Reduced implicit and explicit sequence learning in first-episode schizophrenia

A high prevalence of deficits in explicit learning has been reported for schizophrenic patients, but it is less clear whether these patients are impaired in implicit learning. Deficits in implicit learning indicative of a fronto-striatal dysfunction have been reported using a serial reaction-time task (SRT), but the impact of typical neuroleptic medication and chronicity remains controversial. The present study compared 37 patients with first-episode schizophrenia treated with atypical neuroleptics and 37 healthy matched control participants on two sequence learning tasks: a modified SRT for implicit sequence learning and a serial generation task (SGT) for explicit sequence learning. The two tasks were designed to be procedurally equivalent, in order to provide better comparability between implicit and explicit performance. Although unaffected in global cognitive functioning, schizophrenic patients were significantly impaired in implicit and explicit sequence learning. Deficient sequence learning in schizophrenic patients was neither related to psychopathology nor to chlorpromazine equivalent daily dosage. As performance was impaired even though patients were exclusively treated with atypical neuroleptics, the present findings concur with converging evidence of a sequence learning deficit inherent in schizophrenia. This deficit would be consistent with a fronto-striatal dysfunction and might constitute a crucial factor for the acquisition of new information.