Patient H.M., known as Henry Molaison, was a man who underwent a surgical procedure in 1953 to treat his severe epilepsy. During the surgery, parts of his hippocampus and surrounding areas were removed, which significantly impacted his memory. As a result of the surgery, H.M. developed profound anterograde amnesia, meaning he could not form new memories after the operation. He could remember events from before the surgery but struggled to recall anything that happened afterward, making him a key subject in the study of memory and the brain.