Scientists wouldn't go near these cells. They called them "annoying neurons." They were frustrated by how hard it was to excite the cells; the neurons seemed to activate randomly. The hesitance wasn't entirely unwarranted. As researchers from the University of Salamanca and the University of Washington recently discovered, randomness is exactly what these neurons need in order to be activated.
In a paper published in the European Journal of Neuroscience, three researchers pinpoint neurons at the base of the brain stem that respond to novel sounds. When a sound is sustained or a pattern is repeated, these neurons lie dormant. But when pitch changes even slightly, or the sound suddenly grows a bit louder, or if a sound in a pattern endures a bit too long, these neurons go haywire.
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"It's been generally thought that this kind of semi-cognitive processing doesn't occur until the level of the cortex in a mammal," said Ellen Covey, psychology professor at the University of Washington and coauthor of the study. "One of the interesting things about this is that it implies that there's some kind of cognition occurring at a preconscious level."
David Perez-Gonzalez, lead author of the study, noted that these neurons have the capacity to detect a wide variety of changes in sound that falls within the auditory range.
"These neurons habituate rapidly when a repetitive sound is presented, but fire again when the sound is 'new,'" he said via e-mail. "They are sensitive to changes in the frequency, intensity or duration of the sounds, and also to more complex features, like the modulation rate of amplitude modulated sounds."
The Spanish and American team studied these neurons in rats. It used saline solution to record the action potential, a spike in a neuron's electrical activity when it's sending information. Different auditory stimuli were presented to the rats and activity was observed in these neurons. For a simple sound repetition, the signal only spiked in response to the initial noise, and, occasionally, one repetition. The neurons showed activity in response to randomized stimuli but did not react to stimuli that changed at a predictable rate.
Covey said the researchers are now using a program that presents the same stimulus over and over and then throws in an "oddball stimulus" to observe how the neurons react. This will allow them to determine how much a sound needs to change in order to be considered "novel" and garner a response from these neurons.
Covey said the research almost certainly applies to humans, although the study was conducted in rats, because of remarkable similarities in brain structure and circuitry between rats and humans.
These neurons may play a key role in other mysterious psychological processes.
"Due to their properties, these neurons could provide a model to explore some of the neural mechanisms underlying memory, prediction and selective attention," Perez-Gonzalez said. "Because of their ability to detect the onset of a new sound, these neurons may also be involved in breaking down an ongoing stream of sound into segments.