I started the Laboratory for Perceptual Dynamics at RIKEN, Japan in 2000 and in 2011 brought it to the University of Leuven supported by an Odysseus grant of FWO, the Flemish Science Foundation. The lab is interested in a Complex Systems approach to visual perception, in particular to high and mid-level vision. We combine experimental, computational and theoretical models in our studies. Until recently, the laboratory consisted of approximately 15 academic staff, but is now winding down in anticipation of my retirement in 2022. Phone: +32.16.326069 Address: Katholieke Universiteit Leuven Faculteit PPW Tiensestraat 102 B-3000 Leuven Belgium
Analyzing single trial brain activity remains a challenging problem in the neurosciences. We gain... more Analyzing single trial brain activity remains a challenging problem in the neurosciences. We gain purchase on this problem by focusing on globally synchronous fields in within-trial evoked brain activity, rather than on localized peaks in the trial-averaged evoked response (ER). We analyzed data from three measurement modalities, each with different spatial resolutions: magnetoencephalogram (MEG), electroencephalogram (EEG) and electrocorticogram (ECoG). We first characterized the ER in terms of summation of phase and amplitude components over trials. Both contributed to the ER, as expected, but the ER topography was dominated by the phase component. This means the observed topography of cross-trial phase will not necessarily reflect the phase topography within trials. To assess the organization of within-trial phase, traveling wave (TW) components were quantified by computing the phase gradient. TWs were intermittent but ubiquitous in the within-trial evoked brain activity. At most task-relevant times and frequencies, the within-trial phase topography was described better by a TW than by the trial-average of phase. The trial-average of the TW components also reproduced the topography of the ER; we suggest that the ER topography arises, in large part, as an average over TW behaviors. These findings were consistent across the three measurement modalities. We conclude that, while phase is critical to understanding the topography of event-related activity, the preliminary step of collating cortical signals across trials can obscure the TW components in brain activity and lead to an underestimation of the coherent motion of cortical fields.
We investigated visual working memory encoding across saccadic eye movements, focusing our analys... more We investigated visual working memory encoding across saccadic eye movements, focusing our analysis on refixation behavior. Over 10-s periods, participants performed a visual search for three, four, or five targets and remembered their orientations for a subsequent change-detection task. In 50% of the trials, one of the targets had its orientation changed. From the visual search period, we scored three types of refixations and applied measures for quantifying eye-fixation recurrence patterns. Repeated fixations on the same regions as well as repeated fixation patterns increased with memory load. Correct change detection was associated with more refixations on targets and less on distractors, with increased frequency of recurrence, and with longer intervals between refixations. The results are in accordance with the view that patterns of eye movement are an integral part of visual working memory representation.
Sense of agency can be defined as the self-awareness of bodily movement, whereas extended agency ... more Sense of agency can be defined as the self-awareness of bodily movement, whereas extended agency as the self-awareness of affecting, through movement, events concomitant with movement. As a distinctive manifestation of agency, we review Spizzo's effect. This effect arises when agents coordinate their rhythmic movements with visual pulses. Once coordination is established, agents feel controlling the onset or the offset of the pulses through their movements. Spizzo's effect, therefore, constitutes a manifest case of extended agency, in which agents are aware of controlling , through movement, the pulses temporally concomitant with movement. We propose that sense of agency requires continuity of kinesthesia, which in turn requires the continuity of self-hood. The continuity of kinesthesia is necessarily deduced from the consistent kinematics observed in movements, whereas the continuity of selfhood may stem from the patterns of rhythmic coordination that humans encounter ever since intrauterine life. The primacy of these patterns in adults is in accordance with phenomena such as Spizzo's effect, which require coordination to be induced. We, therefore, propose coordination as the fundamental interaction from which selfhood, kinesthesia, and agency arise.
