Visuo-spatial working memory abilities modulate mental rotation: Evidence from event-related potentials

In the present study, we investigated whether differences in spatial working memory (SWM) abilities - assessed through the Corsi block task (CBT) - impact the processes of mental rotation (MR) engaged during a classic letter rotation task. Based on the median split of their scores in the CBT, participants were divided into a higher and a lower SWM group. Behavioral and electrophysiological data were recorded while participants completed the MR task and were compared across groups. Higher error rates were observed in individuals with lower than higher SWM scores, while no RT differences emerged. Systematic group differences were observed before and during the MR process of canonical letters. A delayed onset of the event-related potential (ERP) rotation-related negativity (RRN), a reliable psychophysiological marker for MR processes, was observed in the lower SWM group for all rotation angles, suggesting that a longer time is needed to generate a mental representation of familiar stimuli in individuals with lower SWM scores. Furthermore, a delayed RRN offset indicating the end of the MR process and longer RRN durations suggesting longer MR processes were found for letters with larger rotation angles (i.e. 120°, 150°) in individuals with lower SWM scores on canonical character trials. These observed group differences provided evidence for the debated issue of the interaction between SWM and MR, suggesting that SWM plays a role in both the initial phase to generate the mental representation of familiar objects and during the MR process, especially for larger angles.

The neuro-oscillatory profiles of static and dynamic music-induced visual imagery

Visual imagery, i.e., seeing in the absence of the corresponding retinal input, has been linked to visual and motor processing areas of the brain. Music listening provides an ideal vehicle for exploring the neural correlates of visual imagery because it has been shown to reliably induce a broad variety of content, ranging from abstract shapes to dynamic scenes. Forty-two participants listened with closed eyes to twenty-four excerpts of music, while a 15-channel EEG was recorded, and, after each excerpt, rated the extent to which they experienced static and dynamic visual imagery. Our results show both static and dynamic imagery to be associated with posterior alpha suppression (especially in lower alpha) early in the onset of music listening, while static imagery was associated with an additional alpha enhancement later in the listening experience. With regard to the beta band, our results demonstrate beta enhancement to static imagery, but first beta suppression before enhancement in response to dynamic imagery. We also observed a positive association, early in the listening experience, between gamma power and dynamic imagery ratings that was not present for static imagery ratings. Finally, we offer evidence that musical training may selectively drive effects found with respect to static and dynamic imagery and alpha, beta, and gamma band oscillations. Taken together, our results show the promise of using music listening as an effective stimulus for examining the neural correlates of visual imagery and its contents. Our study also highlights the relevance of future work seeking to study the temporal dynamics of music-induced visual imagery.

Predicting the subjective intensity of imagined experiences from electrophysiological measures of oscillatory brain activity

Most people can conjure images and sounds that they experience in their minds. There are, however, marked individual differences. Some people report that they cannot generate imagined sensory experiences at all (aphantasics) and others report that they have unusually intense imagined experiences (hyper-phantasics). These individual differences have been linked to activity in sensory brain regions, driven by feedback. We would therefore expect imagined experiences to be associated with specific frequencies of oscillatory brain activity, as these can be a hallmark of neural interactions within and across regions of the brain. Replicating a number of other studies, relative to a Resting-State we find that the act of engaging in auditory or in visual imagery is linked to reductions in the power of oscillatory brain activity across a broad range of frequencies, with prominent peaks in the alpha band (8-12 Hz). This oscillatory activity, however, did not predict individual differences in the subjective intensity of imagined experiences. For audio imagery, these were rather predicted by reductions within the theta (6-9 Hz) and gamma (33-38 Hz) bands, and by increases in beta (15-17 Hz) band activity. For visual imagery these were predicted by reductions in lower (14-16 Hz) and upper (29-32 Hz) beta band activity, and by an increase in mid-beta band (24-26 Hz) activity. Our data suggest that there is sufficient ground truth in the subjective reports people use to describe the intensity of their imagined sensory experiences to allow these to be linked to the power of distinct rhythms of brain activity. In future, we hope to combine this approach with better measures of the subjective intensity of imagined sensory experiences to provide a clearer picture of individual differences in the subjective intensity of imagined experiences, and of why these eventuate.

Mental rotation ability and spontaneous brain activity: a magnetoencephalography study

We performed experiments using magnetoencephalography to clarify the relationship between three-dimensional visuospatial abilities and spontaneous visual thinking characteristics. Subjects were divided into two groups based on the rate of correct answers to mental rotation tasks: those with good performances (Group G) and those with bad performances (Group B). We found the followings: (1) in the mental rotation tasks, the 25-35 Hz lower γ band activities in the superior parietal lobule/intraparietal sulcus regions and in the occipitotemporal region were significantly larger in Group G than in Group B and (2) in the spontaneous mental imagery tasks, the 20-Hz band activity in the left premotor cortex and the 35-Hz band activity in the supplementary motor area were significantly larger in Group G.

Visual Imagery and Perception Share Neural Representations in the Alpha Frequency Band

To behave adaptively with sufficient flexibility, biological organisms must cognize beyond immediate reaction to a physically present stimulus. For this, humans use visual mental imagery [1, 2], the ability to conjure up a vivid internal experience from memory that stands in for the percept of the stimulus. Visually imagined contents subjectively mimic perceived contents, suggesting that imagery and perception share common neural mechanisms. Using multivariate pattern analysis on human electroencephalography (EEG) data, we compared the oscillatory time courses of mental imagery and perception of objects. We found that representations shared between imagery and perception emerged specifically in the alpha frequency band. These representations were present in posterior, but not anterior, electrodes, suggesting an origin in parieto-occipital cortex. Comparison of the shared representations to computational models using representational similarity analysis revealed a relationship to later layers of deep neural networks trained on object representations, but not auditory or semantic models, suggesting representations of complex visual features as the basis of commonality. Together, our results identify and characterize alpha oscillations as a cortical signature of representations shared between visual mental imagery and perception.

