10 December 2015

Music and language processing in the brain even more complex than previously thought

Moerel, who conducted research on human sound processing at the Center for Magnetic Resonance Research (CMRR) in Minneapolis (USA). This study was made possible with a Rubicon grant by the NWO Social Sciences (MaGW). She recently published her findings in Scientific Reports

Michelle Moerel investigated how people process sound in two small subcortical areas of the human brain (situated under the cerebral cortex). These inferior colliculi, of which everybody has two, and the auditory part of the thalamus are part of the hearing system in the brain. As the areas are just a few cubic millimetres in size, Moerel used MRi scanners with an ultra-strong magnetic field of 7 Tesla. This allowed the researchers to view both the brain and brain processes at a far higher resolution than is possible with conventional MRI scanners.

'The Netherlands also has powerful scanners like these, but at CMRR I could learn from experts who know how to use these scanners to investigate the function of auditory brain areas,' says Moerel. 'We still know so little about the areas of the human brain that deal with sound. Much more is known about this in animals. However, what takes place in the human brain is far more complex. After all, we process intricate sounds like language and music. How, exactly, we are able to do this is largely unknown.'
During the experiment, subjects in the MRI scanner heard a range of sounds, like music, animal sounds and speech. These sounds came from different locations with respect to the participant. The study revealed that sounds originating from the left were processed in the right inferior colliculus and thalamus and vice versa. This result was not surprising, as earlier animal studies had also shown this to be the case. 

The researchers, however, discovered something else. In both the inferior colliculi and the thalamus, sound processing is tonotopically organised, or organised by pitch. Each part of the areas investigated responded best to a specific frequency. Researchers could see this because the part concerned consumed the most oxygen. Successive pitches were found to be processed next to each other, similar to the keys on a piano. Even more striking: a single representation (full keyboard) for all pitches was found in the inferior colliculi and two of these 'keyboards' were found in the auditory part of the thalamus. Why the thalamus processes all pitches twice remains unclear.
This is the first time that sound processing in the auditory thalamus has been studied. In the past, researchers tended to focus on visual processing in the brain instead of sound processing.

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