Scientists are beginning to understand the neurological patterns behind mental repression, suppression, and dissociation. Multiple personality disorder, now known as dissociative identity disorder (DID), can actually cause complete changes to the structure of the brain dependent on which personality a patient is expressing. A person suffering from DID was diagnosed as blind for 15 years:

There was nothing wrong with the patient’s eyes per se, but she claimed she couldn’t see, and testing at the ophthalmologist bore this out. During the experiment reported here, one personality state had essentially normal eyesight, whereas a younger, male personality—which could be summoned momentarily by calling out his name—was blind. When in her sighted personality, the EEG showed normal brain waves in response to a checkerboard pattern that alternated its squares 10 times each second—from white to black and back again. But visually evoked activity was much reduced in her blind personality state. There is no known mechanism that allows someone to consciously block vision with open eyes. This remarkable finding implies that the brain can rapidly intervene at a very early stage of the visual system, preventing visual information from reaching the patient’s cortex. How it does so remains a mystery.