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Perception and imagery share neural representations in the alpha frequency band

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Shared representations stem from parieto-occipital sources

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Modeling suggests contents of shared representations are complex visual features

Modalities of Thinking: State and Trait Effects on Cross-Frequency Functional Independent Brain Networks

Functional states of the brain are constituted by the temporally attuned activity of spatially distributed neural networks. Such networks can be identified by independent component analysis (ICA) applied to frequency-dependent source-localized EEG data. This methodology allows the identification of networks at high temporal resolution in frequency bands of established location-specific physiological functions. EEG measurements are sensitive to neural activity changes in cortical areas of modality-specific processing. We tested effects of modality-specific processing on functional brain networks. Phasic modality-specific processing was induced via tasks (state effects) and tonic processing was assessed via modality-specific person parameters (trait effects). Modality-specific person parameters and 64-channel EEG were obtained from 70 male, right-handed students. Person parameters were obtained using cognitive style questionnaires, cognitive tests, and thinking modality self-reports. EEG was recorded during four conditions: spatial visualization, object visualization, verbalization, and resting. Twelve cross-frequency networks were extracted from source-localized EEG across six frequency bands using ICA. RMANOVAs, Pearson correlations, and path modelling examined effects of tasks and person parameters on networks. Results identified distinct state- and trait-dependent functional networks. State-dependent networks were characterized by decreased, trait-dependent networks by increased alpha activity in sub-regions of modality-specific pathways. Pathways of competing modalities showed opposing alpha changes. State- and trait-dependent alpha were associated with inhibitory and automated processing, respectively. Antagonistic alpha modulations in areas of competing modalities likely prevent intruding effects of modality-irrelevant processing. Considerable research suggested alpha modulations related to modality-specific states and traits. This study identified the distinct electrophysiological cortical frequency-dependent networks within which they operate.

Brain activities of visual thinkers and verbal thinkers: A MEG study

In this study we measured activation patterns in the primary visual cortex and the frontal language areas and compared them in individuals with strong and weak capacities to mentally visualize information during spontaneous thinking. Subjects were first administered a 5-item questionnaire to assess their ability to create mental pictures, and were divided into two groups (strong and weak visualizers) on this basis. They then performed tasks requiring visual imagery and verbal recollection, and their local neural activities were measured, using magnetoencephalography (MEG). Notably in the high beta-band (25Hz), the visual area (BA 17) was more strongly activated in strong visualizers, whereas, the frontal language areas were more strongly activated in weak visualizers. Strong visualizers are considered to be visual thinkers, and weak visualizers are verbal thinkers.

Spatial attention boosts short-latency neural responses in human visual cortex

In a previous study of visual-spatial attention, Martinez et al. (2007) replicated the well-known finding that stimuli at attended locations elicit enlarged early components in the averaged event-related potential (ERP), which were localized to extrastriate visual cortex. The mechanisms that underlie these attention-related ERP modulations in the latency range of 80-200 ms, however, remain unclear. The main question is whether attention produces increased ERP amplitudes in time-domain averages by augmenting stimulus-triggered neural activity, or alternatively, by increasing the phase-locking of ongoing EEG oscillations to the attended stimuli. We compared these alternative mechanisms using Morlet wavelet decompositions of event-related EEG changes. By analyzing single-trial spectral amplitudes in the theta (4-8 Hz) and alpha (8-12 Hz) bands, which were the dominant frequencies of the early ERP components, it was found that stimuli at attended locations elicited enhanced neural responses in the theta band in the P1 (88-120 ms) and N1 (148-184 ms) latency ranges that were additive with the ongoing EEG. In the alpha band there was evidence for both increased additive neural activity and increased phase-synchronization of the EEG following attended stimuli, but systematic correlations between pre- and post-stimulus alpha activity were more consistent with an additive mechanism. These findings provide the strongest evidence to date in humans that short-latency neural activity elicited by stimuli within the spotlight of spatial attention is boosted or amplified at early stages of processing in extrastriate visual cortex.

A comparison of methods for assessing alpha phase resetting in electrophysiology, with application to intracerebral EEG in visual areas

There are two competing views on the mechanisms underlying the generation of visual evoked potentials/fields in EEG/MEG. The classical hypothesis assumes an additive wave on top of background noise. Another hypothesis states that the evoked activity can totally or partially arise from a phase resetting of the ongoing alpha rhythm. There is no consensus however, on the best tools for distinguishing between these two hypotheses. In this study, we have tested different measures on a large series of simulations under a variety of scenarios, involving in particular trial-to-trial variability and different dynamics of ongoing alpha rhythm. No single measure or set of measures was found to be necessary or sufficient for defining phase resetting in the context of our simulations. Still, simulations permitted to define criteria that were the most reliable in practice for distinguishing additive and phase resetting hypotheses. We have then applied these criteria on intracerebral EEG data recordings in the visual areas during a visual discrimination task. We investigated the intracerebral channels that presented both ERP and ongoing alpha oscillations (n=37). Within these channels, a total of 30% fulfilled phase resetting criteria during the generation of the visual evoked potential, based on criteria derived from simulations. Moreover, 19% of the 37 channels presented dependence of the ERP on the level of pre-stimulus alpha. Only 5% of channels fulfilled both the simulation-related criteria and dependence on baseline alpha level. Our simulation study points out to the difficulty of clearly assessing phase resetting based on observed macroscopic electrophysiological signals. Still, some channels presented an indication of phase resetting in the context of our simulations. This needs to be confirmed by further work, in particular at a smaller recording scale.

