New research from the US suggests that the brain uses separate pathways to process the start and the end of sounds, a discovery that could change our ideas about how we hear and understand speech and lead to improvements in how we help children with speech and hearing problems and the design of hearing aids.

You can read a report on the study by researchers at the University of Oregon (UO) in Eugene in the 11 February online issue of the journal Neuron.

There is a long-held belief among scientists that the signalling of a sound’s appearance (“onset”) and its subsequent disappearance (“offset”) are both handled by the same pathway in the brain.

Co-author Dr Michael Wehr, a professor of psychology and member of the UO Institute of Neuroscience, explained to the media that this study now challenges that belief, suggesting a separate set of synapses deals with sound offsets:

“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.”

“The two channels finally come together in a brain region called the auditory cortex, situated in the temporal lobe,” he added.

Wehr said this could lead to new therapies and hearing devices that target the pathways separately, for instance, depending on which one is the main problem.

For the study, which was part funded by UO’s Robert and Beverly Lewis Center for Neuroimaging Fund, Wehr and two UO undergraduate students, Xiang Gao and Ben Scholl, lead author of the paper and now a graduate student at the Oregon Health and Science University in Portland, observed what happened to neurons and their connecting synapses in rats when they were exposed to short millisecond bursts of sound tones. They were particularly interested in what happened at the start and the end of each tone and used different lengths and frequencies.

They found that one set of synapses was strongly active at the onsets or start of tones but a different group of synapses responded strongly at the offsets, or the sudden ending of the tones.

They also found no overlap of the two responding sets: the end of one tone did not affect the response to a new one, which stressed the point that it must be due to separate pathways.

The researchers noted that:

“.. on and off responses appear to be driven by distinct sets of synapses, because they have distinct frequency tuning and different excitatory-inhibitory balance.”

“Furthermore, an on-on sequence causes complete forward suppression, whereas an off-on sequence causes no suppression at all, they added, concluding that:

“… on and off responses are driven by largely nonoverlapping sets of synaptic inputs.”

Wehr said that being able to notice when a sound stops is very important for processing speech, explaining that one of the hardest problems is:

“Finding the boundaries between the different parts of words. It is really not well understood how the brain does that.”

Wehr gave the example of the difficulty some people have when they try to have a conversation at a cocktail party and they struggle to listen to what the other person is saying against a range of 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,” said Wehr, who suggested the discovery could improve how we help children with deficits in speech and learning.

It could also lead to better hearing aids and cochlear implants, and Wehr also suggested it could improve therapies that help people with dyslexia improve their reading skills because they also have problems identifying boundaries of sounds in speech.

“Nonoverlapping Sets of Synapses Drive On Responses and Off Responses in Auditory Cortex.”
Ben Scholl, Xiang Gao, Michael Wehr
Neuron, 65(3) pp. 412 – 421, published online 11 February 2010.
DOI:10.1016/j.neuron.2010.01.020

Source: University of Oregon.

Written by: Catharine Paddock, PhD