Visual working memory (VWM) is used to maintain 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 behavioral capacity limit have remained unknown. We mapped the anatomical and dynamic structures of network synchrony supporting VWM by using a neuro informatics approach and combined magnetoencephalography and electroencephalography. Interareal phase synchrony was sustained and stable during the VWM retention period among frontoparietal and visual areas in α-(10-13 Hz), β-(18-24 Hz), and γ-(30-40 Hz) frequency bands. Furthermore, synchrony was strengthened with increasing memory load among the frontoparietal 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 in which the intraparietal sulcus was the most central hub. These data suggest that interareal phase synchrony in the α-, β-, and γ-frequency bands among frontoparietal and visual regions could be a systems level mechanism for coordinating and regulating the maintenance of neuronal object representations in VWM.cortical synchrony | graph theory | magnetoencephalography | source modelling | functional connectivity F unctional MRI (fMRI) studies have shown that human visual working memory (VWM) is supported by neuronal activity in several cortical regions in the frontal, parietal, occipital, and temporal lobes (1-6), where the frontoparietal regions mediate attentional and central executive functions (2-4, 7, 8) and the visual areas underlie the formation of neuronal object representations (9-11) and sustain them in VWM (8). However, fMRI does not have the subsecond temporal precision required for revealing the neuronal mechanisms that integrate and coordinate the processing in the functionally distinct regions during VWM maintenance. These functions could be carried out by oscillatory synchrony (i.e., rhythmical millisecond-range temporal correlations of neuronal activity), which modulates neuronal interactions and regulates network communication (12)(13)(14)(15)(16). The functional role of oscillatory synchrony can be studied noninvasively by combining magnetoencephalography and electroencephalography (MEEG) recordings with source reconstruction techniques that reveal the anatomical structures producing the MEEG signals. Earlier studies have considered interactions among approximately three to nine cortical regions of interest and revealed attentional modulations of interareal synchrony (17-19). The interactions underlying VWM have remained uncharacterized. We hypothesized that neuronal synchronization is instrumental for the maintenance of object representations in VWM. To have this role, s...

Several studies show that the amplitudes of human brain oscillations are modulated during the performance of visual working memory (VWM) tasks in a load-dependent manner. Less is known about the dynamics and identities of the cortical regions in which these modulations take place and hence their functional significance has remained unclear. We used magneto- and electroencephalography (M/EEG) together with minimum-norm-estimate- (MNE) based source modeling to study the dynamics of ongoing brain activity during a parametric VWM task. Early stimulus processing and memory encoding were associated with a memory-load dependent spread of neuronal activity from occipital to temporal, parietal, and frontal cortical regions. During the VWM retention period, the amplitudes of oscillations in θ/α- (5–9 Hz), high-α- (10–14 Hz), β- (15–30 Hz), γ- (30–50 Hz), and high-γ- (50–150 Hz) frequency bands were suppressed below baseline levels and yet, in fronto-parietal regions, load dependently strengthened. However, in occipital and occipito-temporal structures, only β, γ, and high-γ amplitudes were robustly strengthened by memory load. Individual behavioral VWM capacity was predicted by both the magnitude of the N1 evoked-response component in early visual regions and by the amplitudes of fronto-parietal high-α and high-γ band oscillations. Thus both early stimulus processing and late retention period activities may influence the behavioral outcome in VWM tasks. These data support the notion that β- and γ-band oscillations support the maintenance of object representations in VWM whereas α-, β-, and γ-band oscillations together contribute to attentional and executive processing.

Time is a fundamental dimension of everyday experiences. We can unmistakably sense its passage and adjust our behavior accordingly. Despite its ubiquity, the neuronal mechanisms underlying the capacity to perceive time remains unclear. Here, in two experiments using ultrahigh-field 7-Tesla (7T) functional magnetic resonance imaging (fMRI), we show that in the medial premotor cortex (supplementary motor area [SMA]) of the human brain, neural units tuned to different durations are orderly mapped in contiguous portions of the cortical surface so as to form chronomaps. The response of each portion in a chronomap is enhanced by neighboring durations and suppressed by nonpreferred durations represented in distant portions of the map. These findings suggest duration-sensitive tuning as a possible neural mechanism underlying the recognition of time and demonstrate, for the first time, that the representation of an abstract feature such as time can be instantiated by a topographical arrangement of duration-sensitive neural populations.

Estimation of time is central to perception, action, and cognition. Human functional magnetic resonance imaging (fMRI) and positron emission topography (PET) have revealed a positive correlation between the estimation of multi-second temporal durations and neuronal activity in a circuit of sensory and motor areas, prefrontal and temporal cortices, basal ganglia, and cerebellum. The systems-level mechanisms coordinating the collective neuronal activity in these areas have remained poorly understood. Synchronized oscillations regulate communication in neuronal networks and could hence serve such coordination, but their role in the estimation and maintenance of multi-second time intervals has remained largely unknown. We used source-reconstructed magnetoencephalography (MEG) to address the functional significance of local neuronal synchronization, as indexed by the amplitudes of cortical oscillations, in time-estimation. MEG was acquired during a working memory (WM) task where the subjects first estimated and then memorized the durations, or in the contrast condition, the colors of dynamic visual stimuli. Time estimation was associated with stronger beta (β, 14 - 30 Hz) band oscillations than color estimation in sensory regions and attentional cortical structures that earlier have been associated with time processing. In addition, the encoding of duration information was associated with strengthened gamma- (γ, 30 - 120 Hz), and the retrieval and maintenance with alpha- (α, 8 - 14 Hz) band oscillations. These data suggest that β oscillations may provide a mechanism for estimating short temporal durations, while γ and α oscillations support their encoding, retrieval, and maintenance in memory. Hum Brain Mapp 37:3262-3281, 2016. © 2016 Wiley Periodicals, Inc.