When we synchronize finger tapping with a visual metronome, we experience a strikingly robust phe... more When we synchronize finger tapping with a visual metronome, we experience a strikingly robust phenomenon of extended agency known as Spizzo’s effect. This effect is the compelling sense that we are controlling the metronome. The effect arises even though the agent knows that the metronome operates autonomously. We propose that the extended agency here established over metronome pulses results from sensorimotor coordination. To test this hypothesis, we operationalize sensorimotor coordination in terms of the correlation structures in series of asynchronies or reaction times from two finger-tapping tasks. Analyses reveal that, whereas correlation structures vary across individuals and show a systematic drift towards nonstationarity with increasing metronome frequency conditions, the presence of correlation structure is co-extensive with Spizzo’s effect. We interpret this result as supporting the view that extended agency relies on sensorimotor coordination. Sensorimotor coordination, we suggest, may induce the effect by integrating the perception of visual pulses and the agency over tapping into a synesthetic experience.
Human connectome studies suggest that the brain has a modular small world network structure with ... more Human connectome studies suggest that the brain has a modular small world network structure with rich-club effect. Such structure emerges spontaneously in simple model neural networks, (e.g. coupled maps), through adaptive rewiring according to the dynamic functio na l connectivity. The utility of adaptive rewiring has so far exclusively been demonstrated for unweighted networks; it is anything but guaranteed to work as well for weighted networks. We investigate adaptive rewiring in weighted networks, comparing various right-skewed, symmetrical, and left-skewed fixed weight distributions. We examine how network clustering, path length, modularity, and rich club coefficients develop for weakly, intermediate and strongly coupled networks. At low coupling strength, the weight distribution, as well as episodes of functio na l synchrony, have a significant effect on network evolution. With increased coupling strengths, all weighted networks robustly develop architectures similar to the unweighted ones. Adaptive rewiring appears relative ly ineffective in networks with (biologically implausibly) extreme rightskewed weight distributions but performed most economically in biologically plausible log-normal distributions.
In order to consolidate the dissociation in feature integration found in earlier studies between ... more In order to consolidate the dissociation in feature integration found in earlier studies between letters of the Roman alphabet and corresponding pseudo-letters, the present study used kanji and kana and corresponding pseudo-letters, presented visually, either in isolation or surrounded by congruent or incongruent shape, as targets in a choice-response task with three different response criteria. When the criterion was shape, congruence effects were obtained for both real and pseudo-letters. With the second and third response criteria this result was found for pseudo-letters, but not for letters. These criteria either involved distinguishing between letters and visually similar pseudo-letters or distinguishing between visually similar letters. The dissociation of congruence effects between letters and pseudo-letters was therefore shown to depend on visual similarity between targets, independent of their category. This effect was found to be robust for kanji but not for kana, which is related to distinctions between these two writing systems.
A computational theory of consciousness should include a quantitative measure of consciousness, o... more A computational theory of consciousness should include a quantitative measure of consciousness, or MoC, that (i) would reveal to what extent a given system is conscious, (ii) would make it possible to compare not only different systems, but also the same system at different times, and (iii) would be graded, because so is consciousness. However, unless its design is properly constrained, such an MoC gives rise to what we call the boundary problem: an MoC that labels a system as conscious will do so for some—perhaps most—of its subsystems, as well as for irrelevantly extended systems (e.g., the original system augmented with physical appendages that contribute nothing to the properties supposedly supporting consciousness), and for aggregates of individually conscious systems (e.g., groups of people). This problem suggests that the properties that are being measured are epiphenomenal to consciousness, or else it implies a bizarre proliferation of minds. We propose that a solution to the boundary problem can be found by identifying properties that are intrinsic or systemic: properties that clearly differentiate between systems whose existence is a matter of fact, as opposed to those whose existence is a matter of interpretation (in the eye of the beholder). We argue that if a putative MoC can be shown to be systemic, this ipso facto resolves any associated boundary issues. As test cases, we analyze two recent theories of consciousness in light of our definitions: the Integrated Information Theory and the Geometric Theory of consciousness.