The transliminal brain at rest: baseline EEG, unusual experiences, and access to unconscious mental activity

Transliminality reflects individual differences in the threshold at which unconscious processes or external stimuli enter into consciousness. Individuals high in transliminality possess characteristics such as magical ideation, belief in the paranormal, and creative personality traits, and also report the occurrence of manic/mystic experiences. The goal of the present research was to determine if resting brain activity differs for individuals high versus low in transliminality. We compared baseline EEG recordings (eyes-closed) between individuals high versus low in transliminality, assessed using The Revised Transliminality Scale of Lange et al. (2000). Identifying reliable differences at rest between high- and low-transliminality individuals would support a predisposition for transliminality-related traits. Individuals high in transliminality exhibited lower alpha, beta, and gamma power than individuals low in transliminality over left posterior association cortex and lower high alpha, low beta, and gamma power over the right superior temporal region. In contrast, when compared to individuals low in transliminality, individuals high in transliminality exhibited greater gamma power over the frontal-midline region. These results are consistent with prior research reporting reductions in left temporal/parietal activity, as well as the desynchronization of right temporal activity in schizotypy and related schizophrenia spectrum disorders. Further, differences between high- and low-transliminality groups extend existing theories linking altered hemispheric asymmetries in brain activity to a predisposition toward schizophrenia, paranormal beliefs, and unusual experiences.

Alpha reduction and event-related potentials, theta and gamma increase linked to letter selection

We examined evoked and induced modulations in theta, alpha and gamma oscillations, and also the P2 and late positive component of event-related potentials, during a visual discrimination task with target and nontarget letters. Results for target letters showed a decrease in the amplitude of alpha-band (10-11 Hz) activity and an increase in theta (4-7 Hz) and gamma (40-44 Hz) activities around 350 ms after stimulation. P2 and late positive component presented a higher amplitude to target than to repeated nontarget letters. Alpha reduction was inversely related to theta and late positive component increase. Moreover, gamma oscillation amplitude was directly related to theta amplitude. The findings suggest the importance of occipitoparietal alpha reduction for the development of task-related neuronal activity.

EEG alpha oscillations: the inhibition-timing hypothesis

The traditional belief is that the event-related alpha response can solely be described in terms of suppression or event-related desynchronization (ERD). Recent research, however, has shown that under certain conditions alpha responds reliably with an increase in amplitudes (event-related synchronization or ERS). ERS is elicited in situations, where subjects withhold or control the execution of a response and is obtained over sites that probably are under, or exert top-down control. Thus, we assume that alpha ERS reflects top-down, inhibitory control processes. This assumption leads over to the timing aspect of our hypothesis. By the very nature of an oscillation, rhythmic amplitude changes reflect rhythmic changes in excitation of a population of neurons. Thus, the time and direction of a change - described by phase - is functionally related to the timing of neuronal activation processes. A variety of findings supports this view and shows, e.g., that alpha phase coherence increases between task-relevant sites and that phase lag lies within a time range that is consistent with neuronal transmission speed. Another implication is that phase reset will be a powerful mechanism for the event-related timing of cortical processes. Empirical evidence suggests that the extent of phase locking is a functionally sensitive measure that is related to cognitive performance. Our general conclusion is that alpha ERS plays an active role for the inhibitory control and timing of cortical processing whereas ERD reflects the gradual release of inhibition associated with the emergence of complex spreading activation processes.

EEG gamma coherence and other correlates of subjective reports during ayahuasca experiences

The current study examined QEEG power and coherence of ayahuasca experiences with two experienced participants in a Brazilian jungle setting. An exploratory case series design was adopted for naturalistic field research. EEGs recorded during visual imagery was compared to eyes-closed baselines. The most important findings were increases in global EEG coherence in the 36-44 Hz and 50-64 Hz frequency bands for both subjects. Widely distributed cortical hyper-coherence seems reasonable given the intense synesthesia during ayahuasca experiences. Other findings include increased modal EEG alpha frequency and global power decreases across the cortex in most frequency bands, which concur with the EEG of psychedelics literature. Exploratory analysis revealed the usefulness of analyzing single Hz bins over the standard wide-band analysis. The discovery-oriented naturalistic approach developed for this study resulted in potentially important findings. We believe that finding increases in global gamma coherence during peak psychedelic experiences might contribute to the discussion of binding theory. Also, in light of recent research with gamma coherence during advanced meditative conditions, our findings might further the comparison of shamanic psychedelic practices with meditation.

Upper alpha ERD and absolute power: their meaning for memory performance

A variety of studies have shown that EEG alpha activity in the upper frequency range is associated with different types of cognitive processes, memory performance, perceptual performance and intelligence, but in strikingly different ways. For semantic memory performance we have found that resting or reference power is positively associated with performance, whereas during actual processing of the task, small power--reflected by a large extent of event-related desynchronization (ERD)--is related to good performance. We also have shown that the induction of large alpha reference power by neurofeedback training or repetitive transcranial magnetic stimulation (rTMS) at individual alpha frequency mimicked exactly the situation which is typical for good memory performance under normal situations: increased alpha reference power is associated with large ERD and good performance. Recent studies have demonstrated that this relationship holds true only for memory and not perceptual tasks that require the identification of simple visual stimuli under difficult conditions. In contrast to good memory performance, good perceptual performance is related to small pre-stimulus alpha power and a small ERD. We interpret this finding in terms of cortical inhibition vs. activation preceding task performance by assuming that large rhythmic alpha activity reflects inhibition. We assume that small reference alpha enhances perceptual performance because the cortex is activated and prepared to process the stimulus, whereas memory performance is enhanced if the cortex is deactivated before a task is performed because in typical memory tasks selective processing can start only after the to-be-remembered item or cue is presented. We also suggest that conflicting results about alpha ERD and the neural efficiency hypothesis (which assumes that highly intelligent exhibit a small ERD) can also be interpreted in terms of inhibition. Only if an intelligence test actually requires the activation of (semantic) memory, a large (because task specific) ERD can be observed. If other processing systems are required, the semantic memory system may even become suppressed, which is reflected by alpha event-related synchronization (ERS) or at least a largely decreased ERD.

