I don't know what others might have experienced, but during my period of ultra panic attack anxiety last year, I was unable to think clearly. It was as if my head was full of cotton or "fog" and I felt like I had lost 50 IQ points. My memory was obliterated and I was frozen like the proverbial deer in headlights. In my case, I think it was a symptom of anxiety more than depression. I have been on a sabbatical from work and as my temperature has come down, the brain fog has abated and I think clearly again. I'm still depressed, although not to the extent I was earlier in the year yet I can think fairly clearly. Are you under extreme duress right now?
When I get "brain fog", mine is all anxiety, like astroboy's. It's usually on a day where I either don't take my medicine, I change doses of medication, I've had to memorize a lot of information for a test and my mind is buzzing with the facts, or (the most common) when I have a day where I have a lot of obsessions, and I know that I'm going to have a lot of obsessions. Stupid OCD!
"Not everything that steps out of line, and thus 'abnormal,' must necessarily be 'inferior.'"
I am not sure what causes my brain fog as it tends to last for hours and sometimes days on end. The best way to explain it would be my brain is on time delayed as I see something or someone says something and 5 seconds later my brain says hey something just happenned or a response is needed. Basically it feels like you have been awake for days on end and have almost reached zombie status. I just answered the same question twice without realizing it, blame it on the brain fog