Abstract: We investigated the relationship between visual experience and temporal intervals of synchronized brain activity. Using high-density scalp electroencephalography, we examined how synchronized activity depends on visual stimulus information and on individual observer sensitivity. In a perceptual grouping task, we varied the ambiguity of visual stimuli and estimated observer sensitivity to this variation. We found that durations of synchronized activity in the beta frequency band were associated with both stimulus ambiguity and sensitivity: the lower the stimulus ambiguity and the higher individual observer sensitivity the longer were the episodes of synchronized activity. Durations of synchronized activity intervals followed an extreme value distribution, indicating that they were limited by the slowest mechanism among the multiple neural mechanisms engaged in the perceptual task. Because the degree of stimulus ambiguity is (inversely) related to the amount of stimulus information, the durations of synchronous episodes reflect the amount of stimulus information processed in the task. We therefore interpreted our results as evidence that the alternating episodes of desynchronized and synchronized electrical brain activity reflect, respectively, the processing of information within local regions and the transfer of information across regions.

Abstract: Visual integration between target and irrelevant features leads to effects of irrelevant feature congruency (Stroop) or variation (Garner) on target classification performance. Presenting closed geometrical shapes as stimuli, we obtained Stroop and Garner effects of one part of their contour on another, in response times and error rates. The correlates of these effects in brain activity were observed in event-related potentials (ERP). Stroop effects occurred in ERP amplitude of the N1 and N2 components, starting about 170 ms after stimulus onset; Garner effects occurred in amplitude of the rising part of the P3 component, starting about 330 ms after stimulus onset. A subsequent point-wise analysis of Stroop and Garner effects in ERP showed that they belong to different, cascaded processing stages. The difference in time course between Stroop and Garner effects in ERP is in accordance with the view that both are produced by different mechanisms, the former sensitive to interference within presentations and the latter sensitive to interference between presentations. The brief interval of 330-370 ms after stimulus onset when these two mechanisms overlap may correspond to the central processing bottleneck, responsible for the combinations of Stroop and Garner effects generally found in response times.

Abstract: Selection of visual short-term memory content takes place during perceptual and postperceptual processing. We studied postperceptual selection in a change detection task, in which spatially distributed cues preceding the memory array marked potential change targets. In the 550-730 ms interval after the memory array onset but not in the following retention interval, event-related potentials showed larger contralateral delay activity when the spatial cues were selective than when they were unselective. This finding implies that distributed precueing strengthens the visual short-term memory representation during an extended memory consolidation phase.

Abstract: We investigated the brain responses to the transitions from the static to moving audiovisual stimuli using magnetoencephalography. The spatially congruent auditory and visual stimuli moved in the same direction whereas the incongruent stimuli moved in the opposite directions. Using dipole modeling we found that the static-to-moving transitions evoked a neural response in the primary auditory cortex bilaterally. The response started about 100 ms after the motion onset from a negative component (mvN1) and lasted during the entire interval of the stimulus motion. The mvN1 component was similar to the classical auditory N1 response to the static sound, but had smaller amplitude and later latency. The coordinates of the mvN1 and N1 dipoles in the primary auditory cortex were also similar. The amplitude of the auditory response to the moving stimuli appears to be sensitive to spatial congruency of the audiovisual motion; it was larger in the incongruent than congruent condition. This is evidence that the moving visual stimuli modulate the early sensory activity in the primary auditory cortex. Such early audiovisual integration may be specific for motion processing.

Abstract: We used high-density event-related potentials (ERP) in a modified flanker paradigm to study the role of anticipated action consequences in action planning and the role of anticipation in the perception of action consequences. Prior to the experiment, participants were trained to classify target letters in a four-alternative forced-choice task; another letter was presented as an effect following each response. After participants had thus acquired the response-effect contingencies, in the experiment effect letters were presented as flankers to target letters. Effect-compatible flankers were letters that were learned as effects of the correct response to the target; effect-incompatible ones were learned as effects of other responses; neutral flankers were never presented as action effects. To help distinguish early and late effects of flankers on target processing, flankers were presented either simultaneously with the target or after a delay. We found that effect-incompatible flankers resulted in longer, than other flankers, time between the onset of the response-locked lateralized readiness potential and the response, indicating extended motor processing. ERP evoked by the effect-incompatible flankers differed from those evoked by other flankers in early perceptual component P1 and in later frontal component P2 reflecting stimulus evaluation and conflict detection. These results show that anticipating action consequences involves brain systems ranging from perceptual to executive; anticipated action effects constitute a link between perception and action.

