The loss of thinking and memory problems was linked to the extent of white matter damage.

Wikipedia:

White matter is a component of the central nervous system, in the brain and superficial spinal cord, and consists mostly of glial cells and myelinated axons that transmit signals from one region of the cerebrum to another and between the cerebrum and lower brain centers. https://en.wikipedia.org/wiki/White_matter

Some good news:

Prof. Blamire says these results "show that thinking skills were recovering over time. The areas of brain damage were not as widespread across the brain as previously, but focused in certain areas of the brain, which could indicate that the brain was compensating for the injuries."

Not surprisingly, brain trauma is problematical.