This article first appeared in The Checkup, MIT Technology Review’s weekly biotech newsletter. To receive it in your inbox every Thursday, and read articles like this first, sign up here.

Justin Graves was managing a scuba dive shop in Louisville, Kentucky, when he first had a seizure. He was talking to someone and suddenly the words coming out of his mouth weren’t his. Then he passed out. Half a year later he was diagnosed with temporal-lobe epilepsy.