Event-related neural activities: what about phase?

The main topic of this overview is an analysis of the concepts of phase and synchrony, as used in neurophysiology, in their various meanings. A number of notions related to the concepts of phase and synchrony, which are incorporated in contemporary neurophysiology, particularly in the domain of neuro-cognitive physiology are discussed. These notions need a critical examination, since their use sometimes is not clear, or it may even be ambiguous. We present some of these concepts, namely (a) (des)synchronization, (b) phase resetting, (c) phase synchrony and phase/time delays, and (d) phase clustering within one signal, while discussing what type of neuronal activities may underlie these EEG phenomena.

Reduced Alpha power associated with the recall of mentation from Stage 2 and Stage REM sleep

Relationships between Alpha (8-12 Hz) activity and cognitive processes during wakefulness raise the possibility of similar relationships between Alpha and cognitive activity during sleep. We hypothesized that Alpha power decreases during both Stage 2 and REM sleep would index the presence of sleep mentation in these stages. Absolute power for six classical EEG bands and three Alpha subbands was calculated for Stage 2 and REM sleep awakenings both with and without mentation recall. In both stages, recall was associated with lower Alpha power, especially with middle Alpha power (9.5-11.5 Hz). Unexpectedly, a similar effect for Delta power (0.5-4.0 Hz) was also observed. The Alpha effect may reflect cognitive elaboration active in the minutes preceding awakening; however, attention and memory processes cannot be excluded. The Delta effect is consistent with prior observations of regular linkages between Alpha and Delta power during sleep.

Human brain oscillatory activity phase-locked to painful electrical stimulations: a multi-channel EEG study

The main aims of this study were 1) a fine spatial analysis of electroencephalographic (EEG) oscillations after galvanic painful stimulation (nonpainful stimulation as a reference) and 2) a comparative evaluation of phase- and nonphase-locked component of these EEG oscillations. Preliminary surface Laplacian transformation of EEG data (31 channels) reduced head volume conductor effects. EEG phase values were computed by FFT analysis and the statistical evaluation of these values was performed by Rayleigh test (P < 0.05). About 50% of the EEG single trials presented statistically the same FFT phase value of the evoked EEG oscillations (phase-locked single trials), indicating a preponderant phase-locked compared to nonphase-locked component. The remaining single trials showed random FFT phase values (nonphase-locked single trials), indicating a preponderant nonphase-locked compared to phase-locked component. Compared to nonpainful stimulation, painful stimulation increased phase-locked theta to gamma band responses in the contralateral hemisphere and decreased the phase-locked beta band response in the ipsilateral hemisphere. Furthermore, nonphase-locked alpha band response decreased in the ipsilateral fronto-central area. In conclusion, both decreased and increased EEG oscillatory responses to galvanic painful stimulation would occur in parallel in different cortical regions and in the phase- and nonphase-locked EEG data sets. This enriches the actual debate on the mapping of event-related oscillatory activity of human brain.

Theta band power changes in normal and dyslexic children

OBJECTIVE

Tonic and phasic (event-related) theta band power changes were analyzed in a sample of 8 dyslexic and 8 control children. Previous research with healthy subjects suggests that electroencephalograph (EEG) theta activity reflects the encoding of new information into working memory. The aim of the present study was to investigate whether the processing deficits of dyslexics are related to a reduced phasic theta response during reading.

METHOD

The EEG was recorded while subjects were reading numbers, words and pseudowords and analyzed in a lower and upper theta band (4--8 Hz). A phasic response is measured in terms of an increase in event related band power during reading with respect to a reference interval. Tonic power is measured in terms of (log) band power during a reference interval.

RESULTS

Large group differences in tonic and phasic lower theta were found for occipital sites where dyslexics show a complete lack of pseudoword processing. For words, only controls show a highly selective left hemispheric processing advantage.

CONCLUSIONS

Dyslexics have a lack to encode pseudowords in visual working memory with a concomitant lack of frontal processing selectivity. The upper theta band shows a different pattern of results which can be best interpreted to reflect the effort during the encoding process.

Stimulus- and frequency-specific oscillatory mass responses to visual stimulation in man

Oscillatory mass responses centered at about 20-35 Hz or 100-120 Hz occur (after contrast or luminance visual stimulation, respectively) in the retina and cortex of animals and man and are recorded by electrical or magnetic methods. These oscillatory events reflect stimulus-related uni/multicellular oscillations of the firing rate/membrane potential and result from synchronization of neuronal assemblies selectively responding to the stimulus characteristics. Methodological problems in the study of these events derive from the contiguity in frequency between the ERG or VEP and the oscillatory responses and from the need to reliably define oscillatory events in time and frequency. Two methods (time-frequency analysis by matching pursuit and locking index) have been implemented to approach this issue. Theory and application are reviewed.

Theta oscillations and the ERP old/new effect: independent phenomena?

OBJECTIVES

The hypothesis is examined whether a memory-related change in induced band power (oscillatory old/new effect) is functionally related to a memory-related increase in ERP positivity (ERP old/new effect).

METHODS

In order to avoid a confounding on the measurement level, induced band power (IBP) was used as a measure that is devoid of the influence of evoked components. The EEG was recorded during a recognition memory task.

