EMILY BORGHARD has a computer inside her skull, but you wouldn’t know it to look at her. A small bump behind her left ear, the only external evidence of her implant, is partially covered by a tuft of hair that’s still growing in from the last time she had the batteries changed.

Before Borghard received a brain implant, she was having as many as 400 “spikes” of seizure-like activity a day, along with multiple seizures. This unrelenting storm of abnormal neural activity turned her teenage years into a semiconscious nightmare....