How does brain catch up with sound of silence?

February 11th, 2010 - 6:39 pm ICT by IANS  

Washington, Feb 11 (IANS) Researchers have isolated an independent processing channel of synapses (nerve junctions) in a brain area that deals with turning off sound when required.
Such regulation is vital for hearing and for understanding speech. The discovery overturns a long-held assumption that the signalling of a sound’s appearance and its subsequent disappearance are both handled by the same pathway.

The new finding, which supports an emerging theory that a separate set of synapses is responsible, could lead to new, distinctly targeted therapies such as improved hearing devices, said Michael Wehr, psychology professor and member of the University of Oregon (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 specialised 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 (part of the cerebral cortex in either hemisphere of the brain lying inside the temples of the head).

Wehr and two UO undergraduate students — study co-author Ben Scholl 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 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, said an UO release.

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.

These findings appeared in the Thursday edition of Neuron.

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