In a nutshell: Damage to one hemisphere of the brain can lead to decreased connectivity across the brain, beyond the site of injury.

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If a person has damage to the right side of their brain, they often lose awareness on the left side of their body. This effect, known as ‘neglect’, is common after a stroke.

The symptoms of neglect are often similar, regardless of what part of the right hemisphere has been injured. This suggests that neglect is the result of changes across the whole brain, not just in the damaged part.

An international team of researchers sought to test this hypothesis. The team was led by Martin Dietz at Aarhus University, Denmark, and Brain Function CoE investigator Marta Garrido from the University of Melbourne.

The researchers recruited 21 participants: ten people who had left-sided neglect after damage to the right side of the brain, and 11 people who were healthy. They used electroencephalography (EEG) to measure the participants’ brain activity during an auditory oddball task. This task tests how the brain responds to unexpected sounds played on either the left or the right side of the head. In particular, the researchers used the EEG recordings to calculate the connectivity between two regions of the brain: the parietal and frontal cortex. These regions, which span both hemispheres, are involved in processing unexpected stimuli.

In healthy participants, regardless of which side of the body an unexpected sound was played on, the connectivity between the parietal and frontal cortex was higher on the opposite side of the brain. This confirmed results that the group had found previously.

But in participants with left-sided neglect, the brain connectivity increased only in the left hemisphere in response to an unexpected sound to their right. When a sound appeared to their left, there was no increase in brain connectivity in their right hemisphere. However, there was an increase in connectivity between two other brain regions in the left hemisphere. This suggests that the brain might have strengthened intact pathways to compensate for damaged ones.

These results support the idea that neglect is caused by a disruption to the network of connections across the brain, rather than to just one region.

Next steps:
The researchers plan to repeat this study in a much larger group of people. They will also combine different imaging techniques to see how brain connectivity changes during recovery from a brain injury.

Dietz, M. J., Nielsen, J. F., Roepstorff, A., & Garrido, M. (2020). Reduced effective connectivity between right parietal and inferior frontal cortex during audiospatial perception in neglect patients with a right-hemisphere lesion. Hearing Research, doi: 10.1016/j.heares.2020.108052

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