Schizophrenia patients and their first-degree relatives exhibit impaired working memory (WM) performance, possibly reflecting the disorder’s genetic diathesis (Glahn et al., 2003). Attempts to determine the neural bases of these WM deficits suggest that reduced efficiency of certain critical neurocognitive mechanisms may be involved (Callicott et al., 2003; Manoach, 2003; Karlsgodt et al., 2007). Consistent with an inefficiency hypothesis, we demonstrated that compared to controls, patients and their non-schizophrenic co-twins exhibit relatively larger changes in the electrophysiological signatures of the stimulus-encoding and memory-consolidation stages of WM performance, per unit increase in memory demands (Bachman et al., submitted). Along with encoding and consolidation, successful maintenance of information over a delay requires action of WM control functions, including suppression of potential interference when target stimuli are no longer displayed. Inhibiting cognitive processing of potential distracters is critical for preventing competition between task-relevant and task-irrelevant representations (Postle, 2005), and has been shown behaviorally to be differentially impaired in schizophrenia (Oltmanns & Neale, 1975).
Event-related desynchronization (ERD) and synchronization (ERS) of upper, or “fast,” EEG alpha frequency band activity, a measure of power decrease (ERD) or increase (ERS) as a percentage of pre-stimulus power (Neuper & Pfurtscheller, 2001), have been associated consistently with individual differences in neurocognitive efficiency measured during performance of challenging cognitive tasks, including paradigms that place heavy demands on WM (Neubauer et al., 2006). Research into the mechanisms underlying these fluctuations in power has demonstrated that alpha ERD reflects increased excitability of active cerebral cortex, whereas, alpha ERS appears to reflect decreased excitability, or large-scale inhibition, of cortex (Neuper & Pfurtscheller, 2001).
Consistent with this inhibitory function, alpha ERS is evident during the delay period of WM tasks, correlates in magnitude with memory load, and peaks over cortical areas responsible for processing task-modality specific sensory information (Sauseng et al., 2005; Jensen et al., 2002). Collectively, these findings suggest that alpha ERS reflects ‘top-down’ gating of sensory areas to prevent encoding of goal-irrelevant stimuli while task-relevant information is held actively in mind (reviewed, Klimesch et al., 2007).
Fast alpha ERS, therefore, should be greater in amplitude during the delay period of a delayed-match-to-sample WM task among individuals less efficient at gating task-irrelevant stimuli. It was predicted that, across groups, delay-period ERS would increase with memory load; moreover, patients and their co-twins were expected to display stronger load effects than controls, reflecting a hypothesized decrease in neurocognitive efficiency. Alternatively, inefficient inhibition of posterior cortical areas not directly related to WM would be associated with increased posterior ERS among patients and their co-twins, insensitive to WM load manipulation.