@Bertie68 - Like the article talked about, problems with the brain are a combination of both the amount of gray matter and how weak that gray matter is in a particular area. For example, when we get older, our short term memory starts fading because the gray matter in the memory area of the brain starts shrinking and doesn't work as well.
Also, scientists think that people who have bipolar disorder may have less gray matter in one part of the brain and poorly functioning gray matter in another area.
I'm sure scientists need to do a lot more studies to get clear evidence of how the gray matter works. Hope this helps.