New insights into the mechanisms of language recovery after stroke

It would be a nightmare for anyone: suffering a stroke and losing the ability to communicate properly. In many cases, speech will recover to some extent in the days and weeks that follow. This is because the brain, stimulated by its own efforts and speech therapy, tries to restore speech as far as possible on its own. Until now, it was not known exactly what processes were involved in language recovery.

“In our study, we examined stroke patients at the university hospital at three stages: immediately after their stroke, then two weeks and six months later,” says author Professor Gesa Hartwigsen from the Wilhelm Wundt Institute of Psychology at Leipzig University and the Max Planck Institute for Cognitive and Brain Sciences. While previous studies have focused on the activity of classical language centres in the brain, the authors of this study went a step further: for the first time, they investigated the interactions between different areas of the brain at network level. “This is because language involves many areas of the brain that form functional networks,” says the scientist. “But it was still unclear exactly how these areas of the brain work together and influence each other during language recovery."

Quick help from other network areas

The authors of the study identified three principles: “Firstly, language-specific network areas of the left hemisphere of the brain that are affected by the stroke very quickly receive functional reinforcement from other network areas,” says Hartwigsen. “These ‘domain-general’ areas are present in both sides of the brain and perform cognitive support functions here.” Secondly, the scientists found that “the mirror-image areas of the right side of the brain, which are normally less involved in language processing than those on the left side damaged by the stroke, step in,” says Dr Philipp Kuhnke. These mirror-image areas are also called homologues. “And thirdly, we saw that network communication between the language areas in the left hemisphere of the brain also intensifies during language recovery,” says the scientist.

Adaptation processes themselves are flexible

Patients’ functional adaptation processes for regaining lost language skills changed over several months, in some cases significantly. How this happened depended, among other things, on whether the tissue damaged by the stroke was in the front or back of the patient’s left brain hemisphere. Because the distribution of language-specific areas differs between right-handed and left-handed people, the researchers only looked at right-handed people who had suffered a stroke in the left hemisphere.

A total of 51 participants – 34 patients and 17 healthy controls – were examined at the Department of Neurology at the University of Leipzig Medical Center, led by Professor Dorothee Saur. Their brain activity was measured using functional MRI while they performed language tasks. The researchers then analysed the data using a modelling method that takes causal relationships into account. This method makes it possible to determine the direction of network communication between different areas of the brain. “Our method allowed us to determine not only which areas are activated at the same time, but also which part influences which other part in which recovery phase,” says Dr Philipp Kuhnke.

Future potential for personalised treatment

“The findings hold the potential for personalised treatment of patients in the future, for example with targeted neurostimulation,” says Professor Gesa Hartwigsen. Until then, she adds, more research is needed, with more subjects and more extensive and more detailed analyses. At the same time, the scientists are working to identify key factors that can be used to predict good speech recovery shortly after a stroke.