For human working memory the neural correlates of the phonological loop and the visuospatial sketch pad are well explored. In contrast, less is known about central executive processes. Neuroimaging studies suggest that central executive processes are related to a complex fronto-parietal network. In the present study we investigate the question whether varying demands on central executive processes are reflected by differences in coherent activity between and within a fronto-parietal network. We calculated coherence during a visuospatial working memory task. Under an easy executive condition subjects had to mentally imagine previously studied abstract patterns, whereas in the difficult condition, subjects had to mentally manipulate these patterns. The results indicate the involvement of prefrontal areas in executive functions reflected by a decrease of anterior upper alpha short-range connectivity and a parallel increase of fronto-parietal long-range coherence mirroring activation of a fronto-parietal network.