RESULTS

The results show that compared to correctly rejected words, targets (remembered words) elicit a significantly larger P300. An oscillatory old/new effect was found for the delta and theta but not for the alpha band. It is manifested by an increase in delta and theta IBP which is significantly larger for targets than for correctly rejected words. It can be observed during the same time interval and shows the same topographic distribution as the ERP old/new effect. Most importantly, however, the ERP old/new effect (as well as the P300 itself) is generated by very slow frequencies which lie below the delta band.

CONCLUSIONS

These findings demonstrate that the two types of old/new effects are functionally related. Possible physiological mechanisms underlying this relationship are discussed in terms of a threshold change in the cortex (generating the P300) that occurs during an increase in hippocampal theta activity (generating an increase in induced theta power).

Principal component elimination method for the improvement of S/N in evoked neuromagnetic field measurements

In the study of magnetoencephalography, it is important to obtain evoked fields with good signal-to-noise ratios (S/N) and with a small number of epochs in averaging. The noises are considered to be mainly spontaneous neuromagnetic fields. In the present study, we propose a method to improve the S/N. The basic principle of this method is the elimination of a principal component (PC) of multichannel-recorded neuromagnetic fields, utilizing the synchronized characteristics of spontaneous rhythmic activities dominating the fields. The proposed method is, therefore, called the principal component elimination method (PCEM). PCEM was applied to neuromagnetic fields measured by a 37-channel superconducting quantum interference device system, on which computer-generated evoked fields were superposed, in order to examine possible improvement in S/N. It was found that elimination of the first PC could improve the S/N of the evoked fields. The improvement in S/N with elimination of the first PC, compared to conventional simple averaging, increased with increases in the number of epochs and reached more than 50% after averaging over 128 epochs. PCEM also reduced the number of epochs needed in averaging to about half of that needed in conventional simple averaging.

Evidence for image-scanning eye movements during transitive inference

Contrary to earlier work, recent studies have demonstrated a reduction in eye movements during the solution of tasks that seem to require visual imagery, relative to verbal tasks. The present study provides evidence that the nature of the visual imagery required by a task determines whether saccades are evoked and in which spatial pattern. In two experiments, subjects solved transitive inference problems with the relational terms left/right and above/below, while the horizontal and vertical EOG were recorded. Subjects made more horizontal and fewer vertical saccades while solving problems with the left/right terms than while solving identical problems with above/below. The results of silent counting tasks showed that the rate of subvocalization can also influence saccadic rate, especially in the horizontal plane, but cannot explain the eye-movement patterns observed during transitive inference. The results are discussed in terms of a motor theory of voluntary thinking.

Responsiveness of human cortical activity to rhythmical stimulation: a three-modality, whole-cortex neuromagnetic investigation

We developed numerical indicators to quantify stimulus-related changes in cortical magnetic signals recorded from nine healthy subjects while they received 1- to 2.5-s trains of 15 stimuli (somatosensory, visual, or auditory in separate runs) at rates from 6 to 14 Hz, intermingled with 1.6-s pauses. A locking index (L) was introduced to quantify how well the responses are time locked to the stimuli and a global change factor (GC) to indicate changes in the whole-cortex oscillatory activity in the 5- to 25-Hz frequency range. The responses were visualized with color-coded images illustrating cortical reactivity for all stimulus rates simultaneously. These color maps clearly showed that the modality-specific cortical signals were enhanced at frequencies corresponding to the stimulus rate during the trains. For somatosensory stimulation the activity in the vicinity of the somatosensory hand area was enhanced at most stimulus rates, suggesting mainly superposition of evoked responses. In individuals with strong posterior resting rhythm, visual stimuli typically entrained activity in the parietooccipital sulcus at stimulus rates close to the main frequency of the spontaneous activity, probably reflecting driving of the intrinsic cortical rhythm, whereas in subjects with little spontaneous parietooccipital rhythm the cortical signal appeared to be composed mainly of visual evoked responses. No modality-specific enhancement was observed during auditory stimulation. During the pauses between the trains, the cortical signals were significantly suppressed compared with the resting condition: The peak activity (7-13 Hz) was modulated within, but also outside, the modality-specific areas, and the signals outside the frequency peaks of maximum power were consistently and reproducibly suppressed over the whole cortex by all stimuli.

A method for the calculation of induced band power: implications for the significance of brain oscillations

A method for the calculation of significant changes in induced band power (IBP) is presented. In contrast to traditional measures of event-related band power (ERBP) which are composed of evoked and not evoked EEG components, the proposed measure for IBP is deprived from phase locked (or evoked) EEG activity. It is assumed that changes in IBP reflect the modulation of brain oscillations that are largely independent from ERPs. The results of a visual oddball task show that significant changes in IBP can be observed in response to the presentation of a warning signal (preceding a target or nontarget) and the imperative stimulus (i.e. a target or nontarget) in the alpha, theta and delta band. Only a few significant changes in IBP were obtained for the warning signal in the theta band although highly significant changes in ERBP were found. Our findings document that changes in IBP may be considered a phenomenon that is largely independent from the occurrence of ERPs. They underline the significance of oscillatory processes and suggest that induced rhythms are modulated by stimuli and/or events in a not phase locked way.

Modulation of the parieto-occipital alpha rhythm during object detection

Changes in the human neuromagnetic alpha rhythm were monitored during an object detection task to study the effects of visual shape processing on the parieto-occipital activity. Pictures of coherent meaningful objects, which the observers had to detect, and of disorganized meaningless non-objects were presented briefly between masks. The non-objects were systematically followed by a higher level of alpha than the objects, the difference emerging on average 400 msec after the stimulus, with a median delay of 130 msec after evoked response onsets in the occipital, temporal, and parietal cortices. Without attention to visual shape, the alpha levels did not differ between objects and non-objects. The alpha level was higher after non-objects than missed objects, and higher after missed than correctly detected objects, suggesting that the alpha level is inversely related to saliency or familiarity of the object and does not directly reflect visual awareness. The reactive alpha rhythm was generated in the parieto-occipital sulcus, which in several primate species includes areas belonging to the dorsal visual pathway. According to current views, the parietal cortex produces attentional signals that filter out irrelevant information in the ventral visual stream. Our results reinforce the idea of bidirectional interaction: information derived from visual shape can rapidly modify activity in the parieto-occipital region. The synchronized alpha oscillations may reflect attenuation of occipito-parietal information transfer and disengagement of parietal cortex from object selection.

