Out with Old Sounds, In with New

You’re visiting a friend whose grandfather clock ticks so loudly it is maddening. But your friend seems not to notice. How has he managed to block out the sound?

Credit goes to neurons in the mammalian brainstem that enable humans and other animals to ignore ongoing, predictable sounds while focusing on new, novel sounds. These “novelty detector neurons” quickly stop firing if a sound or sound pattern is repeated, but will briefly resume firing whenever some aspect of the sound changes.

According to Ellen Covey, UW professor of psychology, the neurons can detect changes in the pitch, loudness, or duration of a single sound and can even detect changes in the pattern of a complex series of sounds. Covey has been studying the neurons with colleagues Dr. Manuel Malmierca of the University of Salamanca and doctoral student David Perez-Gonzalez, a visiting scientist in the UW psychology department.

The neurons are located under the cortex in a part of the brain called the inferior colliculus. The research implies that these cells can “remember a frequently occurring pattern and perform relatively sophisticated cognitive tasks such as discriminating a novel pattern from a frequently occurring one,” says Covey.
She adds that cognitive processes for sorting and identifying sounds occur very early in the auditory pathway, and that novelty detector neurons could be involved in directing attention to unexpected sounds, possibly evoking rapid reflex responses.

Novelty detector neurons seem to act as gatekeepers, preventing information about unimportant sounds from reaching the cortex, thus allowing people to ignore sounds that do not require attention.

“It is probably a good thing to have this ability because it allows us to tune out background noises like the humming of a car’s motor while we are driving,” Covey said. “But at the same time, these neurons would instantly draw a person’s attention if their car’s motor suddenly made a strange noise.”

Because novelty detector neurons are able to store information about a pattern of sound, they may also be involved in breaking down an ongoing stream of sound into segments and making predictions about what sounds are expected to occur next.

Covey believes these neurons provide a unique model that can be used to explore some of the neural mechanisms underlying memory, prediction, and selective attention. She is particularly interested in the role of prediction in cognitive tasks such as bats’ use of echolocation and humans’ understanding of speech.

“Speech fluency requires a predictive strategy. Whatever we have just heard allows us to anticipate what will come next, and violations of our predictions are often surprising or humorous,” Covey explains. “Without prediction we would be listening syllable by syllable and we would not have an idea when a word began or ended. We want to understand the neural processes that allow us to do this, and how much change in a sound is necessary for novelty detector neurons to identify a change.”

[Winter-Spring 2006 - Table of Contents]