You are lost in a crowd, when someone calls your name. Automatically, you turn toward the direction of the caller – the location of the sound. This ability, called “localizing,” is important to the survival of animals and humans.
The ability of potential prey to hear predators’ sounds, detect their whereabouts and choose a path of escape is as vital as the predators’ need to localize prey via sound cues. Localization is equally important for humans. We use it to find a ringing telephone or an acquaintance among a crowd, and to distinguish between conflicting noises in order to understand an individual speaker among the din of a cocktail party – a difficult task for many hearing-impaired people.
Understanding another’s speech amid interfering and multi-directional sounds requires separating the main speech from the interfering sounds and sound localization is an intimate part of that process.
Normally, the location of an object heard by the ears coincides with what is seen by the eyes. But when the ears or eyes are impaired due to disease or trauma, the sound location of an object sometimes does not coincide with the visible location. The brain’s adaptive process to resolve this disagreement between hearing and vision is known as plasticity, the brain’s ability to learn from experience. An important component of normal brain development and maturation, plasticity declines with age so an adult’s plasticity is naturally less than a juvenile’s.
Animal research helps us better understand how a person’s ears and brain work together to localize objects. The barn owl is particularly well-suited for studies on sound localization because it localizes and captures its prey, small rodents, by combining hearing and vision at dusk and dawn in dim light conditions.
Because its survival depends on successful hunting, barn owls have adapted well for precise localization of sounds. The animal’s brain stem has systematically organized “maps” comprised of neurons that are highly sensitive to sound (or light) located in a restricted region of space. Each neuron represents a particular location and adjacent neurons represent adjacent spatial locations, thus forming a “map.”
These auditory and visual neuron maps enable the study of adaptive plasticity at a cellular level. Current research supported by the Deafness Research Foundation, “Adaptive Plasticity of the Barn Owl Auditory Localization System,” is concerned with understanding how the barn owl’s brain learns to adjust the auditory space map when prolonged conflict between hearing and vision exists. Specifically, the study hopes to determine whether plasticity might be increased, even in adulthood, by exposing the owl to an environment with increased motivation to reshape the discordant relationship between hearing and vision. Such an environment is created by engaging the owl in active hunting and comparing the results to those of owls maintained in a passive environment. The “passive” owls are fed without any effort on their part and, therefore, need not relearn the hearing and vision relationship to survive.
Initially, a young owl is taught to turn its head toward a sound or light. After reaching adulthood, the owl is fitted with prismatic spectacles causing its vision to shift to one side, which has an immediate effect on the owl’s ability to localize visual stimuli. Because the prisms affect only the eyes, the auditory localization is initially the same, but over time, the owl’s response to sound begins to shift toward its response to light – the owl exhibits adaptive plasticity.
To understand the cellular changes forming the basis of adaptive plasticity, we record the physiological responses of neurons in the owl’s brain stem to sound and light stimuli, and the relationship between the maps of sound and light is determined. We observe that the owl’s brain demonstrates the results of adaptive plasticity after it has adjusted the hearing response to closely match the altered vision.
The results of this study should aid the efforts of clinicians, such as otolaryngologists and audiologists, working to interpret the relationship between sound localization and speech recognition amid a noisy background. The results should support development of future rehabilitative strategies to maximize the brain’s adaptive auditory plasticity to enable enhanced localization of sound and isolation of speech amid loud and conflicting noises.
Duck O. Kim, D.Sc., has been a professor of neuroscience at the University of Connecticut Health Center in Farmington, Conn., since 1986. His areas of research are signal processing in the mammalian inner ear and brain stem and auditory plasticity of the barn owl. In 1972, he received a Doctorate of Science in biomedical engineering from Washington University, where he taught for 11 years.
Andrew Moiseff, Ph.D., is currently a professor at the University of Connecticut in Storrs, Conn., where he studies barn owl hearing, as well as photic communication by fireflies. He received his doctorate in neurobiology and behavior from Cornell University in Ithaca, N.Y., where he studied acoustical communication in crickets. As a postdoctoral associate at the California Institute of Technology in Pasadena, Calif., he studied barn owl sound localization.
