In HM, declarative memory was impaired while nondeclarative memory was?

• A. Also impaired
• B. Slightly impaired
• C. Relatively intact
• D. Absent

Answer: (C)

The main concept here is the distinction between declarative (explicit) memory and nondeclarative (implicit) memory. Declarative memory includes memories you can consciously recall, such as events and facts. Nondeclarative memory covers things like skills, habits, priming, and conditioned responses—memories that don’t require conscious recollection.

In HM, the brain damage affected the hippocampus and surrounding medial temporal structures, which are essential for forming new declarative memories. That’s why he developed severe anterograde amnesia: he couldn’t create new episodic or semantic memories. However, the systems supporting nondeclarative memory—like the basal ganglia and cerebellum involved in skills and motor learning, or cortical circuits involved in priming—remained largely intact. As a result, HM could still learn new motor skills and show improvements with practice, even though he wouldn’t remember practicing them or know he had learned them.

So the statement that declarative memory was impaired while nondeclarative memory was relatively intact best captures what his case demonstrates: a dissociation between memory systems, with explicit memories severely disrupted but implicit, procedural memory preserved to a substantial degree.