Neural activity differs between explicit and implicit learning of artificial grammar strings : An fMRI study

An influential neuropsychological finding in the field of human memory has been the dissociation of implicit from explicit memory. Explicit, or declarative, memory is memory for facts and events; it is dependent on the hippocampal and diencephalic brain systems (Cohen, Poldrack, & Eichenbaum, 1997; Squire, 1994). Implicit memory is a heterogeneous set of phenomena that have in common that they are preserved in patients with global amnesia and thus are independent of the hippocampal– diencephalic brain systems. These phenomena include motor skills and sequencing (Corkin, 1968; Nissen & Bullemer, 1987), perceptual and conceptual priming (Graf, Squire, & Mandler, 1984; Warrington & Weiskrantz, 1970), and implicit learning of patterns, such as artificial grammar learning (Knowlton, Ramus, & Squire, 1992; A. S. Reber, 1967). In the artificial-grammar-learning task (A. S. Reber, 1967, 1989), participants study letter strings generated using an artificial grammar (see Figure 1) without being informed of the structured nature of the strings. Participants are then told of the existence of the underlying grammar and are asked to judge novel strings as to whether they are grammatical or not (i.e., whether the string follows the same rules as the strings in the study task). Participants typically report relying on their intuition or “gut feelings” in making these judgments. Grammaticality judgments are influenced by previous experience with rulegenerated stimuli and thus reflect learning; this learning is implicit in that participants are not aware of the specific rules underlying the stimuli (Seger, 1994). The artificial-grammar-learning task is well suited to direct comparison with explicit memory tasks, such as recognition. Participants can be shown the same sorts of stimuli (letter strings) and can indicate their judgments via the same sort of responses (keypresses indicating “yes” or “no”) in both recognition and grammatical judgment conditions. What differs is the memory resources drawn on to make each judgment. For recognition tasks, participants attempt to recall a specific episodic memory of the stimulus in order to verify that the stimulus was previously experienced. For grammatical judgments, participants make a decision on the basis of feelings of correctness or familiarity. Grammatical judgment has been shown to be independent of the hippocampal–diencephalic memory systems in research performed with patients with global amnesia. Knowlton and Squire (1994, 1996; Knowlton et al., 1992) found that amnesic patients exhibit normal performance on grammatical judgments of strings but