Implicit vs Explicit Memory: Two Distinct Systems?
Perhaps the most compelling evidence for this distinction comes from amnesia patients, who show intact implicit memory despite a profound lack of explicit memory. For example, amnesic patients with selective damage to the hippocampus and related structures in the medial temporal lobe frequently show intact priming on both conceptual and perceptual repetition priming tasks (Levy et al., 2004).
However, the case of new-associate priming is more equivocal, in which amnesics show impaired priming in word-stem completion, but not speeded reading or lexical decision making, of unrelated word-pairs. In this case it appears that the hippocampus (and other MTL structures) may be necessary for normal conceptual priming, but not perceptual priming, of arbitrary information in particular (Keane & Verfaellie, 2006). Thus, the critical distinction between what amnesics can and can’t do appears to relate to the arbitrariness of the information to be remembered: if the task depends on the linking of unrelated items, amnesics will likely be impaired.
Nonetheless, the idea that a single exposure is enough to facilitate the processing of a related word pair in amnesics and healthy subjects alike has a profound implication for the type of systems that may support these functions. Because the facilitation resulting from priming is accompanied by inhibition for closely related items (Ratcliff, 2000), it seems unnecessary to propose a separate memory system for priming in general as for normal cognition.
Instead, one need only propose that the bias in processing that results from priming procedures is frequently below the threshold of awareness. Neuroimaging evidence also supports this idea (Schacter, 2005), in that perceptual priming tasks frequently activate areas in the “cortical-perceptual representation system” as opposed to hippocampal or other regions implicated only in memory tasks.What about declarative memory?
Some evidence suggests that successful conceptual new-associate priming is usually accompanied by conscious recollection of having seen those pairs before (McClelland, et al., 2002), suggesting that amnesics are impaired in conceptual new-associate priming to the extent that their explicit memory is impaired. More to the point, extensive damage to hippocampus and surrounding structures results in chance performance on some tests of recognition memory (Levy, Stark & Squire, 2004) and but not equal impairments in recall. This distinction – between familiarity and recollection – is one that has engendered much debate, and will be reviewed in the next post.
Note: This post is part 2 of a series of posts, in which the traditional distinctions between memory systems are reviewed. The final post in this series will propose a three-system model of memory, which I argue is the minimum number of distinct systems required to explain current behavioral, neuropsychological, and neuroimaging evidence on the nature of human memory.
Keane, M. M., & Verfaellie, M. (2006). Amnesia II: Cognitive issues. In M. J. Farah & T. E. Feinberg (Eds.), Patient-based approaches to cognitive neuroscience (pp. 303-314). Cambridge, MA: MIT Press
Levy, D. A., Stark, C. E., & Squire, L. R. (2004). Intact conceptual priming in the absence of declarative memory. Psychol Sci, 15, 680-686.
Ratcliff, R., & McKoon, G. (2000). Memory models. In E. Tulving & F. I. M. Craik (Eds.), The Oxford handbook of memory (pp. 571-581). New York: Oxford University Press
Schacter, D. L., Dobbins, I. G., & Schnyer, D. M. (2004). Specificity of priming: a cognitive neuroscience perspective. Nat Rev Neurosci, 5, 853-862.
Vargha-Khadem, F., Gadian, D. G., Watkins, K. E., Connelly, A., Van Paesschen, W., & Mishkin, M. (1997). Differential effects of early hippocampal pathology on episodic and semantic memory [see comments] [published erratum appears in Science 1997 Aug 22; 277(5329):1117]. Science, 277, 376-380