Tomohisa Asai’s research while affiliated with Advanced Telecommunications Research Institute International and other places

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Publications (42)


Immediate Modulation of the Blood Oxygenation Level-Dependent Signals by Dual-Site Transcranial Alternating Current Stimulation Propagates Across the Whole Brain
  • Preprint

September 2024

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16 Reads

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Tomohisa Asai

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Hiroshi Kadota

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Transcranial alternating current stimulation (tACS) is assumed to target specific brain regions and modulate their activity. Recent discussions of tACS propose that, entraining the phase of brain activity to the stimulation current, stimulation effects extend globally across the whole brain based on phase differences. However, immediate online spatiotemporal propagation of resting-state blood oxygenation level-dependent (BOLD) signals within the brain due to multi-region stimulation remains unclear. The objectives of the present study were three-fold: 1) to elucidate the immediate online effect of tACS on BOLD signal, 2) to examine the extent of the influence on the brain when applying tACS, and 3) to explore whether variations in the phase difference between two brain regions result in differential effects on the stimulated areas and the whole brain. Through two experiments involving high-definition tACS with simultaneous measurements using a functional magnetic resonance imaging (fMRI), we revealed that the immediate online stimulation effects not only altered BOLD signals in the stimulated regions but also propagated across the whole brain in specific spatiotemporal patterns (functional networks). Stimulation effects were observed specifically in regions rich in neural fibres, including the grey and white matter, with no effect in regions containing cerebrospinal fluid. The timing of the signal value peaks depended on the stimulated region and functional networks, with a notable trend observed. Thus, tACS with a specific phase difference in two anatomically connected brain regions can immediately modulate online neural dynamics at both local and global scales. Graphical abstract


Figure 3. Traditional view of microstate transitions-transition probability matrices and 1397 directed graphs of microstate transition pairs ignoring topographical polarity in Study 1398 1. 1399 Each transition pair's average probability of occurrences was shown by placing the "from" 1400 states in the row direction and the "to" states in the column direction. Twenty transition pairs 1401 were included, except for five self-recurrences on the diagonal. In each directed graph, the 1402 nodes were the respective template maps, and the edge color indicated the transition 1403 probabilities between nodes or the magnitude of the differences between age groups. The 1404 edges of the directed graphs in (a) and (b) were only displayed if the transition probability was 1405 greater than 0.05 (= the chance level probability defined as random transitions between all 1406 states). Black squares in each transition matrix indicated transitions with valid edges in the 1407 directed graph. The distribution of nodes was defined based on the spatial correlations 1408 between templates and determined based on the dissimilarity distances compressed by 1409 classical MDS (Asai et al., 2023; Koenig et al., 2024). (a) Average transition probabilities for 1410 the older and (b) younger groups. (c) Differences in the average transition probabilities for 1411 each transition pair for older and younger groups. Red indicates more frequent transitions in 1412 the older group, and blue indicates less frequent transitions. (d) Standardized mean 1413 differences for comparisons between age groups (Hedge's g; Zanesco et al. (2020)). The 1414 symbol of the effect size g and red-blue colors were used to indicate the direction of the 1415 difference, with red indicating more frequent transitions and blue indicating less frequent 1416 transitions in the older group. 1417
Figure 5. Examples of microstate transition matrices and directed graphs for different 1440 node definitions. 1441 (a) Example with 14 template maps defined as nodes. (b) Addition of four template maps, 1442 including midpoints between A-B and D-E, enabling the arrangement of template maps to 1443 evenly divide each of the A-C-B and D-C-E axes. (c) Mean transition probabilities for each of 1444 the older and (d) younger groups, (e) differences between the two groups, and (f) standardized 1445 mean differences for comparisons between age groups. Details of (c)-(f) are the same as in 1446 Figures 4a-d. 1447
Figure 7. Inter-participant correlation matrix between the number of transitions between 1458 EEG microstate templates in Study 1. 1459 We divided the EEG topographical dynamics into 100 distinct transitions by applying ten 1460 polarity-sensitive EEG microstate templates. Correlations between transition pairs were then 1461 calculated based on the number of transitions counted from each participant's EEG data. The 1462 rows and columns were sorted according to the results of Varimax-rotated PCA, in order of 1463 absolute loading values, for each of the eight components formed by collecting the items with 1464 the highest (most contributing) absolute loading values. Dotted delimiters indicated the 1465 boundaries of the components. Each of the eight blocks of diagonal components surrounded 1466 by dotted lines corresponded to eight transition components found in the rotated PCA. For 1467 simplicity in the axis labels in the figure, positive polarity templates were denoted by uppercase 1468 letters (A, B, C, D, E) and negative polarity templates by lowercase letters (a, b, c, d, e), in 1469 "transition from" state/"transition to" state order (e.g., "Ab" label meant the transition from A+ 1470 to B-). 1471
Figure 10. Updated view of microstate transitions-transition probability matrices and 1495 directed graphs of microstate transition pairs when considering topographical polarity 1496 in Study 2. 1497 Each transition pair's average probability of occurrences was shown by placing the "from" 1498 states in the row direction and the "to" states in the column direction. Ninety transition pairs 1499 were included, except for ten self-recurrences on the diagonal. In each directed graph, the 1500 nodes were the respective template maps, and the edge color indicated the transition 1501 probabilities between nodes or the magnitude of the differences between age groups. The 1502 edges of the directed graphs were only displayed if the transition probability was greater than 1503 0.011 in (a) and (b) (≒ the chance level probability defined as random transitions between all 1504 states) and greater than 0.0011 in (c) for visualization purposes. (a) Average transition 1505 probabilities for the older and (b) younger groups. (c) Differences in the average transition 1506 probabilities for each transition pair for older and younger groups. Red indicates more frequent 1507 transitions in the older group, and blue indicates less frequent transitions. (d) Standardized 1508 mean differences for comparisons between age groups (Hedge's g; Zanesco et al. (2020)). 1509 The symbol of the effect size g and red-blue colors were used to indicate the direction of the 1510 difference, with red indicating more frequent transitions and blue indicating less frequent 1511 transitions in the older group. 1512
Topographical polarity reveals continuous EEG microstate transitions and electric field direction in healthy aging
  • Preprint
  • File available

