Sleep-related cognitive function and the K-complex in schizophrenia

Sleep parameters have been reported to be related to cognitive function in a variety of ways. Problem solving and procedural learning were found to be improved after sleep but training also affected subsequent sleep and some parameters were related to cognitive trait variables, e.g. IQ. Additional to rapid-eye-movement (REM) and slow wave sleep (SWS), micro-architectural features such as spindle activity and K-complexes have recently been the focus of interest. The study aimed at investigating the relationship of neuropsychological variables, problem solving and procedural learning with sleep parameters in stably medicated schizophrenia patients.Twenty schizophrenia out-patients participated in the study. Learning and testing occurred over a randomly balanced waking and sleep interval. The tasks were the Tower of London (ToL) and mirror tracing. Sleep EEG was analysed together with spindle activity and K-complexes.Performance improved with regard to both tasks from learning to testing irrespective of type of interval. Increasing density of K-complexes was related to a higher number of solved ToL tasks pre and post night whereas longer SWS was related to faster completion of the ToL. A higher age was related to less overnight improvement in regard to number of solved ToL tasks.K-complexes are thought to indicate intra-cortical activity paving the way for the uptake of new information. As ToL is considered a test of executive function, K-complexes appear to be linked to this domain, deficits of which are thought to be a core feature of schizophrenia.

► Increasing density of K-complexes is related to enhanced problem solving performance in schizophrenia.
► Increasing age is related to decreased overnight improvement in problem solving performance in schizophrenia.
► Longer SWS is related to shorter duration of completing the problem solving task.