FRANKFURT. Speaking requires both sides of the
brain. Each hemisphere takes over a part of the complex task of forming sounds,
modulating the voice and monitoring what has been said. However, the
distribution of tasks is different than has been thought up to now, as an
interdisciplinary team of neuroscientists and phoneticians at Goethe University
Frankfurt and the Leibniz-Centre General Linguistics Berlin has discovered: it
is not just the right hemisphere that analyses how we speak – the left
hemisphere also plays a role.
Until now, it has been assumed that the
spoken word arises in left side of the brain and is analysed by the right side.
According to accepted doctrine, this means that when we learn to speak English
and for example practice the sound equivalent to “th", the left side of the
brain controls the motor function of the articulators like the tongue, while
the right side analyses whether the produced sound actually sounds as we
intended.
The division of labour actually follows
different principles, as Dr Christian Kell from the Department of Neurology at
Goethe University explains: “While the left side of the brain controls temporal
aspects such as the transition between speech sounds, the right hemisphere is
responsible for the control of the sound spectrum. When you say 'mother', for
example, the left hemisphere primarily controls the dynamic transitions between
“th" and the vowels, while the right hemisphere primarily controls the sounds themselves."
His team, together with the phonetician Dr Susanne Fuchs, was able to
demonstrate this division of labour in temporal and spectral control of speech
for the first time in studies in which speakers were required to talk while
their brain activities were recorded using functional magnetic resonance
imaging.
A possible explanation for this division
of labour between the two sides of the brain is that the left hemisphere
generally analyses fast processes such as the transition between speech sounds
better than the right hemisphere. The right hemisphere could be better at
controlling the slower processes required for analysing the sound spectrum. A
previous study on hand motor function that was published in the scientific
publication “elife" demonstrates that this is in fact the case. Kell and his
team wanted to learn why the right hand was preferentially used for the control
of fast actions and the left hand preferred for slow actions. For example, when
cutting bread, the right hand is used to slice with the knife while the left hand
holds the bread.
In the experiment, scientists had
right-handed test persons tap with both hands to the rhythm of a metronome. In
one version they were supposed to tap with each beat, and in another only with
every fourth beat. As it turned out, the right hand was more precise during the
quick tapping sequence and the left hemisphere, which controls the right side
of the body, exhibited increased activity. Conversely, tapping with the left
hand corresponded better with the slower rhythm and resulted in the right
hemisphere exhibiting increased activity.
Taken together, the two studies create a
convincing picture of how complex behaviour – hand motor functions and speech –
are controlled by both cerebral hemispheres. The left side of the brain has a
preference for the control of fast processes while the right side tends to
control the slower processes in parallel.
Publications:
Floegel M, Fuchs S, Kell CA (2020)
Differential contributions of the two cerebral hemispheres to temporal and
spectral speech feedback control. Nature Communications, 11:2839. https://doi.org/10.1038/s41467-020-16743-2
Pflug A, Gompf F, Muthuraman M, Groppa S,
Kell CA (2019) Differential contributions of the two human cerebral hemispheres
to action timing. eLife, 8:48404 https://doi.org/10.7554/eLife.48404
Further
information: Dr. Christian Kell, Cognitive Neuroscience Group, Clinic for Neurology, Goethe University Frankfurt/ University
Hospital Frankfurt, Tel.:
+49 69 6301-6395, E-mail:
c.kell@em.uni-frankfurt.de
