Nerve cells in the brain region planum temporale have more synapses in the
left hemisphere than in the right hemisphere – which is vital for rapid
processing of auditory speech, according to the report published by researchers
from Ruhr-Universität Bochum and Technische Universität Dresden in the journal
“Science Advances”. There has already been ample evidence of left
hemisphere language dominance; however, the underlying processes on the
neuroanatomical level had not yet been fully understood.
A new form of magnetic resonance imaging (MRI) in combination with
electroencephalography (EEG) measurements has made it possible to bundle insights
into the microstructure of planum temporale with the speed of auditory speech
processing. The team headed by Dr Sebastian Ocklenburg, Patrick Friedrich,
Christoph Fraenz, Prof Dr Dr h. c. Onur Güntürkün and Dr Erhan Genç outlines
their findings in an article published in the scientific journal Science Advances from July 11, 2018.
Left hemisphere language dominance
Using a simple experiment, researchers can demonstrate just how superior
the left hemisphere is when it comes to the processing of auditory speech: when
playing two different syllables – for example “Da” and “Ba” – to a person’s
left and right ear via headphones, most people will state that they only heard
the syllable in the right ear. The reason: language that is perceived via the
right ear is processed in the left hemisphere. When brainwaves are measured
using EEG, it emerges that the left hemisphere processes auditory speech
information more rapidly.
“Researchers have long determined that a brain region that is crucial for
the processing of auditory speech, i.e. planum temporale, is frequently larger
in the left hemisphere than in the right one,” says Sebastian Ocklenburg from
the biopsychology research unit in Bochum. In the brains of deceased
individuals who had donated their bodies to science, Frankfurt-based
researchers later discovered that the nerve cells in the left planum temporale
have a larger number of neuronal synapses than those in the right hemisphere.
New measurement method facilitates hitherto impossible insights
“However, it had previously not been understood if that asymmetrical
microstructure is the decisive factor for the superiority of the left
hemisphere when it comes to the processing of auditory speech,” explains Erhan
Genç, likewise a member of the biopsychology research unit. Since a method for
counting the number of neural synapses in living humans had not existed until
very recently, that number could not be conclusively linked to the performance
of auditory speech processing. The researchers have now closed this gap with
the aid of so-called neurite orientation dispersion and density imaging.
By deploying this highly specific MRI technology, the bio-psychologists
measured the density and spatial arrangement of planum temporale neurites in
almost one hundred test participants. At the same time, they used EEG
measurements to analyse the processing speed of auditory speech information in
both the left and the right hemispheres in the same individuals.
Higher speed thanks to more neurites
The result: test participants who were capable of processing auditory
speech in the left hemisphere at a high speed possessed an extraordinarily high
number of densely packed neurites in the left planum temporale. “It is because
of this microstructure that processing of auditory speech is faster in the left
hemisphere; those individuals are presumably also able to decode what they hear
at higher temporal precision,” concludes Ocklenburg. “Higher connectivity
density thus appears to be a crucial component for the linguistic superiority
of our left hemisphere,” adds Genç.
Source: https://myfusimotors.com/2019/02/20/why-the-left-hemisphere-understands-language-better-than-the-right/
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