Henry Gustav Molaison or better known as the HM patient has been fundamental in understanding how brain and memory are related. MH suffered from amnesia caused by surgery that involved the removal of much of the hippocampus and adjacent areas, areas that we now know are related to memory.

According to its chronology we can refer to anterograde amnesia when there's an inability to retain new learning or retrograde amnesia when the difficulty occurs in recovering from events prior to the injury. Although HM had amnesia, not all of his memory was affected. He had anterograde amnesia and wasn't able to accumulate new information after the operation.

He suffered to a lesser degree from retrograde amnesia and didn't remember events that lasted approximately 2 years before the surgery, although his remote memory was perfectly preserved. The capacity of his short-term memory was also intact, being able to keep a conversation going smoothly. He was born in 1926 and died in 2008, but his memory of him remained stagnant at the age of 27.