In the course of perceptual organization, incomplete optical stimulation can evoke the experience... more In the course of perceptual organization, incomplete optical stimulation can evoke the experience of complete objects with distinct perceptual identities. According to a well-known principle of perceptual organization, stimulus parts separated by shorter spatial distances are more likely to appear as parts of the same perceptual identity. Whereas this principle of proximity has been confirmed in many studies of perceptual grouping in static displays, we show that it does not generalize to perception of object identity in dynamic displays, where the parts are separated by spatial and temporal distances. We use ambiguous displays which contain multiple moving parts and which can be perceived two ways: as two large objects that gradually change their size or as multiple smaller objects that rotate independent of one another. Grouping over long and short distances corresponds to the perception of the respectively large and small objects. We find that grouping over long distances is often preferred to grouping over short distances, against predictions of the proximity principle. Even though these effects are observed at high luminance contrast, we show that they are consistent with results obtained at the threshold of luminance contrast, in agreement with predictions of a theory of efficient motion measurement. This is evidence that the perception of object identity can be explained by a computational principle of neural economy rather than by the empirical principle of proximity.
Culture-related differences in visual creativity were investigated, comparing Italian and Japanes... more Culture-related differences in visual creativity were investigated, comparing Italian and Japanese participants in terms of divergent (figural completion task) and product-oriented thinking (figural combination task). Visual restructuring ability was measured as the ability to reinterpret ambiguous figures and was included as a covariate. Results showed that in divergent thinking, Italians only outperformed Japanese participants in elaboration and in product-oriented thinking in terms of originality of products. Visual restructuring ability was found to play a key role both in originality and practicality of products. Both groups scored the same in terms of fluency, originality, and flexibility of visual divergent thinking, as well as in term of practicality of creative products. These findings are consistent with the idea that Italians and Japanese have the same creative potential, although from the early stages of the design Japanese seem to show a greater tendency to take practicality constraints into account when creating in the visual domain.
Stroop interference is deterioration of a behavioral response to a target feature resulting from ... more Stroop interference is deterioration of a behavioral response to a target feature resulting from an effect of incongruent but irrelevant other feature. Garner interference is an effect of variation of an irrelevant feature across trials. van Leeuwen & Bakker (1995) predicted that Stroop ...
ABSTRACT We tested the memory characteristics of visual search in an event-related potentials stu... more ABSTRACT We tested the memory characteristics of visual search in an event-related potentials study. Participants searched for a target letter presented among nontarget letters. We varied target identity (switch to new vs swap with nontarget from previous trial), nontarget identity (switch to new vs swap with target from previous trial), target location (repeat, switch, swap with nontarget), and nontarget location (repeat, switch, swap with target). Repeated target locations were responded to faster and more accurately than switched ones. There was a slowdown in performance when nontarget and target swapped their locations. The amplitude in the N2pc component was smaller when target location switched than repeated. The amplitude in the N1 component was highest when target and nontarget swapped their roles and target location switched; and smallest when both target and nontarget switched to new identity and target location repeated. These data show that visual search has memory for both target location and identity, but also nontarget identity and location play a role in processing.
Spatiotemporal characteristics of spontaneous alpha EEG activity patterns are analyzed in terms o... more 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.
Studies in Perception and Action: Fourteenth International Conference on Perception and Action, Jul 9, 2007
Method We adopted an approach developed by Ziessler and his colleagues based on Eriksen's fl... more Method We adopted an approach developed by Ziessler and his colleagues based on Eriksen's flanker paradigm (Ziessler & Nattkemper, 2002; Ziessler & Nattkemper, 2003; Ziessler et al., 2004). Eriksen and Eriksen (1974) showed that letters flanking a target letter on both sides were processed together with the target letter and activated their own responses. Responses to the target were facilitated or inhibited depending on the compatibility between the response to the target and the response activated by the ...
Internal psychophysics is concerned with lawful mind-brain relationships. Too few of these have b... more Internal psychophysics is concerned with lawful mind-brain relationships. Too few of these have been discovered so far. All we have been able to observe are correlations; for instance between a mental event and an electrophysiological signal. Is this because we are looking at ...
van, Traveling waves and trial averaging: the nature of single-trial and averaged brain responses... more van, Traveling waves and trial averaging: the nature of single-trial and averaged brain responses in large-scale cortical signals, NeuroImage (2013),
Objective: Data from a previous event-related potential (ERP) study in visual-perceptual grouping... more 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 (w2 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.
