Implicit memory functioning in schizophrenia: explaining inconsistent findings of word stem completion tasks

Relationship between effortful motivation and neurocognition in schizophrenia

Effortful motivation and reward valuation learning deficits are associated with negative symptoms and impaired cognition in schizophrenia (SZ) patients. Whereas clinical assessments of motivation and reward value typically rely upon clinician ratings or self-report scales, behavioral measures often confound these constructs. Simple reverse-translated behavioral tasks that independently quantify motivation and reward valuation-which could then be linked to cognition-may facilitate the development of pro-cognitive therapeutics by bridging the "preclinical-to-clinical" gap. This study determined whether novel behavioral measures of effortful motivation and reward valuation are associated with impaired cognition in SZ patients (n=36). Patients completed the Progressive Ratio Breakpoint task (PRBT; physical effort motivation) and the Probabilistic Learning Task (PLT; reward learning/valuation) in conjunction with the MATRICS Consensus Cognitive Battery (MCCB). SZ patients exhibited statistically significant deficits in global cognition and all individual MCCB subdomains. Significant correlations were observed between PRBT and MCCB global cognition (r=0.52), speed of processing (r=0.56) and attention vigilance (r=0.48) subdomains, but not with PLT or clinical symptoms. Results indicate that effort and reward learning deficits are dissociable targets that can improve our understanding of cognitive impairments associated among patients with SZ. More importantly, the results support the long-standing notion that the measurement of cognitive impairments in SZ is highly linked to a willingness to expend effort. The availability of a PRBT designed for use in both rodents and humans could improve our understanding of the nature of cognitive impairments in neuropsychiatric disorders and accelerate the development of novel pro-cognitive therapeutics.

Effectiveness of fast mapping to promote learning in schizophrenia

Fast mapping (FM), a process that promotes the expeditious incidental learning of information, is thought to support rapid vocabulary acquisition in young children through extra-medial temporal lobe (MTL) regions. A recent study suggested that patients with MTL damage resulting in profound amnesia were able to learn novel word–image associations using an FM paradigm. The present study investigated whether FM would be an effective strategy to promote learning for individuals with schizophrenia, a severe mental illness associated with compromised MTL functionality. Twenty-five patients with schizophrenia and 27 healthy control subjects completed trials of incidental FM encoding (experimental condition) and explicit encoding (EE, control condition) over the course of three visits spaced one week (± 2 days) apart. All participants were evaluated for recognition 10 min after each encoding condition was presented, and again one week (± 2 days) later. Results indicate that both groups performed better on the EE recognition trials when compared to FM (p's < 0.05). For the FM recognition trials, both groups performed similarly. However, participants with schizophrenia performed significantly worse on the EE recognition trials than healthy control participants (p's < 0.05). While participants with schizophrenia did not perform significantly worse when assessed for FM recognition, these results do not provide enough evidence to suggest that FM facilitates learning to a greater extent in schizophrenia when compared to EE. Whether FM may benefit a subgroup of patients with schizophrenia remains a focus of further investigation.

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.