Reason for brain fog is that in addition to inhibiting DHT, finasteride also inhibits other 5AR pathways. These pathways involve neurosteroid production (Progesterone --> Allopregnanolone, Deoxycorticosterone --> THDOC), which normally would act on GABA-A receptors/GABAergic system in the brain.
Diagram on Page 3
http://www3.interscience.wiley.com/cgi- ... 4/PDFSTART
Loss of allopregnanolone especially can cause severe depression and cognitive dysfunction. Low levels have been correlated with Alzheimer's disease.
THDOC is an anti-stress/anti-anxiety neurosteroid and reducing its levels can cause severe anxiety, panic attacks.
Interference with the GABAergic system has been implicated in schizophrenia, sleep disorders including insomnia and loss of REM sleep, and sexual dysfunction.
Your problems are due to Finasteride use and due to near flat dose reponse rate of drug, .25mg will make no difference as it inhibits almost same amount of DHT as 1mg, 5mg, 10mg:
http://www.physics.upenn.edu/facultyinf ... peciafda2/
Thus, you can expect the same side effects to continue even at reduced dosages. The only solution is not inhibiting 5AR.