With right-handed people, it is positioned in the left side of the brain; left-handed people have it (usually) in the right side: the location of speech production has been known for quite some time. But it is not that simple, states psychologist Gesa Hartwigsen, Professor at Kiel University. In her current scientific publication, published in the magazine Proceedings of the National Academy of Science (PNAS), she investigates which areas in the brain really are in charge of speech, and how these interact. Her findings are supposed to help patients who have speech production problems or aphasia following a stroke.
Comprehending & Speaking
Gesa Hartwigsen and her team started by analysing speech production. They let healthy right-handed test persons listen to words, which they should then repeat. "These were pseudo words such as `beudo`. In German, they don't have any associated meaning. Therefore, when hearing and repeating these words, no areas of the brain that had a connection to the meaning of what had been heard were activated," said Hartwigsen.
The psychologist applies a combination of non-invasive methods (fMRI- functional magnetic resonance imaging and TMS -- transcranial magnetic stimulation) to deduce what happens in the brain during the test. "We thus proved that the left hemisphere, as expected, was activated during speech production, while the right hemisphere did not actively contribute to language function," explains Hartwigsen. This is the regular functionality within a healthy brain. From these results as well as others, scientists had up to now deduced that the right hemisphere did not contribute to speech production in the healthy system and was therefore suppressed.
Interfering & Measuring
With a second test, the Kiel University scientists simulated a dysfunction in the brain comparable to a stroke. A magnetic coil transmits a current pulse that interrupts the function of the area responsible for producing speech (Broca's Area) in the left hemisphere. This completely harmless method influences the speech production of the volunteers for about 30 to 45 minutes. "During this period, the ability to listen and repeat was tested again. While we observed a suppressed activity in the left hemisphere during repeating, with some test persons taking longer to repeat the pseudo words, we also found unexpected activities in the right hemisphere," reports Hartwigsen.
The right hemisphere showed increased activity during pseudo word repetition. The more the activity in the right Borca's Area increased, the faster the volunteers were able to solve their speech tests. The right hemisphere also increased its facilitatory influence on the right hemisphere, a finding that was not observed prior to the TMS-induced lesion. "This reaction lends further support to the notion that the right hemisphere area reacts to the dysfunction of the left hemisphere and tries to compensate for the lesion." Does the right hemisphere have a supporting influence and does it play an active role in speech production? So far, the common opinion was that it does not.
Result & Outlook
The findings of Gesa Hartwigsen and her team show an interaction of both hemispheres during speech repetition. When the left hemisphere is suppressed for example by a stroke, the right hemisphere could actively facilitate speech production. "By stimulating the right hemisphere, it could be possible to support speech recovery," speculates the scientist. Here, timing would be very important. "Right after a stroke, we could support the right hemisphere. But when the remaining areas of the left hemisphere are ready to do their work again, it might be more helpful if the right hemisphere was suppressed. During this phase, we could stimulate the left hemisphere instead. The correct timing can therefore be crucial for recovery of speech after a stroke."
In collaboration with the Department of Neurology at Kiel University, a stroke specialist from Leipzig and doctoral students of Medicine and Psychology, Gesa Hartwigsen has started a follow-up study on the recent publication. "We would like to find out more about the collaboration of the hemispheres and the right timing in helping stroke patients to recover," says Hartwigsen. Her field of research is fairly new within the cognitive neuroscience. Nevertheless, she is positive that it will offer practical help in the form of concrete therapies within the next ten to fifteen years.
- G. Hartwigsen, D. Saur, C. J. Price, S. Ulmer, A. Baumgaertner, H. R. Siebner. Perturbation of the left inferior frontal gyrus triggers adaptive plasticity in the right homologous area during speech production. Proceedings of the National Academy of Sciences, 2013; 110 (41): 16402 DOI: 10.1073/pnas.1310190110
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