Abstract: Perceptual grouping is a multi-stage process, irreducible to a single mechanism localized anatomically or chronometrically. To understand how various grouping mechanisms interact, we combined a phenomenological report paradigm with high-density event-related potential (ERP) measurements, using a 256-channel electrode array. We varied the relative salience of competing perceptual organizations in multi-stable dot lattices and asked observers to report perceived groupings. The ability to discriminate groupings (the grouping sensitivity) was positively correlated with the amplitude of the earliest ERP peak C1 (about 60 ms after stimulus onset) over the middle occipital area. This early activity is believed to reflect spontaneous feed-forward processes preceding perceptual awareness. Grouping sensitivity was negatively correlated with the amplitude of the next peak P1 (about 110 ms), which is believed to reflect lateral and feedback interactions associated with perceptual awareness and attention. This dissociation between C1 and P1 activity implies that the recruitment of fast, spontaneous mechanisms for grouping leads to high grouping sensitivity. Observers who fail to recruit these mechanisms are trying to compensate by using later mechanisms, which depend less on stimulus properties such as proximity.

Abstract: We studied an effect of predictability in an audio-visual apparent motion task using magnetoencephalography. The synchronous sequences of audio-visual stimuli were self-triggered by subjects. The task was to detect the direction of the apparent motion in experimental blocks in which the motion either started from the side selected by subjects (predictable condition) or was random (unpredictable condition). Magnetic fields yielded three patterns of activity in the motor, auditory, and visual areas. Comparison of the dipole strength between predictable and unpredictable conditions revealed a significant difference of the preparatory motor activity in the time interval from -450 to -100 ms before self-triggering the stimulus. Perception of the audio-visual apparent motion was also modulated by predictability. However, the modulation was found only for the auditory activity but not for the visual one. The effect of predictability was selective and modulated only the auditory component N1 (100 ms after stimulus), which reflects initial evaluation of stimulus meaning. Importantly, the preparatory motor activity correlates with the following auditory activity mainly in the same hemisphere. Similar modulation by predictability of the motor and auditory activities suggests interactions between these two systems within an action-perception cycle. The mechanism of these interactions can be understood as an effect of anticipation of the own action outcomes on the preparatory motor and perceptual activity.

Abstract: In three experiments, participants determined the orientation of a global triangle formed by three Gabor patches of a target spatial frequency in a field of distracters. The orientation of the target patches and their proximity were varied between conditions. When all the target patches had the same orientation this facilitated the response compared to random orientations. This effect occurred only when the patches were in close proximity. When the orientations of the target patches were different but aligned to the global triangle, facilitation occurred regardless of proximity. These contrasting types of facilitation were attributed to different early perceptual integration mechanisms that enable the perception of holistic structure.

Abstract: Phase patterns of human scalp alpha EEG activity show spontaneous transitions between different globally phase-synchronized states. We studied the dynamical properties of these transitions using the method of symbolic dynamics. We found greater predictability (deterministicity) and heterogeneity in the dynamics than what was expected from corresponding surrogate series in which linear correlations are retained. A possible explanation of these observations within the framework of chaotic itinerancy is discussed.

Abstract: We investigate patterns of collective phase synchronization in brain activity in awake, resting humans with eyes closed. The alpha range of human electroencephalographic activity is characterized by ever-changing patterns, with strong fluctuations in both time and overall level of phase synchronization. The correlations of these patterns are reflected in power-law scaling of these properties. We present evidence that the dynamics underlying this fluctuation is type-I intermittency. We present a model study illustrating that the scaling property and the collective intermittent dynamics are emergent features of globally coupled phase oscillators near the critical point of entering global frequency locking.

