Introduction
The central role of semantic organization in verbal episodic memory (Tulving 1983) has been increasingly appreciated. Early studies showed that as information encoding moves from shallow perceptual processing to more elaborate semantic-associative encoding, the strength of the memory trace increases (Craik and Lockhart 1972; Kintsch 1968). The benefit of semantic organization is reflected by better recall of semantically related word lists that can be categorized. It has also been well established that patients with schizophrenia tend not to spontaneously use semantic information to categorize related word lists to benefit encoding and retrieval (Gold et al 1992; Iddon et al 1998; Koh et al 1973; Koh and Peterson 1978; Paulsen et al 1995). Failure to utilize semantic information has contributed to putative differential impairment of episodic memory in schizophrenia (Gold et al 1992; McKenna et al 1990; Rund 1989; Saykin et al 1991; Saykin et al 1994; see Blanchard and Neale 1994 for exception). The mechanism accounting for this impaired use of semantic information is less clear.
A leading explanation for why patients do not use semantic information to facilitate memory is that they have difficulty self-generating organizational strategies (Brébion et al 1997; Iddon et al 1998; Stone et al 1998). These strategic memory explanations conclude that executive problems and related frontostriatal dysfunction interfere with patients’ ability to adopt higher-level organizational strategies. A number of attempts have been made to remediate these strategic impairments. McClain (1983) examined free recall of 18 words following either random or blocked presentation (i.e., grouping words belonging to three different semantic categories together), with or without retrieval cues. Patient performance improved with blocking and equaled control performance only if blocked presentation and retrieval cues were provided. Gold et al (1992) contrasted three encoding presentations: a random list of 20 unrelated nouns; an unblocked list containing five exemplars, each from four randomly presented semantic categories; and a blocked list. Patients benefited from the blocked list but showed no difference between random and unblocked presentation. In contrast, controls benefited from both unblocked and blocked list presentation. Brébion et al (1997) compared recall of two types of word lists, random and unblocked, and obtained similar results. A study comparing blocked versus unblocked recall established that the beneficial effect of blocking was the same for acutely ill and chronically ill patients (Chan et al 2000). Finally, Iddon et al (1998) examined verbal list learning as part of a larger test battery before, during, and after training sessions where patients were instructed in semantic organizational strategies. Patients did not benefit from training and continued to organize the words based on serial order rather than semantic content.
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Findings that patients benefit from blocked word presentation provide some support for the strategic memory explanation and suggest that semantic memory (i.e., knowledge) is grossly intact; however, results of below average blocked word performance (Gold et al 1992) and nonresponse to training (Iddon et al 1998) imply that patients have deficits in semantic or other information processing during word encoding apart from higher-level organizational strategies. Evidence of semantic processing deficits in schizophrenia comes primarily from verbal fluency paradigms in which participants generate words belonging to a specific category (e.g., animals). These studies show a consistent performance impairment (Allen and Frith 1983; Goldberg et al 1998) that is among the strongest neuropsychological discriminators of patients from healthy controls (Arango et al 1999). In a recent study (Moelter et al 2001), we found that impaired animal fluency was related to both aberrant automatic semantic-associative network activation and to impaired controlled processes such as search, access, and selection. If these semantic processing impairments are prominent, they may interfere with patients’ ability to engage in deep semantic processing and thereby result in a reduced levels-of-processing effect.
The role of semantic and organizational processes will be investigated in the current study through use of a levels-of-processing (Craik and Lockhart 1972) word encoding and recognition paradigm previously used in functional magnetic resonance imaging (fMRI) research (Buckner et al 1998a, 1998b; Demb et al 1995; Rugg 1998; Wagner et al 1998). In this paradigm, participants are explicitly instructed to process words on either a shallow level (i.e., perceptual features) or deep level (i.e., semantic content). This results in a levels-of-processing effect in which recognition is better for words that undergo deep encoding. By providing explicit instruction, strategic memory demands are reduced. Assuming that patients are able to switch between shallow and deep encoding strategies, this should normalize patient performance if semantic processing is intact; however, we hypothesize that patients have difficulties in both strategic memory and semantic processing and will, therefore, demonstrate a reduced levels-of-processing effect.
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