Phase patterns of human scalp alpha EEG activity show spontaneous transitions between different g... more 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.
Analyzing single trial brain activity remains a challenging problem in the neurosciences. We gain... more Analyzing single trial brain activity remains a challenging problem in the neurosciences. We gain purchase on this problem by focusing on globally synchronous fields in within-trial evoked brain activity, rather than on localized peaks in the trial-averaged evoked response (ER). We analyzed data from three measurement modalities, each with different spatial resolutions: magnetoencephalogram (MEG), electroencephalogram (EEG) and electrocorticogram (ECoG). We first characterized the ER in terms of summation of phase and amplitude components over trials. Both contributed to the ER, as expected, but the ER topography was dominated by the phase component. This means the observed topography of cross-trial phase will not necessarily reflect the phase topography within trials. To assess the organization of within-trial phase, traveling wave (TW) components were quantified by computing the phase gradient. TWs were intermittent but ubiquitous in the within-trial evoked brain activity. At most task-relevant times and frequencies, the within-trial phase topography was described better by a TW than by the trial-average of phase. The trial-average of the TW components also reproduced the topography of the ER; we suggest that the ER topography arises, in large part, as an average over TW behaviors. These findings were consistent across the three measurement modalities. We conclude that, while phase is critical to understanding the topography of event-related activity, the preliminary step of collating cortical signals across trials can obscure the TW components in brain activity and lead to an underestimation of the coherent motion of cortical fields.
We investigated visual working memory encoding across saccadic eye movements, focusing our analys... more We investigated visual working memory encoding across saccadic eye movements, focusing our analysis on refixation behavior. Over 10-s periods, participants performed a visual search for three, four, or five targets and remembered their orientations for a subsequent change-detection task. In 50% of the trials, one of the targets had its orientation changed. From the visual search period, we scored three types of refixations and applied measures for quantifying eye-fixation recurrence patterns. Repeated fixations on the same regions as well as repeated fixation patterns increased with memory load. Correct change detection was associated with more refixations on targets and less on distractors, with increased frequency of recurrence, and with longer intervals between refixations. The results are in accordance with the view that patterns of eye movement are an integral part of visual working memory representation.
Sense of agency can be defined as the self-awareness of bodily movement, whereas extended agency ... more Sense of agency can be defined as the self-awareness of bodily movement, whereas extended agency as the self-awareness of affecting, through movement, events concomitant with movement. As a distinctive manifestation of agency, we review Spizzo's effect. This effect arises when agents coordinate their rhythmic movements with visual pulses. Once coordination is established, agents feel controlling the onset or the offset of the pulses through their movements. Spizzo's effect, therefore, constitutes a manifest case of extended agency, in which agents are aware of controlling , through movement, the pulses temporally concomitant with movement. We propose that sense of agency requires continuity of kinesthesia, which in turn requires the continuity of self-hood. The continuity of kinesthesia is necessarily deduced from the consistent kinematics observed in movements, whereas the continuity of selfhood may stem from the patterns of rhythmic coordination that humans encounter ever since intrauterine life. The primacy of these patterns in adults is in accordance with phenomena such as Spizzo's effect, which require coordination to be induced. We, therefore, propose coordination as the fundamental interaction from which selfhood, kinesthesia, and agency arise.
When we synchronize finger tapping with a visual metronome, we experience a strikingly robust phe... more When we synchronize finger tapping with a visual metronome, we experience a strikingly robust phenomenon of extended agency known as Spizzo’s effect. This effect is the compelling sense that we are controlling the metronome. The effect arises even though the agent knows that the metronome operates autonomously. We propose that the extended agency here established over metronome pulses results from sensorimotor coordination. To test this hypothesis, we operationalize sensorimotor coordination in terms of the correlation structures in series of asynchronies or reaction times from two finger-tapping tasks. Analyses reveal that, whereas correlation structures vary across individuals and show a systematic drift towards nonstationarity with increasing metronome frequency conditions, the presence of correlation structure is co-extensive with Spizzo’s effect. We interpret this result as supporting the view that extended agency relies on sensorimotor coordination. Sensorimotor coordination, we suggest, may induce the effect by integrating the perception of visual pulses and the agency over tapping into a synesthetic experience.