Abstract: Spatiotemporal characteristics of spontaneous alpha EEG activity patterns are analyzed in terms of large-scale phase synchronization. During periods with strong phase synchronization over the entire scalp, phase patterns take either of two forms; one is a gradual phase shift between frontal and occipital regions and the other is a stepwise pattern with a sudden phase shift in the central region. The former is regarded as a traveling wave of electrocortical activity, of which the direction of propagation is predominantly from anterior to posterior in three out of four subjects, and opposite in the remaining one. The other activity pattern observed may correspond to a standing wave composed of two traveling waves propagating in opposite directions. The duration distributions of these patterns have similar forms within a subject, which suggests that they share the same mechanism for their generation.

Abstract: The attentional blink (AB) phenomenon occurs when perceivers must report two targets embedded in a sequence of distracters; if the first target precedes the second by 200-600 msec, the second one is often missed. We investigated AB by measuring dynamic cross-lag phase synchronization for 565 electrode pairs in 40-Hz-range EEG. Phase synchrony, on average, was higher in experimental conditions, where two targets are reported, than in control conditions, where only the second target is reported. The effect occurred in electrode pairs covering the whole head. Timing of the synchrony was crucial: Brief episodes of enhanced synchrony occurred 100-500 msec before expected target onset in AB conditions where the second target was correctly reported. These results show that intrinsic brain dynamics produce anticipatory synchronization in transient assemblies of cortical areas. Enhanced levels of anticipatory synchronization occur in response to the demands of the task in conditions where the system's limited capacity is under strain.

Abstract: OBJECTIVE: Data from a previous event-related potential (ERP) study in visual-perceptual grouping [Nikolaev AR, van Leeuwen C. Flexibility in spatial and non-spatial feature grouping: an event-related potentials study. Brain Res Cogn Brain Res 2004;22:13-25] were re-analyzed to identify event-related dynamics of phase-synchronization. METHODS: In 20 Hz activity, uniform spreading of phase synchronization in closely spaced (approximately 2 cm) scalp electrodes appears and disappears spontaneously. The lengths of synchronized activity intervals and how they vary as a function of stimulus presentation were compared between task and control conditions. RESULTS: Synchronization reached a maximum in the task condition about 180 ms post-stimulus onset, coinciding with the peak N180 ERP marking the deployment of task-specific attention. Synchronized intervals were longer in the task than in the control condition. Long (above 80 ms) intervals occurred at a stable rate before and just after stimulus onset, but steeply decreased 200-400 ms afterwards. CONCLUSIONS: Perceptual tasks lead to longer synchronized intervals in early visual areas. Attention deployment resets the ongoing synchronization. Event-related activity, besides low-frequency ERP, consists of high-frequency short and long synchronized intervals corresponding to evoked bursts and ongoing oscillations, respectively. SIGNIFICANCE: High-density scalp recorded EEG revealed synchronization dynamics in a local, early visual area of cortex that can be interpreted as modulation of spontaneous ongoing task-related processes by attention.

Abstract: Early perceptual grouping was studied using 256-channels event-related potentials in a choice response task. The task involved the detection of a triangle configuration of Gabor patches among patches of different spatial frequencies. The influence of two task-irrelevant factors was compared. One was the spatial proximity relation between the target patches and the other their relative orientation, a non-spatial relationship. Non-spatial effects were predominant in early peaks N64 and P100 in the occipital areas, and were reduced in size for later peaks. Spatial effects started from N180 in the occipital areas and continued in P250 and P430 in the central areas, increasing in size with time. These findings constitute a case of reversal of the usual order of spatial and non-spatial feature processing, illustrating that the flexibility in the early visual system may be greater than previously assumed.

