There was someone behind her; she was sure of it. A malignant presence: always a man, and always to her right. But whenever she looked around, he was gone.

It was 2006. The woman was in University Hospital, Geneva, awaiting surgery to remove parts of her brain that were causing epileptic seizures. The surgeons, led by neurologist Olaf Blanke, prepared for the operation by electrically stimulating her brain, to identify important regions that they needed to preserve. When they shocked one specific area—the left temporoparietal junction—she immediately felt a phantom menace behind her.

She turned to look and when Blanke asked her why, she said that she felt a “shadow” behind her, silent and still. She did the same thing every