March 2024

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51 Reads

EEG microstate sequences, representing whole-brain spatial potential distribution patterns of the EEG, offer valuable insights for capturing spatiotemporally continuous and fluctuating neural dynamics with high temporal resolution through appropriate discretization. Recent studies suggest that EEG microstate transitions are gradual and continuous phenomena, contrary to the classical view of binary transitions. This study aimed to update conventional microstate analysis to reflect continuous EEG dynamics and examine differences in age-related electrophysiological state transitions. We considered the relative positions of EEG microstates on the neural manifold and their topographical polarity. Transition probability results showed fewer transitions on the microstate D-C-E axis in older adults. In contrast, transitions among microstates A, D, and B increased in the older group and were mainly observed within polarity. Furthermore, the 100 microstate transitions, which are variations of the shortest transitions between 10 microstates, could be reduced to 8 principal components based on the co-occurrence of each transition, including hubs C and E, planar transitions through msA/B and D, and unidirectional transition components. Several transition components were potentially significant predictors of age group. These features were nearly replicated in independent data, indicating their robustness in characterizing age-related electrophysiological spatiotemporal dynamics.

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Generalized Internal Model of Mental Representations: Thought Insertion, Mental Agency, and the Cerebellum

October 2023

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11 Reads

Delusions of Thought Insertion (TI) are considered as one of the most complex psychotic experiences. More prevalent in schizophrenia, TI involves subjects reporting that entities of different nature have introduced thoughts or ideas into their minds. The particularity of this phenomenon is not that patients have been just caused to have certain unusual thoughts (in fact, this can happen all the time in everyday situations), rather, that certain thoughts are experienced as not being the patients’ but created and inserted by an external agent into their personal space. As such, TI confronts us with an enigmatic type of conscious experience where the boundaries between external and internal reality get blurred. In light of its unique features, over the last 20 years both clinicians and philosophers have found in TI a formidable challenge for the way in which consciousness, self–awareness, agency, the nature of beliefs, and a number of other critical concepts have been characterized in philosophy and cognitive sciences. Intruders in the Mind is an interdisciplinary attempt to bring together high–quality contributions to some of the most fundamental debates arising from the comprehensive study of TI. Making TI its central topic, this compilation gathers a series of essays that, taken as a whole, offer a broad and thoughtful approach to the clinical, phenomenological, conceptual, and experimental aspects of the systematic study of the phenomenon. The compilation is divided into 4 sections, each of them focusing on a specific set of interrelated debates.