Abstract: The dynamical properties of large-scale, long-term phase synchronization behavior in the alpha range of electroencephalographic signals were investigated. We observed dynamical phase synchronization and presented evidence of an underlying spatiotemporal ordering. Fluctuations in the duration of episodes of intermittent synchrony are scale-invariant. Moreover, the exponent used to describe this behavior is stable across different normal subjects. The results provide a new feature of self-organization in human brain activity and constitute a quantitative basis for modeling its dynamics.

Abstract: According to a widely cited finding by Ellis and Stark (1978 Perception 7 575-581), the duration of eye fixations is longer at the instant of perceptual reversal of an ambiguous figure than before or after the reversal. However, long fixations are more likely to include samples of an independent random event than are short fixations. This sampling bias would produce the pattern of results also when no correlation exists between fixation duration and perceptual reversals. When an appropriate correction is applied to the measurement of fixation durations, the effect disappears. In fact, there are fewer actual button-presses during the long intervals than would be expected by chance. Moving-window analyses performed on eye-fixation data reveal that no unique eye event is associated with switching behaviour. However, several indicators, such as blink frequency, saccade frequency, and the direction of the saccade, are each differentially sensitive to perceptual and response-related aspects of the switching process. The time course of these indicators depicts switching behaviour as a process of cascaded stages.

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Abstract: The aim of this study was to investigate an interaction between frontal and left temporo-parietal cortices in tasks requiring word association. A new method was used to examine averaged event-related potentials in different frequency bands by calculating correlation coefficients between wavelet curves in distant cortical areas. This method was applied to previous event-related potentials recordings which found successive activation of frontal and left posterior areas [1]. Correlated activity at 17 Hz was observed between frontal and left temporal (Wernicke's) areas prior to full activation of Wernicke's area.

Abstract: Interactions between cortical areas were studied during the search for verbal associations and reading of words. The functional anatomy and the sequence of involvement of cortical areas during the solution of these tasks have been described previously, in studies using positron emission tomography and multichannel recordings of evoked potentials combined with identification of the locations of dipole sources [8, 9, 19, 25]. Cortical interactions reflected in terms of the synchronization of EEG rhythms were studied by developing a method based on correlating curve wavelets, which allows the moments at which this synchronization occurs to be identified over short periods of time comparable with the speeds of individual thought operations (up to 100 msec). Three main stages were identified in the search for associations. During the first 200 msec after stimulus presentation, cortical connections were seen between the right and left frontal areas; at 200-500 msec, there were connections between the frontal and the temporal-parietal areas; finally, at 450-700 sec, there were connections between the left temporal and the right frontal-central-temporal areas. These results are in good agreement with data obtained previously using other methods and supplement them with mapping data on cortical connections. A number of differences in the mechanisms of information processing during the search for associations and reading were also identified.

Abstract: Cortical connectivity was studied in tasks of generating the use of words in comparison with reading aloud the same words. These tasks were used earlier in PET and high density ERP recordings studies (Posner and Raichle, 1997; Abdullaev and Posner, 1998), in which both the functional anatomy and the time course of cortical areas involved in word processing were described. The wavelet transforms of ERP records and the calculation of correlations between wavelet curves were used to reveal connections between cortical areas. Three stages of intracortical communications while task performance were found. These were: (1) the connections between right and left frontal and central areas which preceded stimulus delivery and persisted up to 180 ms after it; (2) the network connecting right and left frontal with left posterior temporal-parietal junction at 280-450 ms; and (3) communications between left and right temporal zones in 550-800 ms. The data are in good agreement with results of previous PET and ERP studies and supply the earlier findings with circuitry of cortical information transfer.

