Fine-tuning. Animals and humans alike have an uncanny ability to pinpoint the source of a sound. This capacity depends on the ability of auditory neurons in the brain to compute the differences between the time it takes for a sound to reach each ear — measurements known as interaural time differences (ITDs).

A theory known as the Jeffress model, first proposed in 1948, explains how auditory neurons process ITD information. It is still considered reliable to explain how birds localize sound, but recent research questions the ability of the Jeffress model to comprehensively explain how mammals localize sound. A group of researchers at ENG’s Center for Hearing Research has recently proposed a new model of sound localization in mammals. The team includes Steven Colburn, a biomedical engineering professor and center director, Yi Zhou (ENG’05), a biomedical engineering Ph.D. candidate, and Laurel Carney, a former member of the biomedical engineering department, now at Syracuse University.

The researchers developed a computer simulation of auditory neurons in a gerbil brain, located in an area known as the medial superior olive. Neurons generally have three parts, dendrites (which receive stimulation), a soma (cell body), and an axon (which passes stimulation along to the next neuron). Dendrites and axons are usually connected to the cell body. According to anatomical evidence, each neuron has two dendrites of slightly different length, one receiving input from the left ear, the other from the right. If stimulation from each dendrite arrives at the soma at precisely the same time, the neuron fires at maximum strength. Since only one neuron, or subset of neurons, receives simultaneous inputs in response to a sound from a particular location, the source of the sound is derived from the position of the neuron that is firing at maximum strength. This occurs in a matter of microseconds.

To better explain what happens in mammalian hearing, the researchers incorporated anatomic observations not included in previous simulation efforts. Gerbils and other mammals have an asymmetrical cell structure produced when axons connect to dendrites rather than to the cell body. The simulations developed by Zhou and colleagues looked at how the asymmetric cell structure, in conjunction with other factors that excite or inhibit the response of the cells, changed the best ITD, or the timing at which the cell fired at maximum capacity. They found that the three factors (asymmetry, excitation, and inhibition) shifted best ITD within a single neuron, in effect working like a tuning system — similar to tuning a radio to find the strongest frequency of a particular station.

According to Zhou, mammalian auditory neurons may be more individualized than those of other species and therefore able to use several neuronal factors to maximize the ability to locate sound — an ability that contributes to survival on urban streets, just as it did when we depended on hunting and gathering for survival.

This work was published in the March 23 issue of the Journal of Neuroscience.