We measured coherence between the electroencephalogram at different scalp sites while human subjects performed delayed response tasks. The tasks required the retention of either verbalizable strings of characters or abstract line drawings. In both types of tasks, a significant enhancement in coherence in the range (4-7 Hz) was found between prefrontal and posterior electrodes during 4-s retention intervals. During 6-s perception intervals, far fewer increases in coherence were found. Also in other frequency bands, coherence increased; however, the patterns of enhancement made a relevance for working memory processes seem unlikely. Our results suggest that working memory involves synchronization between prefrontal and posterior association cortex by phase-locked, low frequency (4-7 Hz) brain activity.

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A series of recordings in cat visual cortex suggest that synchronous activity in neuronal cell ensembles serves to bind the different perceptual qualities belonging to one object. We provide evidence that similar mechanisms seem also to be observable in human subjects for the representation of supramodal entities. Electroencephalogram (EEG) was recorded from 19 scalp electrodes (10/20 system) in 19 human subjects and EEG amplitude and coherence were determined during presentation of objects such as house, tree, ball. Objects were presented in three different ways: in a pictorial presentation, as spoken words and as written words. In order to find correlates of modality-independent processing, we searched for patterns of activation common to all three modalities of presentation. The common pattern turned out to be an increase of coherence between temporal and parietal electrodes in the 13-18 Hz beta1 frequency range. This is evidence that population activity of temporal cortex and parietal cortex shows enhanced coherence during presentation of semantic entities. Coherent activity in this low-frequency range might play a role for binding of multimodal ensembles.

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