Phase and amplitude dynamics of cortical networks in a visual working memory task.
Human visual working memory (VWM) is used for maintaining sensory information for cognitive operations and its deficits are associated with several neuropsychological disorders. VWM is based on sustained neuronal activity in a complex cortical network of frontal, parietal, occipital, and temporal areas. The neuronal mechanisms that coordinate this distributed processing to sustain coherent mental images and the mechanisms that set the behavioural capacity limit have remained unknown. In my presentation, I will demonstrate one approach for investigating VWM-related cortical network dynamics and discuss its advantages and limitations. Many human magneto- (MEG) and electroencephalography (EEG) studies show that the amplitude and phase interactions of ongoing oscillations are load dependently modulated during working memory (WM) encoding and maintenance. Much less is known about the cortical regions in which these modulations take place. We used source reconstruction techniques and combined MEG and EEG (MEEG) to study the phase and amplitude dynamics of ongoing activity during the performance of a visual working memory (VWM) task with a parametrically varying memory load. Using a novel neuroinformatics approach, we mapped the anatomical structure of inter-areal phase synchrony supporting VWM and estimated the graph properties of these networks. We also identified the brain regions where the event-related phase and amplitude 5-90 Hz oscillations were correlated with task performance, memory load, and behavioral performance. Inter-areal phase synchrony was sustained and stable during the VWM retention period among fronto-parietal and visual areas in alpha- (10-13 Hz), beta- (18-24 Hz), and gamma- (30-40 Hz) frequency bands. The synchrony was strengthened with increasing memory-load among the fronto-parietal regions known to underlie executive and attentional functions during memory maintenance. On the other hand, the subjects’ individual behavioral VWM capacity was predicted by synchrony in a network where the intraparietal sulcus was the most central hub. In terms of intra-areal phase and amplitude dynamics, the VWM encoding was characterized by transient large-scale phase locking in the 5-20 Hz band, which was prominent and memory-load dependent in several occipital, temporal, parietal, and frontal regions. The amplitude of ongoing activity was briefly enhanced in the 5-9 Hz band in a similar network. Thereafter, the amplitudes were suppressed below the baseline level for the entire VWM retention period in all bands from 5 to 90 Hz in widespread cortical areas so that the suppression was greater for trials with correct behavioral performance and negatively correlated with memory load. However, the amplitude of 10-30 Hz activity in fronto-parietal networks was strengthened with increasing memory load. These data thus reveal VWM-task dependent modulations in several dynamic and concurrent oscillatory processes in the visual and fronto-parietal regions that are known from fMRI to be associated with VWM encoding and maintenance. We suggest that inter-areal phase synchrony in α-, β-, and γ-frequency bands among fronto-parietal and visual regions could be a systems-level mechanism for coordinating and regulating the maintenance of neuronal object representations in VWM.