The new finding could lead to new, distinctly targeted therapies such as improved hearing devices, said Michael Wehr, a professor of psychology and member of the University of Oregon Institute of Neuroscience.
Researchers monitored the activity of neurons and their connecting synapses as rats were exposed to millisecond bursts of tones, looking at the responses to both the start and end of a sound. They tested varying lengths and frequencies of sounds in a series of experiments, and one set of synapses responded "very strongly at the onset of sounds," but a different set of synapses responded to the sudden disappearance of sounds.
There was no overlap of the two responding sets, the researchers noted. The end of one sound did not affect the response to a new sound, thus reinforcing the idea of separate processing channels.
"It looks like there is a whole separate channel that goes all the way from the ear up to the brain that is specialized to process sound offsets," Wehr said. The two channels finally come together in a brain region called the auditory cortex, situated in the temporal lobe.
The team also noted that responses to the end of a sound involved different frequency tuning, duration and amplitude than those involved in processing the start of a sound, findings that agree with a trend cited in at least three other studies in the last decade.
"Being able to perceive when sound stops is very important for speech processing," Wehr said. "One of the really hard problems in speech is finding the boundaries between the different parts of words. It is really not well understood how the brain does that."
As an example, he noted the difficulty some people have when they are at a noisy cocktail party and are trying to follow one conversation amid competing background noises. "We think that we've discovered brain mechanisms that are important in finding the necessary boundaries between words that help to allow for successful speech recognition and hearing," he said.
The research could also prove useful in working with children who have deficits in speech and learning, as well as in the design of hearing aids and cochlear implants. He also noted that people with dyslexia have problems defining the boundaries of sounds in speech, and tapping these processing areas in therapy could boost reading skills.
Ben Scholl, Xiang Gao, Michael Wehr, 'Nonoverlapping Sets of Synapses Drive On Responses and Off Responses in Auditory Cortex', Neuron, Febraury 2010, 65(3), 412-421; doi: 10.1016/j.neuron.2010.01.020