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Abstract: Resting-state functional connectivity studies with fMRI showed that the brain is intrinsically organized into large-scale functional networks for which the hemodynamic signature is stable for about 10s. Spatial analyses of the topography of the spontaneous EEG also show discrete epochs of stable global brain states (so-called microstates), but they remain quasi-stationary for only about 100ms. In order to test the relationship between the rapidly fluctuating EEG-defined microstates and the slowly oscillating fMRI-defined resting states, we recorded 64-channel EEG in the scanner while subjects were at rest with their eyes closed. Conventional EEG-microstate analysis determined the typical four EEG topographies that dominated across all subjects. The convolution of the time course of these maps with the hemodynamic response function allowed to fit a linear model to the fMRI BOLD responses and revealed four distinct distributed networks. These networks were spatially correlated with four of the resting-state networks (RSNs) that were found by the conventional fMRI group-level independent component analysis (ICA). These RSNs have previously been attributed to phonological processing, visual imagery, attention reorientation, and subjective interoceptive–autonomic processing. We found no EEG-correlate of the default mode network. Thus, the four typical microstates of the spontaneous EEG seem to represent the neurophysiological correlate of four of the RSNs and show that they are fluctuating much more rapidly than fMRI alone suggests. [ABSTRACT FROM AUTHOR]

Abstract: Much of the variation in both neuronal and behavioral responses to stimuli can be explained by pre-stimulus fluctuations in brain activity. We hypothesized that also errors are the result of stochastic fluctuations in pre-stimulus activity and investigated the temporal dynamics of the scalp topography and their concomitant intracranial generators of stimulus- and response-locked high-density event-related potentials (ERPs) to errors and correct trials in a Stroop task. We found significant differences in ERP map topography and intracranial sources before the onset of the stimulus and after the initiation of the response but not as a function of stimulus-induced conflict. Before the stimulus, topographic differences were accompanied by differential activity in lateral frontal, parietal and temporal areas known to be involved in voluntary reorientation of attention and cognitive control. Differential post-response activity propagated both medially and laterally on a rostral–caudal axis of a network typically involved in performance monitoring. Analysis of the statistical properties of error occurrences revealed their stochasticity. [Copyright &y& Elsevier]

Abstract: In western tonal music, musical phrases end with an explicit harmonic consequent which is highly expected. As such expectation is a consequence of musical background, cerebral processing of incongruities of musical grammar might be a function of expertise. We hypothesized that a subtle incongruity of standard closure should evoke a profound and rapid reaction in an expert''s brain. If such a reaction is due to neuroplasticity as a consequence of musical training, it should be correlated with distinctive activations in sensory, motor and/or cognitive function related brain areas in response to the incongruent closure. Using event related potential (ERP) source imaging, we determined the temporal dynamics of neuronal activity in highly trained pianists and musical laymen in response to syntactic harmonic incongruities in expressive music, which were easily detected by the experts but not by the laymen. Our results revealed that closure incongruity evokes a selective early response in musical experts, characterized by a strong, right lateralized negative ERP component. Statistical source analysis could demonstrate putative contribution to the generation of this component in right temporal–limbic areas, encompassing hippocampal complex and amygdala, and in right insula. Its early onset (~200 ms) preceded responses in frontal areas that may reflect more conscious processing. These results go beyond previous work demonstrating that musical training can change activity of sensory and motor areas during musical or audio-motor tasks, and suggest that functional plasticity in right medial–temporal structures and insula also modulates processing of subtle harmonic incongruities. [Copyright &y& Elsevier]

• Microstate sequences are best studied considering high-order temporal characteristics. • We present a method, Microsynt, to study the syntax of microstate sequences. • Microstate sequences are not random but favor low entropy words with binary loops. • During consciousness, externally oriented microstate binary loops are prominent. • Internally generated binary loops become prominent during unconscious states. Microstates represent electroencephalographic (EEG) activity as a sequence of switching, transient, metastable states. Growing evidence suggests the useful information on brain states is to be found in the higher-order temporal structure of these sequences. Instead of focusing on transition probabilities, here we propose "Microsynt", a method designed to highlight higher-order interactions that form a preliminary step towards understanding the syntax of microstate sequences of any length and complexity. Microsynt extracts an optimal vocabulary of "words" based on the length and complexity of the full sequence of microstates. Words are then sorted into classes of entropy and their representativeness within each class is statistically compared with surrogate and theoretical vocabularies. We applied the method on EEG data previously collected from healthy subjects undergoing propofol anesthesia, and compared their "fully awake" (BASE) and "fully unconscious" (DEEP) conditions. Results show that microstate sequences, even at rest, are not random but tend to behave in a more predictable way, favoring simpler sub-sequences, or "words". Contrary to high-entropy words, lowest-entropy binary microstate loops are prominent and favored on average 10 times more than what is theoretically expected. Progressing from BASE to DEEP, the representation of low-entropy words increases while that of high-entropy words decreases. During the awake state, sequences of microstates tend to be attracted towards "A – B – C" microstate hubs, and most prominently A – B binary loops. Conversely, with full unconsciousness, sequences of microstates are attracted towards "C – D – E" hubs, and most prominently C – E binary loops, confirming the putative relation of microstates A and B to externally-oriented cognitive processes and microstate C and E to internally-generated mental activity. Microsynt can form a syntactic signature of microstate sequences that can be used to reliably differentiate two or more conditions. [ABSTRACT FROM AUTHOR]

• EEG microstates capture discrete spatiotemporal patterns of global neuronal activity. • We studied their temporal dynamics in relation to different states of consciousness. • We estimate the complexity of microstates sequences. • With moderate sedation complexity increases then decreases with full sedation. • Complexity of microstate sequences is sensitive to altered states of consciousness. Evidence suggests that the stream of consciousness is parsed into transient brain states manifesting themselves as discrete spatiotemporal patterns of global neuronal activity. Electroencephalographical (EEG) microstates are proposed as the neurophysiological correlates of these transiently stable brain states that last for fractions of seconds. To further understand the link between EEG microstate dynamics and consciousness, we continuously recorded high-density EEG in 23 surgical patients from their awake state to unconsciousness, induced by step-wise increasing concentrations of the intravenous anesthetic propofol. Besides the conventional parameters of microstate dynamics, we introduce a new implementation of a method to estimate the complexity of microstate sequences. The brain activity under the surgical anesthesia showed a decreased sequence complexity of the stereotypical microstates, which became sparser and longer-lasting. However, we observed an initial increase in microstates' temporal dynamics and complexity with increasing depth of sedation leading to a distinctive "U-shape" that may be linked to the paradoxical excitation induced by moderate levels of propofol. Our results support the idea that the brain is in a metastable state under normal conditions, balancing between order and chaos in order to flexibly switch from one state to another. The temporal dynamics of EEG microstates indicate changes of this critical balance between stability and transition that lead to altered states of consciousness. [ABSTRACT FROM AUTHOR]