Spatio-temporal "global" neurodynamics in both continuous and discrete pictures for human brains

July 2023

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28 Reads

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1 Citation

The neural manifold in state space represents the mass neural dynamics of a biological system. A challenging modern approach treats the brain as a whole in terms of the interaction between the agent and the world. Therefore, we need to develop a method for this global neural workspace. The current study aimed to visualize spontaneous neural trajectories regardless of their measuring modalities (electroencephalography [EEG], functional magnetic resonance imaging [fMRI], and magnetoencephalography [MEG]). First, we examined the possible visualization of EEG manifolds. These results suggest that a spherical surface can be clearly observed within the spatial similarity space. Once valid (e.g., differentiable) and useful (e.g., low-dimensional) manifolds are obtained, the nature of the sphere, such as shape and size, becomes a possible target of interest. Because these should be practically useful, we suggest advantages of the EEG manifold (essentially continuous) or the state transition matrix (coarse-grained discrete). Finally, because our basic procedure is modality-independent, MEG and fMRI manifolds were also compared. These results strongly suggest the need to update our understanding of neural mass representations to include robust "global" dynamics.



Event-related microstate dynamics represents working memory performance

October 2022

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384 Reads

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9 Citations

NeuroImage

In recent years, EEG microstate analysis has attracted much attention as a tool for characterizing the spatial and temporal dynamics of large-scale electrophysiological activities in the human brain. Canonical 4 states (classes A, B, C, and D) have been widely reported, and they have been pointed out for their relationships with cognitive functions and several psychiatric disorders such as schizophrenia, in particular, through their static parameters such as average duration, occurrence, coverage, and transition probability. However, the relationships between event-related microstate changes and their related cognitive functions, as is often analyzed in event-related potentials under time-locked frameworks, is still not well understood. Furthermore, not enough attention has been paid to the relationship between microstate dynamics and static characteristics. To clarify the relationships between the static microstate parameters and dynamic microstate changes, and between the dynamics and working memory (WM) function, we first examined the temporal profiles of the microstates during the N-back task. We found significant event-related microstate dynamics that differed predominantly with WM loads, which were not clearly observed in the static parameters. Furthermore, in the 2-back condition, patterns of state transitions from class A to C in the high- and low-performance groups showed prominent differences at 50–300 ms after stimulus onset. We also confirmed that the transition patterns of the specific time periods were able to predict the performance level (low or high) in the 2-back condition at a significant level, where a specific transition between microstates, namely from class A to C with specific polarity, contributed to the prediction robustly. Taken together, our findings indicate that event-related microstate dynamics at 50–300 ms after onset may be essential for WM function. This suggests that event-related microstate dynamics can reflect more highly-refined brain functions.


I Hear My Voice; Therefore I Spoke: The Sense of Agency Over Speech Is Enhanced by Hearing One’s Own Voice

July 2022

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31 Reads

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7 Citations

Psychological Science

The subjective experience of causing an action is known as the sense of agency. Dysfunctional sense of agency over speech has been suggested as a cause of auditory hallucinations. However, agency over speech has not been extensively characterized in previous empirical studies. Here, we manipulated self-voice identity, an indicator of self, embedded in the acoustic quality of voice and examined implicit and explicit measures of the sense of agency. Experiment 1 ( N = 29 adults) demonstrated more compression of a perceived interval between action and outcome when participants heard their undistorted voices than their pitch-distorted voices. Experiment 2 ( N = 28 adults) revealed a strong top-down effect of self-voice identity: Specifically, the judgment of agency over speech was more resistant than the judgment of agency over hand/limb movement to low-level sensorimotor inconsistency. Our findings provide new insight into the sense of agency over speech and an informative perspective for understanding aberrant experience in auditory hallucinations.


