4 ideas
In 1953 neurosurgeon William Scoville removed most of Henry Molaison's hippocampi, amygdalae and adjacent medial temporal cortex on both sides to treat severe epilepsy. Because the damage was surgical and precisely described, and he was tested for decades, his case became the reference point for linking memory functions to specific brain structures.
H.M. learned mirror-tracing across three days and improved each time, yet each day denied having done the task before. This showed that motor skills and other procedural learning run on circuits outside the medial temporal lobe, separate from declarative memory, which is conscious recall of facts and episodes. Losing one system can leave the other fully working, so memory is a set of dissociable systems rather than a single faculty.
H.M. could hold a number or a conversation in mind for as long as he kept rehearsing it. Once his attention shifted, the information was gone within seconds to minutes. This separated immediate or working memory, which does not need the hippocampus, from consolidation, the hippocampus-dependent process that turns short-lived traces into lasting storage.
H.M. kept a stable sense of self: his character, values, humor, and general knowledge from before the operation. His case suggests identity rests largely on semantic self-knowledge and personality rather than an updated autobiographical story. A person can remain recognizably themselves while living in a permanent present.