Human connectome studies suggest that the brain has a modular small world network structure with ... more Human connectome studies suggest that the brain has a modular small world network structure with rich-club effect. Such structure emerges spontaneously in simple model neural networks, (e.g. coupled maps), through adaptive rewiring according to the dynamic functio na l connectivity. The utility of adaptive rewiring has so far exclusively been demonstrated for unweighted networks; it is anything but guaranteed to work as well for weighted networks. We investigate adaptive rewiring in weighted networks, comparing various right-skewed, symmetrical, and left-skewed fixed weight distributions. We examine how network clustering, path length, modularity, and rich club coefficients develop for weakly, intermediate and strongly coupled networks. At low coupling strength, the weight distribution, as well as episodes of functio na l synchrony, have a significant effect on network evolution. With increased coupling strengths, all weighted networks robustly develop architectures similar to the unweighted ones. Adaptive rewiring appears relative ly ineffective in networks with (biologically implausibly) extreme rightskewed weight distributions but performed most economically in biologically plausible log-normal distributions.
In order to consolidate the dissociation in feature integration found in earlier studies between ... more In order to consolidate the dissociation in feature integration found in earlier studies between letters of the Roman alphabet and corresponding pseudo-letters, the present study used kanji and kana and corresponding pseudo-letters, presented visually, either in isolation or surrounded by congruent or incongruent shape, as targets in a choice-response task with three different response criteria. When the criterion was shape, congruence effects were obtained for both real and pseudo-letters. With the second and third response criteria this result was found for pseudo-letters, but not for letters. These criteria either involved distinguishing between letters and visually similar pseudo-letters or distinguishing between visually similar letters. The dissociation of congruence effects between letters and pseudo-letters was therefore shown to depend on visual similarity between targets, independent of their category. This effect was found to be robust for kanji but not for kana, which is related to distinctions between these two writing systems.
A computational theory of consciousness should include a quantitative measure of consciousness, o... more A computational theory of consciousness should include a quantitative measure of consciousness, or MoC, that (i) would reveal to what extent a given system is conscious, (ii) would make it possible to compare not only different systems, but also the same system at different times, and (iii) would be graded, because so is consciousness. However, unless its design is properly constrained, such an MoC gives rise to what we call the boundary problem: an MoC that labels a system as conscious will do so for some—perhaps most—of its subsystems, as well as for irrelevantly extended systems (e.g., the original system augmented with physical appendages that contribute nothing to the properties supposedly supporting consciousness), and for aggregates of individually conscious systems (e.g., groups of people). This problem suggests that the properties that are being measured are epiphenomenal to consciousness, or else it implies a bizarre proliferation of minds. We propose that a solution to the boundary problem can be found by identifying properties that are intrinsic or systemic: properties that clearly differentiate between systems whose existence is a matter of fact, as opposed to those whose existence is a matter of interpretation (in the eye of the beholder). We argue that if a putative MoC can be shown to be systemic, this ipso facto resolves any associated boundary issues. As test cases, we analyze two recent theories of consciousness in light of our definitions: the Integrated Information Theory and the Geometric Theory of consciousness.
In the course of perceptual organization, incomplete optical stimulation can evoke the experience... more In the course of perceptual organization, incomplete optical stimulation can evoke the experience of complete objects with distinct perceptual identities. According to a well-known principle of perceptual organization, stimulus parts separated by shorter spatial distances are more likely to appear as parts of the same perceptual identity. Whereas this principle of proximity has been confirmed in many studies of perceptual grouping in static displays, we show that it does not generalize to perception of object identity in dynamic displays, where the parts are separated by spatial and temporal distances. We use ambiguous displays which contain multiple moving parts and which can be perceived two ways: as two large objects that gradually change their size or as multiple smaller objects that rotate independent of one another. Grouping over long and short distances corresponds to the perception of the respectively large and small objects. We find that grouping over long distances is often preferred to grouping over short distances, against predictions of the proximity principle. Even though these effects are observed at high luminance contrast, we show that they are consistent with results obtained at the threshold of luminance contrast, in agreement with predictions of a theory of efficient motion measurement. This is evidence that the perception of object identity can be explained by a computational principle of neural economy rather than by the empirical principle of proximity.