Biased self-other attribution changes feedback control: link between the sense of agency and sensorimotor control

April 2022

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31 Reads

Agents must correctly attribute the source of sensory inputs to the self or other for successful interaction with the environment. Many previous research works have revealed that self-other attribution depends on the discrepancy between sensory prediction and actual sensory feedback (sensory prediction error). Self-other attribution is assumed to determine how agents reflect sensory feedback in their feedback control. However, the direct effect of self-other attribution on feedback control is still unclear. In the current study, participants’ self-other attribution was biased through induction training in the selfward or otherward direction at a cognitive-judgment level. Before and after the induction, participants controlled a morphed cursor between self and other movements and evaluated the self-other attribution of the cursor movement. We first confirmed that the induction biased their self-other attributions toward the intended direction (i.e., selfward or otherward). Next, we examined whether the respective bias changed the feedback control of the cursor. For the self-biased group, the more participants attributed their visual feedback to themselves, the more they tried to control the morphed cursor in a specific condition. In contrast, for the other-biased group, the more participants attributed their visual feedback to others, the less they tried to control the morphed visual feedback in a specific condition. These results suggest that self-other attribution is not simply a cognitive process reflecting the sensory feedback of motor control but also a factor affecting motor control in a top-down manner.


Real-Time Detection and Feedback of Canonical Electroencephalogram Microstates: Validating a Neurofeedback System as a Function of Delay

February 2022

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98 Reads

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6 Citations

Frontiers in Systems Neuroscience

Recent neurotechnology has developed various methods for neurofeedback (NF), in which participants observe their own neural activity to be regulated in an ideal direction. EEG-microstates (EEGms) are spatially featured states that can be regulated through NF training, given that they have recently been indicated as biomarkers for some disorders. The current study was conducted to develop an EEG-NF system for detecting “canonical 4 EEGms” in real time. There are four representative EEG states, regardless of the number of channels, preprocessing procedures, or participants. Accordingly, our 10 Hz NF system was implemented to detect them (msA, B, C, and D) and audio-visually inform participants of its detection. To validate the real-time effect of this system on participants’ performance, the NF was intentionally delayed for participants to prevent their cognitive control in learning. Our results suggest that the feedback effect was observed only under the no-delay condition. The number of Hits increased significantly from the baseline period and increased from the 1- or 20-s delay conditions. In addition, when the Hits were compared among the msABCD, each cognitive or perceptual function could be characterized, though the correspondence between each microstate and psychological ability might not be that simple. For example, msD should be generally task-positive and less affected by the inserted delay, whereas msC is more delay-sensitive. In this study, we developed and validated a new EEGms-NF system as a function of delay. Although the participants were naive to the inserted delay, the real-time NF successfully increased their Hit performance, even within a single-day experiment, although target specificity remains unclear. Future research should examine long-term training effects using this NF system.



Citations (25)


... While this interpretation needs to be quantitatively validated, it raises the possibility that tACS not only induces modulation of activity in localised regions but also leads to changes of specific functional patterns (i.e., functional networks, brain states, and neural manifolds). In recent years, an approach has been reported that shifts the focus from local brain activity to global spatiotemporal dynamics, capturing the essence of the brain by reframing overall brain activity as transitions in the state space (Asai et al., 2023). In this sense, future tES research should aim to clarify both the local effects in stimulated regions and the subsequent effects that propagate to other regions. ...

Reference:

Immediate Modulation of the Blood Oxygenation Level-Dependent Signals by Dual-Site Transcranial Alternating Current Stimulation Propagates Across the Whole Brain
Spatio-temporal "global" neurodynamics in both continuous and discrete pictures for human brains
  • Citing Preprint
  • July 2023

... The temporal dynamics of these changes are particularly significant in pDoC (Panda et al., 2022;Panda et al., 2016). In recent years, numerous studies have reported that microstates may be associated with various psychological states (Baradits et al., 2020;Tamano et al., 2022;Bochet et al., 2021;Zanesco et al., 2020), providing insight into neural activity of the brain during resting-state. As a "quantitative indicator" of the distribution pattern of brain topographic maps, microstate analysis divides resting-state EEG signals into a limited number of distinct quasi-stable states (Khanna et al., 2015;Pascual-Marqui et al., 1995). ...

Event-related microstate dynamics represents working memory performance
  • Citing Article
  • October 2022

NeuroImage

... Does having such a belief engender a stronger sense of agency? A recent study investigating the use of self-voice suggests this to be the case, showing that the intentional binding effect-an implicit measure of the sense of agency-was more pronounced for self-voice than for other-voice 16 . However, this phenomenon may be attributed to the relatively low actual level of control and the possibility that self-identification influenced the judgment criterion for the sense of agency in ambiguous conditions. ...

