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It has been claimed that the coordination of neuronal oscillations differing in frequency is relevant for cognition. However, the validity of this claim has scarcely been investigated.Recent studies revealed that cross-frequency phase coupling and modulations of alphapower dissociate between retention of relevant and suppression of irrelevant information in visual working memory (WM). We summarize these important results, and discuss possible implications for understanding the neural mechanisms of WM constraints. IntroductionMainly based on behavioral assessments, cognitive theories traditionally assume a fixed upper limit for the available storage space in WM (Luck and Vogel, 1997; Miller, 1956). Estimates for capacity limits thereby range from 4 (± 1) (Cowan, 2001;Luck and Vogel, 1997) to 7 (±2) items (Lisman and Idiart, 1995; Miller, 1956). Recent neuroscientific investigations point to interacting neuronal processes that jointly limit performance in WM tasks (D'Esposito, 2007;Zimmer, 2008). Accordingly, efficient performance on WM tasks requires at least the maintenance of relevant information over a short time-period and, at the same time, inhibition of distracting, non-task relevant information from entering visual awareness (Gazzaley et al., 2005;Vogel et al., 2005).From the mid 1990s onwards, the so-called 'changedetection paradigm' (CD-paradigm, e.g. Luck and Vogel, 1997) has become a popular tool for studying properties of WM in neuroscience. In CD tasks, observers are presented with a memory display that contains a variable amount of information (e.g. colored squares). After a brief retention interval (e.g., around 1 s) a probe display asks the participant to judge whether the currently presented information has changed relative to the representation retained in WM. Since the initial report (Luck and Vogel, 1997), this paradigm has been repeatedly applied to investigate neuronal activity related to WM retention (for a recent review see, Drew et al., 2006).In a recent development, Vogel and Machizawa (2004) included cues prior to the onset of the memory display to direct the observers' attention to information in only one visual

Transiently storing information and mentally manipulating it is known as working memory. These operations are implemented by a distributed, fronto-parietal cognitive control network in the brain. The neural mechanisms controlling interactions within this network are yet to be determined. Here, we show that during a working memory task the brain uses an oscillatory mechanism for regulating access to prefrontal cognitive resources, dynamically controlling interactions between prefrontal cortex and remote neocortical areas. Combining EEG with non-invasive brain stimulation we show that fast rhythmical brain activity at posterior sites are nested into prefrontal slow brain waves. Depending on cognitive demand this high frequency activity is nested into different phases of the slow wave enabling dynamic coupling or de-coupling of the fronto-parietal control network adjusted to cognitive effort. This mechanism constitutes a basic principle of coordinating higher cognitive functions in the human brain.

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The present study attempts to demonstrate functional similarities between the P1 component of event-related potentials and alpha oscillations that are predicted by the 'alpha inhibition-timing' hypothesis. On the basis of findings showing that the frequency characteristic of the P1 component lies in the alpha range and that alpha oscillation is functionally associated with inhibition, we predict that the P1 component also reflects inhibitory processes. This hypothesis is tested in two experiments, a spatial-cuing task and a visual-semantic categorization task. The results of the cuing task demonstrate that in a similar way as alpha power, the P1 component is larger over task-irrelevant ipsilateral sites. For the categorization task, we found that the P1 component, in a similar way to alpha oscillations, is larger for task-irrelevant, distorted pictures. We conclude that the P1 component may be generated at least in part by evoked alpha oscillations and reflects inhibition in the sense of suppressing task-irrelevant processes.