A very interesting recent piece I found almost at random on the nature of Dostoyevsky's epilepsy.
I had known that he was believed to be epileptic, but I didn't know that the condition was self-diagnosed. Relatively little was understood about the condition in the 19th Century, causing Freud to disbelieve the diagnosis on the grounds that it was found only among the mentally feeble. He thought the condition was a straight case of hysteria. Yes, but then he always believed that...
On the basis of recent comments, it seems the consensus is that it was in fact temporal-lobe epilepsy. He described his seizures in such great detail both in his correspondence and in the mouths of his characters, that modern researchers have a remarkably complete account of his condition.
His auras were ecstatic in nature. The sensation was so powerful, that:
“For several moments,” he said, “I would experience such joy as would be inconceivable in ordinary life - such joy that no one else could have any notion of. I would feel the most complete harmony in myself and in the whole world and this feeling was so strong and sweet that for a few seconds of such bliss I would give ten or more years of my life, even my whole life perhaps.”
The author (I can't find a name attached to this piece), conjectures a connection between the nature of Dostoyevsky's ecstatic states and his religiosity, his pervasive sense of guilt, his feeling of dread.
Because of his epilepsy, Dostoyevsky was, to borrow the title of his second novel, a “double man”; a rational, exalted being on the one hand, and, because of his illness, a mystical and base creature on the other. It seems that as his life progressed, and his epilepsy became more severe, the latter persona prevailed, as evidenced by the increasingly mystical nature of the work produced later in his life.