Questions: Lesion Studies and Double Dissociations

Question 1

Patient A has bilateral hippocampal damage and can learn a new motor skill across sessions but cannot remember meeting the therapist who taught it. Patient B has basal ganglia damage and can describe recent events but fails to improve on mirror drawing even after many sessions. What does this pattern demonstrate?

• A. A single dissociation showing that the hippocampus controls memory generally
• B. A double dissociation showing that declarative and procedural memory depend on anatomically independent neural systems
• C. A double dissociation showing that the hippocampus and basal ganglia are redundant memory systems
• D. A single dissociation showing that procedural memory is more robust than declarative memory

Answer: (B)

This is the canonical double dissociation between declarative and procedural memory. Patient A (hippocampal damage) loses declarative memory but retains procedural learning. Patient B (basal ganglia damage) shows the exact opposite pattern. This crossing pattern rules out the alternative interpretation — that one memory system is simply more fragile than the other — and instead establishes that the two systems are computationally and anatomically independent. A single dissociation (just Patient A) could not rule out the 'fragility' explanation.

Question 2

A researcher finds that patients with frontal lobe damage perform worse on a working memory task than on a simple recognition task. A skeptic argues that this single dissociation could be explained by working memory simply being a harder, more resource-intensive task. What kind of evidence would definitively counter this objection?

• A. More patients with the same frontal lobe lesion showing the same pattern
• B. A neuroimaging study showing the frontal lobe activates during working memory
• C. Patients with non-frontal damage who show impaired recognition but intact working memory
• D. A patient whose working memory improves with rehabilitation

Answer: (C)

The skeptic's objection to a single dissociation is that 'harder' tasks are disrupted by any degraded brain state, not because they use a separate system. The only way to refute this is a double dissociation: find patients with different damage who are impaired on recognition but intact on working memory. This crossing pattern cannot be explained by a single system with differential fragility — it requires two independent systems. Option A (more patients) just replicates the single dissociation. Option B (neuroimaging) shows correlation but not necessity.

Question 3

The fact that H.M. showed intact motor skill learning despite severe anterograde amnesia for declarative memories is evidence that the hippocampus is not required for all forms of long-term memory.

• T. True
• F. False

Answer: True

H.M.'s case is a single dissociation that was the first strong evidence for functionally and anatomically distinct memory systems. His hippocampal removal eliminated declarative memory formation while leaving procedural memory (motor skill learning) intact. This showed that 'long-term memory' is not a single unitary system depending on one brain region — at minimum, there is a component that requires the hippocampus and another that does not. This was a revolutionary finding in 20th-century cognitive neuroscience.

Question 4

A single dissociation — where Patient A is impaired on task X but performs normally on task Y — is sufficient to conclude that X and Y are fully independent neural systems.

• T. True
• F. False

Answer: False

A single dissociation is necessary but not sufficient for concluding neural independence. The critical problem is the 'task difficulty' or 'resource' objection: if X is harder or more resource-intensive than Y, then any generalized brain damage might impair X first without requiring separate systems. The same neural system could underlie both functions but be disrupted at lower levels for the more demanding task. Only a double dissociation — where the impairment pattern crosses (A is impaired on X not Y, B is impaired on Y not X) — rules out this explanation and provides strong evidence for independent systems.

Question 5

Why does a double dissociation provide stronger evidence for independent neural systems than a single dissociation, and what specific alternative explanation does it rule out?

Model answer: A double dissociation shows that damage to system A impairs function X but not Y, while damage to system B impairs Y but not X. The pattern cannot be explained by a single neural system in which X is simply harder than Y: if that were true, impaired patients would always lose X before Y — you would never find patients who lose Y while retaining X. The crossing pattern instead demonstrates that X and Y are doubly independent: each can be fully preserved while the other is destroyed, meaning they are implemented in separate, non-redundant neural substrates.

The power of double dissociation lies in its ability to rule out the most persuasive alternative explanation for single dissociations: differential task difficulty or resource sensitivity. Students should understand that lesion evidence is fundamentally about necessity — showing that a region is *required* for a function — and that the double dissociation design achieves this more rigorously than any single-patient result.