Magnetic fields from human prefrontal cortex differ during two recognition tasks

The present study represents our second successful use of magnetoencephalography to identify different sources of human prefrontal activity corresponding to subjects' engagement in different tasks. We used two visual recognition tasks: a familiar person recognition and an abstract pattern recognition task in the context of a design suitable for eliciting Contingent Negative Variations (CNVs) and their concurrent slow magnetic fields in this preliminary study of 5 subjects. Each trial of either task was started by one of two specific warning symbols (S1), indicating whether a person's picture or an abstract pattern should be attended during the presentation of a second stimulus (S2), and compared to the corresponding person's picture or pattern contained in the third stimulus, (S3) that followed. The S2 and S3 stimuli were common to both tasks, and were composed of patterns made with four line traces superimposed on photographs of persons familiar to each subject. Subjects responded with a right hand button press, following S3, indicating their judgments regarding the identity of the patterns or persons' pictures contained in the S2 and the S3 stimuli, for the two tasks, respectively. Results showed that the sources of the CNV equivalent magnetic fields were localized in different cortical regions depending on the task and that this difference was consistent across all subjects. The sources were localized in the right hemisphere, in medial areas of the prefrontal cortex for the person recognition task and in the dorsolateral prefrontal cortex for the pattern recognition task. The same degree of consistency was not found for the left hemisphere sources. Moreover, as in our previous study, we found no difference between the sources active during the first and the second CNV periods (occurring during the S1-S2 and the S2-S3 intervals, respectively), within each task condition.

Study of human occipital alpha rhythm: the alphon hypothesis and alpha suppression

Alpha rhythm of the parieto-occipital area is comprised of a parade of short-lived cortical excitations (alphons), each of which exhibits oscillations having a stable period within the alpha bandwidth. Strong alpha rhythm is produced by alphons extending over a larger cortical area, although an enhanced cortical current density may also contribute. Local suppression of alpha rhythm indicates when specific cortical areas become engaged in sensory or cognitive functions. Examples are provided for mental imagery, visual memory, auditory memory, and silent rhythming.

Non-invasive detection of ongoing neuronal population activity in normal human hippocampus

A 122-channel magnetoencephalographic array was used to monitor ongoing neuronal population activity in six normal human subjects during the performance of a mental calculation and passive viewing of a picture. Signal-space projection was utilized to obtain waveforms and spectra for activity in anterior hippocampus and sensorimotor cortex. Hippocampal waveforms were complex. Spectral components below 12 Hz in hippocampus included task-dependent peaks superimposed on a broadband background of increasing amplitude at decreasing frequencies. MEG imaging of both waveforms and spectra for ongoing activity within hippocampus makes possible the investigation of the role of rhythmic and non-rhythmic hippocampal activity in normal human cognition.

Magnetoencephalographic evidence for common sources of long latency fields to rare target and rare novel visual stimuli

This study used magnetoencephalography to examine the possibility that different generators account for the long-latency event-related potential (P300), evoked by rare target and by rare non-target, novel visual stimuli, in a visual oddball counting task performed by seven subjects. As expected, P300 peak latency was longer in response to rare targets compared to novel, non-target stimuli. Two main source regions were found for the Target- as well as for the Novel-P300, one in the temporal and one in the occipital lobe. Centers of neural activity were observed in the vicinity of the superior temporal sulcus, in the hippocampal formation and parahippocampal gyrus and in the occipital extrastriate cortex. It appears that the brain structures which contributed to the generation of the P300 response to both the target and the novel visual stimuli overlapped to a great extent.

Magnetoencephalographic study on the cerebral neural activities related to the processing of visually presented characters

Neuromagnetic fields were recorded from normal subjects to study the time course of cerebral neural activation while they performed a matching task of visual stimuli in which sequentially presented Japanese characters or unreadable pseudo-characters were compared according to phonological (reading of the characters) or graphical (geometry of the pseudo-characters) identity. In response to the single real-character or pseudo-character which was presented the latest distinct magnetic field components were observed, from which current dipole sources of the fields were localized in the individual magnetic resonance images of the brain. In the phonological identification, the sources were found in the parieto-occipital extrastriate cortex at 155-210 ms following the character presentation, and in the posterior temporal region (part of the Wernicke's area) and the posterior superior temporal region of the visual/auditory association cortex at 210-410 ms. The activity in these temporal regions was left hemisphere dominant, and may be the neural basis of phonological processing of the visual characters. In the graphical identification, sources occurring at 125-250 ms were noted in the inferior temporo-occipital region, and those at 180-460 ms in the posterior temporal and posterior superior temporal regions of the right hemisphere. These results indicate that the activities in the temporal area are lateralized to the left for the phonological processing and to the right for the graphical processing.

Suppression of EEG rhythmic frequencies during somato-motor and visuo-motor behavior

In a previous study of simulated vehicle performance we found that stationary visual attention and body movements alone produced selective effects on topographic EEG frequency patterns. In the present study we focus on an expanded set of these task components. EEG, EOG and ECG data were recorded from 21 subjects during instructed driving movements and during visual scanning tasks ranging from a stationary to a rapidly moving simulated driving display. Spectral analysis was calculated on ten 2-Hz, partially overlapped frequency bands between 6 and 17 Hz. Body movements produced a selective bilateral suppression of 11-15 Hz activity localized to medial somatosensory cortex, while both slow and rapid visual scanning tasks produced a similar bilateral suppression of 11-15 Hz activity localized to temporo-parietal sites. A generalized suppression of 7-11 Hz activity was also found during the fastest visual scanning task. There were no significant differences in ECG between tasks. Other human and animal findings consistent with these functional observations are discussed.

