Researchers at University of Oregon have discovered that separate brain pathways process the start and end of what we hear.
The team has isolated an independent processing channel of synapses inside the brain's auditory cortex that deals specifically with shutting off sound processing at appropriate times. Such regulation is vital for hearing and for understanding speech.
The finding has been reported in the Feb. 11 issue of the journal Neuron.
The discovery goes against a long-held assumption that the signaling of a sound's appearance and its subsequent disappearance are both handled by the same pathway.
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 UO Institute of Neuroscience.
"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.
To do the research, Wehr and two UO undergraduate students -- lead author Ben Scholl, now a graduate student at the Oregon Health and Science University in Portland, and Xiang Gao -- 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.
It became clear, the researchers found, that 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.