Is conceptual implicit memory impaired in schizophrenia? Evidence from lexical decision and category verification

The effect of training intensity on implicit learning rates in schizophrenia

Cognitive impairments in learning and memory are core symptoms of schizophrenia, associated with reduced self-reported quality of life. The most effective treatment of cognitive impairments is drill and practice cognitive training. Still, to date no study has investigated the effect of varying the frequency of training on cognitive outcomes. Here we utilized a verbal memory based language learning task, tapping into implicit cognitive processes, to investigate the role of training intensity on learning rates in individuals with schizophrenia. Data from 47 participants across two studies was utilized, one with a daily training regimen over 5 days and the other with a more intensive schedule of 5 sessions delivered over 2 days. The primary outcome measure was the change in implicit learning performance across five sessions, quantified with the Matthews Correlation Coefficient (MCC). Participants in the daily training group showed improved performance compared to the intensive group only at session 4. This is the first study to show that implicit learning rates are influenced by training intensity, with daily sessions outperforming a more intensive regimen; a period of consolidation overnight may be necessary to optimize cognitive training for individuals with schizophrenia.

Implicit memory in schizophrenia: a meta-analysis

OBJECTIVE

Previous studies examining implicit memory in schizophrenia yielded inconsistent results. The present meta-analysis aimed at determining whether, compared to healthy controls, schizophrenic patients: (a) exhibited reduced priming in the whole set of studies; (b) were differentially impaired in conceptual/perceptual and production/identification tests; and (c) were less efficient in the use of semantic encoding processes.

METHOD

A systematic search in PsycINFO and PubMed led to the selection of 22 critical studies (31 effect sizes), comparing repetition priming in 836 schizophrenic patients and 760 healthy controls. Moderators were assessed by classifying implicit tasks into the perceptual/conceptual and identification/production categories, and by distinguishing between perceptual and conceptual encoding instructions.

RESULTS

Overall, implicit memory was slightly, but significantly, impaired in schizophrenia (d=0.179). Patients exhibited reduced priming in conceptually-driven tasks (d=0.447), but intact priming in perceptually-driven tasks (d=0.080). No significant difference was observed between identification and production priming (d=0.064 vs. d=0.243). Finally, priming in schizophrenic patients was significantly lower than that of controls when the encoding task required the analysis of the conceptual properties of the stimuli (d=0.261).

CONCLUSION

Results suggest that schizophrenia is associated with a specific deficit in the use of conceptual processes, both at encoding and at retrieval. In contrast with theoretical expectations, high levels of response competition did not disproportionately impair the patients' performance.

Testing the Identification/Production Hypothesis of Implicit Memory in Schizophrenia: The Role of Response Competition

OBJECTIVES

Previous evidence indicates that patients with schizophrenia exhibit reduced repetition priming in production tasks (in which each response cue engenders a competition between alternative responses), but not in identification tasks (in which each response cue allows a unique response). However, cross-task comparisons may lead to inappropriate conclusions, because implicit tests vary on several dimensions in addition to the critical dimension of response competition. The present study sought to isolate the role of response competition, by varying the number of solutions in the context of the same implicit tasks.

METHODS

Two experiments investigated the performance of patients with schizophrenia and healthy controls in the high-competition and low-competition versions of word-stem completion (Exp.1) and verb generation (Exp.2).

RESULTS

Response competition affected both the proportions of stems completed (higher to few-solution than to many-solution stems) and the reaction times of verb generation (slower to nouns having no dominant verb associates than to nouns having one dominant verb associate). Patients with schizophrenia showed significant (non-zero) priming in both experiments: crucially, the magnitude of this facilitation was equivalent to that observed in healthy controls and was not reduced in the high-competition versions of the two tasks.

CONCLUSIONS

These findings suggest that implicit memory is spared in schizophrenia, irrespective of the degree of response competition during the retrieval phase; in addition, they add to the ongoing debate regarding the validity of the identification/production hypothesis of repetition priming.