Reactivity of magnetic parieto-occipital alpha rhythm during visual imagery

Spontaneous MEG signals were recorded during visual imagery from 13 healthy adults with a whole-scalp neuromagnetometer. The parieto-occipital 7-14 Hz alpha activity was suppressed strongly while subjects visualized and evaluated letters. The act of forming a visual image caused a smaller suppression than did inspection of the imaged pattern for a named property. The maximum suppression depended on the baseline alpha level and, for the majority of the subjects, occurred close to the area with the strongest alpha, showing no systematic hemispheric asymmetry. Sources for the alpha activity, modeled with equivalent current dipoles, clustered in the parietal and occipital lobes. The strongest suppression of the activity occurred near the parieto-occipital sulcus.

Characterizing the local oscillatory content of spontaneous cortical activity during mental imagery

We report on the determination of detailed spectra for simultaneously active sources of spontaneous neuronal activity in humans directly from data recorded with a whole-scalp 122-channel magnetometer array. Subjects rested with eyes open and performed two contrasting mental imagery tasks: the imagination of the self-performance of a motor activity and the silent generation of a chain of words. A novel analysis technique, frequency-domain signal-space projection (FDSSP) was utilized to determine the temporal and spectral characteristics of spontaneous brain activity at specific cortical sites. Although intersubject differences were significant, spectra for individual subjects contained task-dependent features which were reproducible over successive 20-s epochs. This result supports the concept of multiple sources of spontaneous cortical activity and suggests that detailed spectra of localized oscillatory activity obtained non-invasively with magnetoencephalographic arrays may provide a useful characterization of cortical involvement in mental imagery.

Discrimination between phase-locked and non-phase-locked event-related EEG activity

Differentiation between phase-locked and non-phase-locked event-related EEG activity is an important task in the evaluation of event-related EEG activity. Event-related changes of EEG activity such as event-related desynchronization (ERD) or event-related synchronization (ERS) can be quantified by either instantaneous band power or intertrial variance calculations. In the former case the ERD or ERS can be masked by an event-related potential while in the latter it is not. Examples from sensory stimulation and movement experiments, where the ERD (ERS) is calculated by both methods, are shown and discussed.

Visualization of sensorimotor areas involved in preparation for hand movement based on classification of mu and central beta rhythms in single EEG trials in man

It is well known that mu and central beta rhythms start to desynchronize > 1 s before active hand or finger movement. To investigate whether the same cortical areas are involved in desynchronization of mu and central beta rhythms, 56-channel EEG recordings were made during right- and left-finger flexions in three normal subjects. The event-related desynchronization (ERD) was quantified in single EEG trials and classified by the Distinction Sensitive Learning Vector Quantization (DSLVQ) algorithm. This DSLVQ selects the most relevant features (electrode positions) for discrimination between the preparatory state for left- and right-finger movements. It was found that the most important electrode positions were close to the primary hand area. However, in all three subjects the focus of the central beta ERD was slightly anterior to the focus of mu desynchronization. This can be interpreted that different neural networks are involved in the generation of mu and central beta rhythms.

Magnetoencephalography in studies of human cognitive brain function

Magnetoencephalography provides a new dimension to the functional imaging of the brain. The cerebral magnetic fields recorded noninvasively enable the accurate determination of locations of cerebral activity with an uncompromized time resolution. The first whole-scalp sensor arrays have just recently come into operation, and significant advances are to be expected in both neurophysiological and cognitive studies, as well as in clinical practice. However, although the accuracy of locating isolated sources of brain activity has improved, identification of multiple simultaneous sources can still be a problem. Therefore, attempts are being made to combine magnetoencephalography with other brain-imaging methods to improve spatial localization of multiple sources and, simultaneously, to achieve a more complete characterization of different aspects of brain activity during cognitive processing. Owing to its good time resolution and considerably better spatial accuracy than that provided by EEG, magnetoencephalography holds great promise as a tool for revealing information-processing sequences of the human brain.

Event-related potentials in silent speech

Event-related potentials (ERPs) in silent speech using the vowel /a/ were recorded from 12 scalp electrodes and three electrodes monitoring eye and throat movements in eight subjects cued by one of two randomly lit light-emitting diodes (LEDs). The average silent-speech potential minus nonsilent-speech potential showed two significant scalp potential distributions--a positive difference in the occipital scalp area at a 0.30-s latency from the LED onset and a negative difference in the frontal scalp area peaking at electrode Fz at a 0.42-s latency. The occipital scalp potential may include an endogenous component like P3. Possible sites of neural activities underlying the frontal negative difference are discussed in relation to the topography of the scalp potential and functions involved in silent speech.

Spatiotemporal characteristics of sensorimotor neuromagnetic rhythms related to thumb movement

To assess the spatial extent and temporal behavior of rolandic rhythms we recorded neuromagnetic signals from four healthy subjects with a 24-channel magnetometer. The subjects performed self-paced thumb movements or the motions were triggered by electrical stimulation of the median nerve at the wrist. The main frequency components of the magnetic mu rhythm signals centered at 10 and 20 Hz. Both components were completely suppressed during the movement and increased substantially 0.5-2.5 s after it; the 20-Hz component reacted about 300 ms faster. The rebound was stronger after self-paced than after stimulated motion, and after contra- than after ipsilateral movement. The reactive source areas were identified for both frequency ranges, and they clustered on partly overlapping cortical areas of 6-8 cm2 wide along the course of the central sulcus. The 10-Hz rhythmic oscillations occurred predominantly at the primary somatosensory hand cortex; the sources of the 20-Hz signals were slightly more anterior. We hypothesize that the 10-Hz signal is a true somatosensory rhythm whereas the 20-Hz activity is essentially somatomotor in origin.