I Hear My Voice; Therefore I Spoke: The Sense of Agency Over Speech Is Enhanced by Hearing One’s Own Voice
  • Citing Article
  • July 2022

Psychological Science

... Other EFFECTS OF ONE SESSION OF THETA OR HIGH ALPHA NEUROFEEDBACK 31 studies have applied micro-state analysis, which identifies short, stable, distinct patterns of electrical activity measured by EEG across the scalp (Férat et al., 2022;Khanna et al., 2015;Michel & Koenig, 2018), representing various cognitive processes (Tarailis et al., 2024). Currently, few studies have attempted to modulate these neural states via neurofeedback (Asai et al., 2022;Diaz Hernandez et al., 2016), and it is necessary to assess larger samples in well-controlled protocols to highlight the potential of these methods compared to the use of conventional brain frequencies. ...

Real-Time Detection and Feedback of Canonical Electroencephalogram Microstates: Validating a Neurofeedback System as a Function of Delay

Frontiers in Systems Neuroscience

... In previous studies, an insightful error was discovered when mark tests were applied using mirrors or video images and some toddler participants tried to locate the mark behind their head while it was placed on their forehead. We named this curious initial search error the "rear-search error" 6,7 . In a mirror mark test, 38% of 2-year-olds and 11% of 3-yearolds demonstrated the rear-search error initially 6 . ...

Cross-referenced body and action for the unified self: Empirical, developmental, and clinical perspectives
  • Citing Chapter
  • July 2021

... A previous study using event-related potentials showed that delayed selfvoice feedback elicited significantly larger neural responses than pitch-shifted self-voice feedback, indicating that the neural processing of auditory self-voice feedback is highly sensitive to its temporal features 29 . A recent preprint investigated the time perception of non-altered and acoustic distorted voice feedback, showing that the perceived interval between a person's speech and non-altered voice feedback was significantly shorter than that between the former and acoustic distorted voice feedback 30 . This phenomenon was explained by a strong sense of agency, which is considered to compress the time perception between voluntary actions and sensory feedback 31 for non-altered self-voice 30 . ...

My voice therefore I spoke: sense of agency over speech enhanced in hearing self-voice
  • Citing Preprint
  • November 2020

... However, the SoA can be reduced by non-self-initiated interventions because the outcomes of the actions of an individual influenced by others differ from those predicted by internal models. In other words, autonomous transition control or shared control in which robots assist human operations can reduce the SoA [23], [24], [25], [26]. Therefore, methods that improve performance and reflect the intentions of operators in the perceived outcomes of their actions, even under time-varying delays, are needed. ...

Sense of Agency Beyond Sensorimotor Process: Decoding Self-Other Action Attribution in the Human Brain

Cerebral Cortex

... With regard to face image development, a study investigated the spatial location of different parts of one's face in children with 2.5 or 3.5 years of age by using augmented reality and 3D face-building technology (Miyazaki et al. 2019). Participants had to locate a sign on their body that they saw in the real-time video in front of them and received a "cheerful" visual and auditory reward in case of correct detection. ...

Touching! An Augmented Reality System for Unveiling Face Topography in Very Young Children

... Moreover, it is essential to examine SoA for generating an event (e.g., acceleration of an object) as well as SoA for terminating an event (e.g., deceleration of the object) in the case of automatic driving systems. A recent study reported that explicit SoA for generating an event is stronger than SoA for terminating the event (Asai et al., 2019). Therefore, it is important to examine SoA other control types to understand SoA in automatic control fully. ...

The self as a generative, teleological, and subjective prior: Mutually-modulated temporal agency
  • Citing Preprint
  • January 2019

... When caring for the needs of people with incurable disease, the goal is to support dignity and meaning in life and death (10). As brain lesions have been associated with an altered sense of self (12)(13)(14), it is especially important for patients with brain tumors to be understood by those who care for them, not just as a patient but as a person. This can help to support a patients' sense of dignity and meaning. ...

Measuring the sense of self in brain-damaged patients: A STROBE-compliant article
  • Citing Article
  • September 2018

Medicine