Abstract: Dimensional complexity (DCx) is an EEG measure derived from nonlinear systems theory that can be indicative of the global dynamical complexity of electrocortical activity. This study examined developmental changes in DCx, as well as the effects of cognitive tasks, gender, and brain topography, and compared DCx with traditional spectral power measures. EEG was recorded in three groups of children at mean age of 7.5 (n = 37), 13.8 (n = 42), and 16.4 (n = 56) years at rest and during the performance of verbal and spatial cognitive tasks. DCx measured both at rest and during tasks increased with age. Specific effects of brain topography, condition, and gender became stronger with age, suggesting an increase in structural and functional differentiation of the cortex. Hemispheric asymmetry of DCx recorded during tasks also increased with age, with the task-induced DCx reduction being stronger in the left hemisphere. Gender differences in DCx suggested faster cerebral maturation in girls over late adolescence. Relationships between DCx and spectral power varied as a function of tasks and scalp locations, suggesting that these EEG measures can reflect different aspects of cortical functioning.

Abstract: Cortical connectivity was studied in tasks of generating the use of words in comparison with reading aloud the same words. These tasks were used earlier in PET and high-density ERP recording studies, which described both the functional anatomy and time course of involvement of cortical areas in word processing. We developed a new method for studying the synchrony of EEG spectral components within the short time intervals compatible with the duration of particular cognitive operations. The wavelet transform of the ERP records and calculation of correlations between the wavelet curves were used to reveal connections between cortical areas. Three stages of intracortical communications developing over the course of task performance were discovered: between the right and left frontal areas (0-200 ms after the stimulus presentation), between the left frontal and left posterior temporo-parietal areas (250-500 ms), and, finally, between the left temporal and right fronto-centro-temporal areas. These findings are in good agreement with the results of the previous PET and ERP studies and supplement them with the circuitry of cortical information transfer. Also, they suggest some differences in information processing during automated reading and performance of more complicated use-generation task.

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Abstract: Spectral EEG powers were compared in 4 frequency ranges (8-13, 15-25, 25-35, and 35-45 Hz) in a group of 20 subjects during the performance of tasks requiring mental rotation of two- and three-dimensional objects. Only those EEG segments corresponding to tasks with identical solution times were analyzed. The spectral powers of oscillations in the alpha range were higher in control conditions than during task performance. Power in the frequency range 15-45 Hz was greater during task performance than in control conditions; this supports the concept that alpha rhythm desynchronization accompanies the synchronization of higher-frequency EEG rhythms. Frequency power during task performance with two-dimensional objects was greater than that during tasks with three-dimensional objects. Since the angle of rotation between two-dimensional objects was greater than that between three-dimensional objects, this factor, rather than the depth of the perceived space, increased the level of cortical activation. In all experimental situations, power at frequencies of 15-45 Hz was significantly greater in the occipital regions than any other regions, reflecting the visual modality of the stimulus. Particular changes were noted in the gamma range (35-45 Hz), where power in the first second of task performance was significantly higher than in the second second; this may provide evidence that this range is more closely associated with perception and recognition processes than with mental transformation of the image.

Abstract: The EEG spectral power in 4 frequency ranges (8-13, 15-25, 25-35, and 35-45 Hz) was studied during mental rotation of two- and three-dimensional objects. Only those EEG segments were analyzed which corresponded to the tasks with similar time of solving. The EEG spectral power of the alpha range was higher under control conditions than during task solving. The spectral power in the range of 15-45 Hz was higher under the operation than under the control conditions. This finding confirms the assumption that EEG desynchronization in the alpha range is accompanied by the high-frequency synchronization. The EEG high-frequency power was higher during mental rotation of two-dimensional objects than three-dimensional ones. Since the angular difference between the two-dimensional objects was larger than between the three-dimensional ones, the rotation angle rather than the "depth" of space increased cortical activation. Under experimental conditions, EEG spectral power in the range of 15-45 Hz was always the highest in the occipital areas, which was associated with the visual modality of stimuli. The EEG spectral power in the gamma range (35-45 Hz) was substantially higher at the first second of operation as compared with the second one testifying that EEG changes in this range are more related with the processes of perception and recognition than with mental operations with images.

Abstract: Reflection in EEG parameters of the process of solving two types of mental tasks, i.e. spatial-imaginary and verbal-logical, was investigated in nine test-subjects. Mental efforts were found to specify EEG spatiofrequency patterns depending on the task type and subject's identity. These patterns were unique and reproducible enough to enable identification of this or the other type of mental task within several seconds of EEG record. Recognition was executed with the use of an artificial neural network algorithm in order to simultaneously weigh a wealth of signs of a specific EEG pattern. Recognition precision was 85% on the average.

