Phase synchronization of oscillatory brain activity and the matching of top-down and bottom up visual information

When information which is stored in memory is compared to new visual information there has to be a matching between provided top-down memory information and bottom-up sensory processing. The neural correlates of such memory matching are not yet completely clear. There is evidence that top-down processing and central executive functions of working memory are associated with distributed fronto-parietal interaction at theta frequency (around 5 Hz) in the human electroencephalogram (EEG). On the other hand gamma oscillations (beyond 30 Hz) were shown to be involved in bottom-up visual information processing. Here, data from an experiment in which subjects had to hold ‘templates’ of visual targets in memory and match them with incoming visual information will be reported. It will be shown that distributed interregional phase coherence at theta frequency is increased during time intervals in which top-down memory information had to be provided by a working memory system. This top-down memory content was then matched at posterior parietal brain sites with incoming visual information indicated by increased cross-frequency phase synchronization between theta and gamma oscillations. This correlate of memory matching was most pronounced in a time interval between 100 and 200 ms after presentation of visual information. Furthermore, it will be shown that phase synchronization between theta and gamma oscillations as brain correlate of memory matching was amplified by visual attention. As a conclusion of the study it can be stated that interregional and cross-frequency phase synchronization in the human EEG are informative about the interplay of top-down and bottom-up processing. They seem to reflect the interaction of distributed and local cortical networks and therefore may play an important role in most higher cognitive functions