DAMAGE to white matter causes worse cognitive outcomes after a brain injury than damage to grey matter, a new study suggests. In the past, neurologists and scientists have exclusively favoured studies involving the role grey matter, the neurons that form the cerebral cortex, due to the belief that grey matter plays a larger role in cognitive health and function than white matter, the myelin-covered axons that physically connect the neuronal regions. This study, however, challenges this belief, highlighting that the connecting fibres between regions of the brain are just as important, if not more important, than the regions themselves.
The study was carried out by researchers at the University of Iowa, Iowa City, Iowa, USA, and analysed the brain scans and cognitive function tests from over 500 people with localised brain damage, caused either by stroke or other forms of brain injury, the findings of which have the potential to help neurologists prognosticate the long-term effects of traumatic brain injuries in patients.
Research on cognition often focuses on networks within the brain and how different network signals influence the different characteristics of overall cognition. The team carried out the cognition research using several well-accepted mathematical models to measure and analyse the connectedness of brain networks and to identify highly connected regions of the brain. They identified the hub locations within both the grey and white matter from brain imaging of normal healthy individuals, with no previous brain injury, and compared it to brains scans from individuals with brain lesions to find cases where areas of damage coincided with hubs. By using data from multiple cognitive tests for the patients with brain lesions, the research team was also able to measure the effect that hub damage had on cognitive outcomes.
The results revealed that damage to dense white matter hubs was strongly linked to impaired cognition. In contrast to current belief, damage to highly connected grey matter hubs did not have a strong association with poor cognitive outcomes. Justin Reber, Postdoctoral Research Fellow in Psychology, University of Iowa, and first author on the study explained: “The most unexpected aspect of our findings was that damage to grey matter hubs of the brain, which are really interconnected with other regions, didn’t really tell us much about how poorly people would do on cognitive tests after brain damage. On the other hand, people with damage to the densest white matter connections did much worse on those tests.”