Why did the patient H.M. keep his old memories but form no new ones? · Grey Matter

The 1953 operation removed most of both hippocampi with the surrounding medial temporal cortex, the structures that bind a new experience into a lasting trace, so nothing new could be stored once his attention moved on.

Why did the patient H.M. keep his old memories but form no new ones?

The 1953 operation removed most of both hippocampi with the surrounding medial temporal cortex, the structures that bind a new experience into a lasting trace, so nothing new could be stored once his attention moved on. Knowledge consolidated years earlier had already been reorganised into the cortex, which was untouched, so his vocabulary, general knowledge and childhood memories remained. The sparing was partial: he lost much of the decade before the operation, and later testing found that even his older personal memories had lost their vivid detail while keeping their facts. That gradient is the evidence that the hippocampus is needed for storing and for a time afterwards, but not forever for every kind of memory.