updated 12/2/2005 7:41:54 PM ET 2005-12-03T00:41:54

Special neurons in the brain stem of rats focus exclusively on novel sounds and help them ignore predictable and ongoing noises, a new study finds.

The same process likely occurs in humans and may affect our speech, and even help us laugh.

The "novelty detector neurons," as researchers call them, quickly stop firing if a sound or a pattern of sounds is repeated. They will briefly resume firing if some aspect of the sound changes. The neurons can detect changes in pitch, loudness or duration of a single sound and can also note shifts in the pattern of a complex series of sounds.

"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 or the regular tick-tock of a clock," said study team member Ellen Covey, a psychology professor at the University of Washington. "But at the same time, these neurons would instantly draw a person's attention if their car's motor suddenly made a strange noise or if their cell phone rang."

Covey said similar neurons seem to be present in all vertebrates and almost certainly exist in the human brain.

The novelty detector neurons seem to act as gatekeepers, Covey and her colleagues conclude, preventing information about unimportant sounds from reaching the brain's cortex, where higher processing occurs. This allows people to ignore sounds that don't require attention.

The results are detailed this month in the European Journal of Neuroscience.

The novelty detector neurons seem able to store information about a pattern of sound, so they may also be involved in speech, which requires anticipating the end of a word and knowing where the next one begins.

"Speech fluency requires a predictive strategy," Covey explained. "Whatever we have just heard allows us to anticipate what will come next, and violations of our predictions are often surprising or humorous."

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