How does the brain choose what to listen to?

How do we manage, without any noticeable effort, to listen to a friend talking in a crowded café, or to follow the melody of a violin in an orchestra? Scientists have developed a new approach to how the brain picks out one particular stream of sound from other, distracting sounds.

A team led by scientists from Carnegie Mellon University and Birkbeck, University of London, has developed a new approach to how the brain singles out a specific stream of sound from other, distracting sounds. Using a new experimental method, the scientists mapped sustained selective auditory attention in the human brain, without any invasive procedure. Published in the Journal of Neuroscience, the study lays the groundwork for tracking auditory attention when it is impaired by ageing, illness or brain injury, and for designing clinical interventions, such as behavioural training, to correct or prevent hearing problems.

“Deficits in selective auditory attention can arise for many reasons: concussion, stroke, autism or even healthy ageing.” They are also associated with social isolation, depression, cognitive dysfunction and poor communication at work. “We now have a clear understanding of the cognitive and neural mechanisms that allow the brain to select what to listen to,” said Lori Holt, professor of psychology in CMU's Dietrich College and a faculty member of the Center for the Neural Basis of Cognition (CNBC).

To find out how the brain can listen out for important information in different acoustic frequency ranges, much as we can focus on the treble or the bass in a music recording, eight adults listened to a series of short melodies while ignoring a distracting sound, and signalled whenever they heard a melody repeat.

To understand how paying attention to the melodies changed brain activity, the researchers made use of the fact that sound information is laid out across the surface, or cortex, of the brain. The cortex holds many “tonotopic” maps of auditory frequency. Each map represents frequency rather like an old radio dial, with low frequencies at one end and high ones at the other. These maps fit together like the pieces of a jigsaw in the upper part of the brain's temporal lobes.

When the people in the MRI scanner listened to melodies at different frequencies, the parts of the maps tuned to those frequencies became active. What was surprising was that simply paying attention to those frequencies activated the brain in a very similar way, not just in a few core areas but across much of the cortex where sound information is known to arrive and be processed.

The researchers then used a new high-resolution brain imaging technique called multi-parameter mapping, to see how the activity produced by listening to, or paying attention to, different frequencies related to another feature of the brain: myelination. Myelin is the brain's electrical insulation, and brain regions vary a great deal in how much myelin surrounds the parts of the neurons that carry information.

By comparing the frequency maps with the myelin maps, the researchers found that the two were linked in specific areas of the brain: where the amount of myelin increased across a small patch of cortex, the neurons' preference for particular frequencies increased too.

“This was an exciting finding because it potentially revealed some common ‘fault lines’ in the auditory brain,” said Frederic Dick, professor of auditory cognitive neuroscience at Birkbeck College and University College London. “Just as agricultural scientists try to understand which combination of soil, water and air makes some land better for growing certain crops, neuroscientists can begin to understand how subtle differences in the brain's functional and structural architecture may make some regions more ‘fertile’ for learning new information such as language or music.”

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Source: https://www.sciencedaily.com/releases/2017/12/171211140358.htm
Carnegie Mellon University. "Selecting sounds: How the brain knows what to listen to: New noninvasive approach reveals brain mechanisms of auditory attention." ScienceDaily. ScienceDaily, 11 December 2017.