Remembering or knowing: electrophysiological evidence for an episodic memory deficit in schizophrenia

Background. In schizophrenia, impaired conscious retrieval of past events and facts may represent a selective cognitive deficit of declarative memory against a background of a generalized neuropsychological impairment. We aimed to disentangle the neural subprocesses leading to this deficit applying the ‘Remember/Know procedure’. Method. Event-related potentials (ERPs) were recorded as 14 schizophrenic patients and 14 controls recognized an equal mixture of previously presented old and new words. For recognized old words, participants were required to judge whether recognition was associated with recollection (‘Remember’) or familiarity (‘Know’), either reflecting episodic or semantic memory. Results. Patients showed a lack of ‘Remember responses’, which led to more opportunities to make ‘Know responses’. ERPs for ‘Remember’ compared to ‘New responses’ differed consistently in controls over left temporo-parietal and right frontal electrode sites. Although schizophrenic patients showed the same topography for this Remember old/new effect, it was apparent over temporo-parietal sites for only 800 ms and over right frontal sites for 1100 ms post-stimulus. For controls, the Know old/new effect was elicited over temporo-parietal sites between 500 and 800 ms. For patients, it showed a widespread maximum over frontal sites between 500 and 1100 ms. Conclusions. The shorter time course of the left temporo-parietal Remember old/new effect suggests that the patients' episodic memory impairment was possibly mediated by a dysfunction of the mediotemporal regions. The more widespread frontal Know old/new effect in the patients suggests that the prefrontly mediated processes associated with retrieval of semantic memory may be enhanced compensatorily.