Abstract: This study investigated age-related changes in the human brain function using both traditional EEG analysis (power spectra) and the correlational dimension, a measure reflecting the complexity of EEG dynamics and, probably, the complexity of neurophysiological processes generating the EEG. Assuming that the accumulation of individual experience is determined by the formation of functionally related groups of neurons showing a repetitive synchronous activation (cell assemblies), an increase in the number of such independently oscillating cortical cell assemblies can be expected, despite a decline of some metabolic and memory functions with normal ageing. Thus, the "wisdom of old age' may find its neurophysiological basis in greater complexity of brain dynamics compared to young ages. The experimental hypothesis was that EEG dimension steadily increases with age. In order to test this hypothesis the resting EEGs of 5 age groups from 7 to 60 were analysed. The results confirm the hypothesis: after a jump in the brain dynamics complexity during puberty a linear increase with age is observed. During maturation (7-25 years), the maximum gain in complexity occurs over the frontal associative cortex.

Abstract: Solution of spatial and verbal tasks was studied in 43 subjects using the advanced version of the method of intracortical interaction mapping (IIM) and analysis of EEG spectra in short (250 ms) intervals. Comparison between the IIM and coherence methods during solving simple motor tasks revealed some advantages of the IIM for studying of labile cortical connections. Several stages were separated in the process of solution of the mental task in accordance with patterns of cortical connections. The duration of the stages was similar in both kinds of tasks and corresponded to that revealed in psychological experiments. Cortical connections in the theta range were generalized and reflected the level of general tension. This unspecific tension was high for the whole period of the spatial task solution while in the verbal task the tension increased to the stage of decision making. Task-specific patterns were revealed in the beta range. The right parietal and temporal regions were the centers of EEG connections during solution of the spatial task while the left central area was essential for the verbal task solution. The left parietal area was involved in solution of both kinds of tasks. The frequency parameters of connections in the beta range were stable during the spatial task solving, and in the process of verbal task solving the dominant frequency changed after the initial (perception) stage probably due to switching to another thinking mechanism.

Abstract: The stages of the mental rotation of complex three-dimensional figures were investigated in nine healthy subjects using the intracortical interaction mapping technique. It was established that mental rotation is accomplished with the participation of both parietal regions; the prerotation setup is associated with the involvement of the frontal, central, and right temporal divisions of the brain; decision-making regarding the result of the rotation and the response is accomplished through the conjunction of both frontal and temporoparietooccipital regions of the left hemisphere. In the case of the unsuccessful solution of the problem the process seems to stop at the stage of the prerotation setup. With verbal control the configuration of the zone of high-level association during the first three stages is reminiscent of that observed in the baseline activity.

Abstract: Effect of sustained mental work on the first stage of mental rotation was studied by the method of intracortical interaction. It was revealed that during sustained mental work the zone of the high level of connections in the alpha-band was considerably reduced due to an increase of specificity, rationality of thinking. In the beta-band the zone of the high connection level moved from the central and parietal areas to the anterior regions because of a switch of intention of the mental work from the spatial analysis to the quick verbalization of the response. Tired subjects in search for a word for the response did not fulfil the spatial analysis of the pictures. Subjects with high working ability searched for the verbal response on the basis of mental operations with images, that turned to be optimal for the mental rotation task solving.

Abstract: The stages of mental rotation of complicated three-dimensional figures were studied using the intracortical interaction mapping. The role of the parietal areas in the mental rotation was shown. The frontal, central and the right temporal areas are involved in the prerotation setup. Both frontal areas and the left temporo-parieto-occipital areas participate in decision making and verbal response. In the case of unsuccessful task solving the process stops at the prerotation setup stage. During the verbal control the zones of the predominant connections at the first 3 stages are close to those in the background activity.