dives into the snow nose first, coming up with his meal. Like most predators, foxes have the ability to combine information from both ears to precisely localize the source of a sound. The same is true for cats, who typically hunt at night, and for humans. Over the last several decades, we’re closer to an understanding about how humans and animals accomplish this remarkable task, due in large part to experiments conducted in the UW-Madison laboratory of Dr. Tom Yin.
Starting from knowledge of the anatomy of ear and brain circuits involved in hearing, Yin and his students and colleagues helped to provide some answers to the puzzle of just how these circuits work. Their model is the domestic cat, whose auditory system is very similar to a human’s. For humans this ability to localize the source of a sound is the same mechanism that also allows us to carry on a conversation in a crowded room filled with many other conversations. The inability to hear in a “cocktail party environment” is also the most common complaint of elderly patients who suffer from hearing loss.
The first studies used cats that were anesthetized for the entire experiment, meaning that they could not feel any of the procedures. Small speakers placed next to the eardrums of the anesthetized cats produced regulated sounds. At the same time, tiny electrodes introduced into specific neurons in the brain auditory circuit recorded electrical activity in those neurons. With this approach, Yin was able to map the particular circuits, and study the physiological responses of cells that are involved in localizing sound. Yin discovered that some nerve cells in the auditory system have unusual features that are not found in any other part of the brain. Connections between these neurons, called synapses, often were exceptionally large, and the transmission of an electric signal along the neurons was much faster than for most other neurons. In other words, the system had evolved to optimize sound localization, especially in hunters like cats and humans.
Because the cats were anesthetized in these experiments, it was possible that anesthesia had interfered with the brain signaling processes. To address this concern, and to allow the scientists to correlate neuronal activity with actual cat behavior, Yin progressed to an awake cat system. This model requires the surgical implanting of stainless steel head posts and eye coils onto the cats to, respectively, stabilize the head during experiments and monitor eye movements to measure the cats’ ability to localize sounds. Later, full head and eye movement was allowed, since it turned out that this greater flexibility enabled the cats to more accurately fix their eyes on the source of a sound. The head posts are nearly identical to part of the “halo apparatus” used in humans to keep the head from moving after a spine injury. For humans and animals, such implants increase the risk of localized infection at the site where it is attached. For this reason, the cats were monitored closely by veterinary, animal care, and investigative staff. If an infection developed, it was treated immediately and vigorously.
The biological findings from these studies have allowed engineers to develop a computer model of sound localization in the auditory system. Placed on a chip and inserted into a cell phone, this technology turns cell phones into “binaural” devices, using two microphone receivers to filter out background noise. The technology enables sound spatial localization through headphones for computer applications, including video gaming, but also for air force pilot/airplane interfaces, where response time is critical. In other words, Yin’s work has been an essential element in the creation of virtual sound reality.
More recently, Yin and colleagues have worked with otolaryngologists to explore whether bilateral cochlear implants provide more effective sound localization, and therefore better hearing, than single implants. In a pilot study, two cats were deafened with a local overdose of the antibiotic neomycin. Each received a surgically implanted electrode in each ear identical to a human artificial cochlea. The Madison scientists were trained in the cat cochlear implant technique by visiting experts from the Bionic Ear Institute in Australia, which supplies many of the human cochlear implants world wide. After the surgery, small microphones were fitted to the head posts so that sounds could be delivered just outside of the ear. Because this procedure was more invasive than earlier procedures, and associated with more medical complications, the scientists decided not to extend the study beyond the pilot stage (no brain electrodes were used). However, they successfully demonstrated that, with bilateral cochlear implants, cats could accurately localize sounds, mirroring the ability of cats with intact cochlea.
What about medical applications? The basic studies in the Yin laboratory have created much of our understanding of binaural hearing mechanisms. The design of hearing aids and cochlear implants rely on this basic information, and more information is needed to further improve these devices. Yin’s work is well-respected across the world and widely cited in the literature with nearly 5000 citations over the years. Thus, any medical advances in this area will use Yin’s work as a starting point.
In the future, the Yin laboratory will continue to study the basic mechanisms of binaural hearing, correlating neural activity in the auditory system with behavioral perception in the cats. He has been a leader in the neuroscience community at UW-Madison for over 35 years, serving as the director of the graduate program in Neuroscience and as the interim chair of the newly-formed Department of Neuroscience as well as an award-winning teacher who teaches a large and popular undergraduate and graduate course in neuroscience.