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, one of the authors of the study and a psychology professor at the University of Washington. 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, she said.
Covey and her colleagues, Dr. Manuel Malmierca of the University of Salamanca and doctoral student David Perez-Gonzalez, who is currently a visiting scientist in the UW psychology department, report their findings in the early December issue of the European Journal of Neuroscience.
The neurons are located under the cortex in a part of the brain called the inferior colliculus. Covey said 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."
She said that, contrary to popular belief, the new findings suggest that some 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 or the regular tick-tock of a clock," 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 or if their cell phone rang." Because novelty detector neurons are somehow 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.
The research was done by studying neurons in the brains of rats, but Covey said similar neurons are almost certainly present in the human brain since they seem to be found in all vertebrates. Such neurons were noted in frogs years ago, but the new research for the first time pinpointed the novelty detector neurons in the mammalian brainstem.
She said these neurons provide a unique model that can be used in the future to explore some of the neural mechanisms underlying memory, prediction and selective attention. Covey 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 said. "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 know what are the neural processes that allow us to do this. We also want to know how the brain remembers a particular pattern of sound and to determine how much change in a sound is necessary for novelty detector neurons to identify a change.
The research was supported by the National Institute on Deafness and other Communication Disorders, the Spanish Ministry of Science and Education and the Spanish Junta de Castilla Y Leon.