I’ve just found a remarkable paper with several cases of epilepsy that were interpreted as voodoo possession. They were all people with roots in Haiti, where voodoo is the predominant religion, and where spirit possession is considered a common spiritual event.
For thousands of years epilepsy has been explained as spirit possession in religions around the world. Epilepsy is also known to trigger intense religious or spiritual experiences in some people but the majority of cases are from the West and so have a distinctly Christian theme.
This 36-year-old woman had several years of recurrent complex partial seizures that manifested as a strong sense of fear and epigastric coldness, followed by loss of awareness, utterances of nonsensical phrases, and complex motor automatisms. The local mambo attributed the events to her being taken by “Melle Charlotte,” a french loa [spirit], with the nonsensical speech being interpreted as a foreign language.
It is said that during the possession by this spirit, a person will speak perfect French or other languages, even though in life, the person has no knowledge of that language. She continued to have seizures despite the mambo’s attempts to conjure the spirit. He explained his failure to the fact that Melle Charlotte is a very particular loa who makes only sporadic appearances. She was not treated with AEDs [anti-epileptic drugs] until she left Haiti at the age of 34.
An EEG revealed a right anterior temporal focus, and magnetic resonance imaging (MRI) showed right hippocampal atrophy. Seizures improved with carbamazepine (CBZ), although compliance with medication was a problem, largely because of family interference.
The cases are interesting as they highlight how easily the ‘possession’ theory fits with the unpredictable course of epilepsy and its effects when it seems to briefly ‘take over’ the body and mind of the affected person.
It raises the question of how much observations of epilepsy, a condition that affects approximately 1% of the population, have contributed to the idea of possession throughout the world.
Link to paper on epilepsy and Voodoo experiences.
Link to PubMed entry for same.
Mind Hacks 
I have a friend with epilepsy, and she occasionally switches personalities. She appears as a 10-year-old girl who claims she is a “being of light,” sent by God. As you mention, being a Westerner, it is considered a religious being. Some friends take her advice without question, and give her information I would never share. The odd thing is that she uses (and misuses) the personal infomration she gets to cause conflict between people. I had always blamed it on her neurological condition, but people just brush me off saying I am skeptical and not spiritual enough.
Raul, your friend’s alternate personality sounds too developed to be due to epilepsy alone. There is an increased risk for schizophrenia among epilepsy patients so that may be what’s going on there. Either way she seems like an interesting person.
I’ve wondered lately if the christian phenomenon of “speaking in tongues” has anything to due with some kind of induced partial seizure. I wouldn’t be surprised.