Questions: Medial Temporal Lobe and Declarative Memory Systems

Question 1

Patient H.M. improved at the mirror-drawing task across multiple sessions yet had no memory of having done it before each session. Which conclusion is most directly supported by this?

• A. Both declarative and procedural memory are impaired by MTL damage, but at different rates
• B. The hippocampus is not required for procedural learning, demonstrating a dissociation between memory systems
• C. Mirror-drawing skill is stored in the MTL but in a sub-region that was partially spared by H.M.'s surgery
• D. H.M.'s declarative memories gradually converted into procedural memories over time

Answer: (B)

H.M.'s intact motor learning with absent episodic awareness is the textbook double dissociation: declarative memory (destroyed by MTL removal) and procedural memory (preserved, because procedural learning depends on the basal ganglia and cerebellum, not the hippocampus). This is not about rate differences or partial preservation — it demonstrates that these are separate neural systems. The case established that 'memory' is not a single faculty but a collection of distinct systems with distinct neural substrates.

Question 2

A patient has selective damage to the perirhinal cortex but an intact hippocampus. What pattern of memory impairment would you predict?

• A. Severe episodic amnesia with intact familiarity-based recognition
• B. Impaired familiarity-based recognition with relatively intact hippocampal recollection
• C. Complete anterograde amnesia with preserved procedural learning
• D. Impaired spatial navigation with intact verbal declarative memory

Answer: (B)

The perirhinal cortex supports familiarity — the sense that something has been encountered before, without full recollection of context. The hippocampus is more critical for full recollection: reconstructing the context, source, and relational details of an episode. Selective perirhinal damage therefore selectively impairs familiarity-based recognition while leaving hippocampal-dependent recollection relatively intact. This dissociation supports the view that recognition memory has two separable components with distinct neural bases.

Question 3

The hippocampus permanently stores most declarative memories; damage to the hippocampus decades after encoding will destroy those old memories.

• T. True
• F. False

Answer: False

Semantic memories (general facts about the world) undergo systems consolidation over months and years, becoming progressively represented in distributed neocortical networks and independent of the hippocampus. Old, well-consolidated semantic memories can survive hippocampal damage largely intact. This explains why H.M.'s earliest childhood memories were relatively preserved despite total hippocampal removal. Episodic memories may remain hippocampus-dependent for longer due to their relational complexity, but even these become more distributed over time. The hippocampus is a binding and indexing system, not permanent storage.

Question 4

The MTL is especially important for binding together the 'who, what, where, and when' of an experience into a coherent episodic memory, which is why hippocampal damage particularly devastates episodic memory and spatial navigation.

• T. True
• F. False

Answer: True

The hippocampus specializes in relational binding — holding multiple elements (object, location, time, context) in association with one another. Episodic memories and spatial navigation both require exactly this capacity: remembering an event means binding its constituent elements; navigating requires binding places to routes and contexts. This is why hippocampal damage produces dense episodic amnesia and severe spatial disorientation (as seen in both H.M. and in rodent models with hippocampal lesions), while more bounded, item-level recognition is less affected.

Question 5

Why is H.M.'s case considered a 'double dissociation' rather than simply evidence of amnesia, and what does this distinction tell us about memory organization in the brain?

Model answer: A double dissociation occurs when damage A destroys function X but spares function Y, while damage B destroys Y but spares X — proving the two functions are subserved by separate systems, not merely that one is harder than the other. H.M. showed that MTL damage destroys declarative memory while leaving procedural learning intact. The complementary dissociation (basal ganglia damage impairing procedural learning while sparing declarative memory) completes the double dissociation. Together, they prove that declarative and procedural memory are neurally distinct systems, not points on a single difficulty spectrum.

Without the double dissociation logic, one could argue that H.M. simply had an overall memory impairment and that procedural tasks were 'easier,' not fundamentally different. The dissociation rules this out: if declarative and procedural memory were the same system, you couldn't selectively destroy one while preserving the other. The double dissociation is the gold standard for arguing neural independence of cognitive functions.