Priming and Recognition in Hippocampal Amnesia
A 68-year-old man with bilateral hippocampal damage after herpes simplex encephalitis has a profound anterograde amnesia. On a neuropsychological battery, his Wechsler Memory Scale delayed recall is below the 1st percentile, and yes/no recognition of 40 studied words is at chance (20/40). However, when given three-letter stems of studied words and asked to complete each stem with 'the first word that comes to mind,' he produces studied words 65% of the time, compared with 25% for unstudied baseline stems. He cannot recall the study episode and denies having seen any of the words. A research assistant concludes that the patient shows 'implicit memory' but no 'explicit memory.' The patient's wife reports that he can still learn to use a new coffee machine after a few demonstrations, although he does not remember the demonstrations.
1. Evaluate the assistant's conclusion. What pattern of results would justify it, and what alternative explanation must be ruled out?
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Reasoning: The conclusion is plausible but not yet established. The stem-completion advantage (65% vs 25%) is a priming effect: the patient is more likely to produce studied words without claiming recollection. At-chance recognition and denial of the study episode argue against explicit contamination. To justify 'implicit memory with no explicit memory,' however, the study must rule out residual explicit memory: if recognition were above chance, the priming could reflect explicit guessing. Here recognition is at chance, which strengthens the implicit interpretation. The assistant should also show that the effect is not due to pre-existing word frequency, stem completability, or a response bias; a baseline condition with unstudied words and matched stems is essential. With those controls, the dissociation is credible.
2. In a second session, the patient is given a free-recall test after studying words in a blocked category list vs random list. He recalls more from the blocked list. Does this show spared explicit memory? Explain.
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Reasoning: Not necessarily. A blocked-list advantage can occur without conscious recollection if category structure supports implicit semantic priming or automatic clustering. The key test is whether the patient can recognize the words or report the study episode; if recognition remains at chance, the free-recall advantage is better interpreted as an implicit or familiarity effect, not spared explicit memory. To make the claim, the researchers should vary the retrieval conditions and include a recognition test after recall. If the patient's recall advantage disappears when category cues are removed, or if it is uncorrelated with recognition, the implicit account is stronger. The finding alone does not establish explicit memory.
3. What would a double dissociation with a patient with damage to the perirhinal cortex add?
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Reasoning: A double dissociation would strengthen the claim that priming and recognition depend on separable systems. If the hippocampal patient shows intact priming but impaired recognition, and a perirhinal patient shows impaired priming but relatively spared recognition, the two functions cannot be reduced to a single memory-strength dimension. That pattern would support a multiple-systems account: hippocampal-dependent explicit memory versus neocortical or perceptual priming. However, the double dissociation must be demonstrated with matched tasks and patients; otherwise differences in task difficulty or lesion extent could produce a spurious dissociation. It would also help specify whether the perirhinal patient's deficit is in familiarity-based recognition or in perceptual representation.