Amnesia and Memory Loss

Think of an injury as a marker dropped on a timeline. Retrograde amnesia erases memories from before that marker — the person loses access to past events but can still learn new ones. Anterograde amnesia does the opposite: it blocks new long-term memories from forming after the marker, while everything learned before stays intact. A patient can greet the same doctor every morning and have no memory of ever meeting them, because the hippocampus, which normally converts new experience into lasting storage, isn't doing its job. Their sense of self, old facts, and language all remain, because those were already consolidated into long-term storage before the injury.

Anterograde amnesia patients can't form new explicit memories — no new names, faces, or events — but they can still learn procedural skills. The famous case is patient H.M., who could practice a mirror-tracing task and improve day after day, even though he never consciously remembered practicing it before. That split shows memory isn't one system: explicit memory depends on the hippocampus, while procedural and implicit learning rely on other circuits, like the basal ganglia and cerebellum, which can stay healthy even when the hippocampus is damaged.

Infantile amnesia fits the same logic. Adults can't consciously recall events from before age two or three because the hippocampus is still immature then, so explicit memories never get properly encoded. But implicit learning — motor skills, attachment patterns, emotional responses — forms fine during infancy and shapes behavior for life. Alzheimer's disease shows the timeline unraveling in a different order: it starts by destroying recent memories and slowly eats backward into older ones, the reverse of normal aging, eventually breaking down both explicit and implicit memory as the disease spreads.