Culture-related differences in visual creativity were investigated, comparing Italian and Japanes... more Culture-related differences in visual creativity were investigated, comparing Italian and Japanese participants in terms of divergent (figural completion task) and product-oriented thinking (figural combination task). Visual restructuring ability was measured as the ability to reinterpret ambiguous figures and was included as a covariate. Results showed that in divergent thinking, Italians only outperformed Japanese participants in elaboration and in product-oriented thinking in terms of originality of products. Visual restructuring ability was found to play a key role both in originality and practicality of products. Both groups scored the same in terms of fluency, originality, and flexibility of visual divergent thinking, as well as in term of practicality of creative products. These findings are consistent with the idea that Italians and Japanese have the same creative potential, although from the early stages of the design Japanese seem to show a greater tendency to take practicality constraints into account when creating in the visual domain.
Stroop interference is deterioration of a behavioral response to a target feature resulting from ... more Stroop interference is deterioration of a behavioral response to a target feature resulting from an effect of incongruent but irrelevant other feature. Garner interference is an effect of variation of an irrelevant feature across trials. van Leeuwen & Bakker (1995) predicted that Stroop ...
ABSTRACT We tested the memory characteristics of visual search in an event-related potentials stu... more ABSTRACT We tested the memory characteristics of visual search in an event-related potentials study. Participants searched for a target letter presented among nontarget letters. We varied target identity (switch to new vs swap with nontarget from previous trial), nontarget identity (switch to new vs swap with target from previous trial), target location (repeat, switch, swap with nontarget), and nontarget location (repeat, switch, swap with target). Repeated target locations were responded to faster and more accurately than switched ones. There was a slowdown in performance when nontarget and target swapped their locations. The amplitude in the N2pc component was smaller when target location switched than repeated. The amplitude in the N1 component was highest when target and nontarget swapped their roles and target location switched; and smallest when both target and nontarget switched to new identity and target location repeated. These data show that visual search has memory for both target location and identity, but also nontarget identity and location play a role in processing.
Spatiotemporal characteristics of spontaneous alpha EEG activity patterns are analyzed in terms o... more 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.
Studies in Perception and Action: Fourteenth International Conference on Perception and Action, Jul 9, 2007
Method We adopted an approach developed by Ziessler and his colleagues based on Eriksen's fl... more Method We adopted an approach developed by Ziessler and his colleagues based on Eriksen's flanker paradigm (Ziessler & Nattkemper, 2002; Ziessler & Nattkemper, 2003; Ziessler et al., 2004). Eriksen and Eriksen (1974) showed that letters flanking a target letter on both sides were processed together with the target letter and activated their own responses. Responses to the target were facilitated or inhibited depending on the compatibility between the response to the target and the response activated by the ...
Internal psychophysics is concerned with lawful mind-brain relationships. Too few of these have b... more Internal psychophysics is concerned with lawful mind-brain relationships. Too few of these have been discovered so far. All we have been able to observe are correlations; for instance between a mental event and an electrophysiological signal. Is this because we are looking at ...
van, Traveling waves and trial averaging: the nature of single-trial and averaged brain responses... more van, Traveling waves and trial averaging: the nature of single-trial and averaged brain responses in large-scale cortical signals, NeuroImage (2013),
Objective: Data from a previous event-related potential (ERP) study in visual-perceptual grouping... more 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 (w2 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.
Phase patterns of human scalp alpha EEG activity show spontaneous transitions between different g... more 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.
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Papers by Cees van Leeuwen