I think Brain Fog and Depersonalisation are very different. Personally I had Brain Fog on and off for years before I even realised what it was or even that it actually had a name. Its that being unable to think straight/as quickly as you usually do, that woolly headed feeling. It’s like trying to think through treacle. If Brain Fog could affect fingers one would say ‘my fingers are all thumbs today’. ‘Derealisation’ is like you aren’t looking at the Real World or only as a bystander, not actually being part of it at the time of looking. Of course it’s each a symptom of the condition. According to the attached link it relates to brain dysfunction somewhere along the line. An interesting phenomenon though, a lot of threads on here relating and possible hypothesis with this link.