MNLS inverse discriminates between neuronal activity on opposite walls of a simulated sulcus of the brain

The minimum-norm least-squares inverse for magnetic field measurements is applied to a representation of a sulcus of the human brain, where one or both walls have regions of neuronal activity. Simulations indicate that the magnetic source image (MSI) is largely confined to the appropriate wall of the sulcus, even for a depth of 4 cm where the distance between walls is only 3 mm. Two nearly oppositely oriented dipoles located 3 mm apart are found to be distinguished. Influences on the quality of the MSI by measurement noise and inaccuracy in determining the image surface are discussed in detail.

Event-related potential maps depend on prestimulus brain electric microstate map

The brain functional microstate immediately before each of about 3000 identical tone stimuli was classified using extracted reference-free descriptors (locations of maximal and minimal potential) of the landscape of the brain's momentary electric field, in 8 volunteers. Six prestimulus microstate map classes occurred more than 30 times in each subject, and were clustered into two map class types (totals of 242 and 283 cases, respectively, on the average per subject). Event-related potential (ERP) map series were averaged for each subject and prestimulus map class. Map descriptors were extracted from the ERP maps at times of maximal Global Field Power during the component time windows N100, P200 and P330. Discriminant functions were estimated; for the maps of N100 and P330, the discriminant scores differed significantly between the maps associated with the two prestimulus map class types (paired t-tests, df = 7, p = .014 and p = .005, respectively). The dominant axis of the poststimulus class type II ERP maps deviated clockwise from that of the type I ERP maps in all components. We conclude that subtle changes in the brain's spontaneous momentary functional microstate (as classified by spatial descriptors of a single map) influence event-related information processing by the brain, following common rules over subjects.

Minimum-norm least-squares estimation: magnetic source images for a spherical model head

This paper extends the minimum-norm least-squares inverse approach to a local spherical model for the conductivity geometry of the human head. In simulations of cortical activity of the human brain, the magnetic field pattern across the scalp is interpreted with prior knowledge of anatomy, and the properties of intraneuronal current flow to yield a unique magnetic source image across a portion of cerebral cortex. Influences on the quality of magnetic source images from the noise in measurements, the position error in determining the image surface, and the number of sensors are evaluated in detail.

Imaging regional changes in the spontaneous activity of the brain: an extension of the minimum-norm least-squares estimate

This paper describes methods for inferring mathematically unique local distributions of primary cortical current that underly changes in the average pattern of power of the ongoing ("spontaneous") extracranial magnetic field of the brain. In previous work we demonstrated that mathematically unique solutions to the inverse problem are possible for current sources of the brain's field, without assuming a small set of current dipoles as a source model. In principle, it is possible to locate and delineate patterns of current of any configuration. In practice this approach applies to synchronized neuronal activity, e.g., activity which is known to underly average evoked or event-related brain responses. This paper extends that approach to local changes in incoherent activity, e.g., activity yielding fields or potentials that tend to be self-cancelling when averaged over time. This includes the spontaneous brain activity normally treated as background noise when it accompanies event-related responses. We demonstrate that local changes in this ongoing incoherent activity may also be uniquely delineated in space and time. The solution is a covariance matrix characterizing activity across an image surface. Its diagonal elements represent the spatial pattern of mean current power. Evidence is reviewed indicating that the distribution of the brain's magnetic field, due to both its synchronized and incoherent neural activity, is affected by early sensory-perceptual processes and by higher cognitive processes. Hence, in principle, the ability to delineate both kinds of sources in space and time makes it possible to form more comprehensive dynamic functional images of the human brain.

Magnetic source imaging based on the minimum-norm least-squares inverse

The flow of ionic currents within the neurons of cerebral cortex produces a magnetic field that can be detected outside the human scalp. The dominant contribution is attributed to pyramidal cells, which are preferentially oriented perpendicular to the cortical surface. In general, it is not possible to deduce a unique representation of the spatial configuration of these cortical sources from a measurement of their field pattern alone. However, accurate a priori knowledge of the geometry of the underlying cerebral cortex makes it possible to infer the spatial configuration of these transcortical current sources, moment by moment, without imposing a simplified model such as a small set of current dipoles. To achieve such a realistic magnetic source image, we have introduced what we call the "Minimum-Norm Least-Squares Inverse" (MNLS inverse) for the magnetic problem. The MNLS inverse provides the least residual error in accounting for the measured field pattern, with a source current distribution having minimum power. An extension of this procedure provides an inverse solution for average field power, as opposed to field per se. This makes it possible to define spatial configurations of spontaneous cortical activity not phase-locked to a sensory stimulus. Rhythmic activity such as the occipital alpha rhythm is one example. Thus, it is possible to determine spatial patterns of enhanced or suppressed cortical rhythms that accompany cognitive processes and some pathological conditions. This paper provides the necessary background for understanding these recent developments, as well as examples of how they might be used.

Changes in cortical activity when subjects scan memory for tones

The magnetoencephalogram (MEG) was used to detect regional changes in spontaneous cortical activity accompanying short-term memory search. This method was chosen because magnetic fields are detectable only within a few centimeters of the projections of their sources onto the scalp. The specific hypothesis that auditory cortex is involved in scanning memory for tones was tested by sensing the field of the magnetic counterpart to N100 (N100m) which is known to originate in auditory cortex. N100m was measured at many different positions and the spontaneous cortical rhythms in the alpha bandwidth (8-12 Hz) were measured at the same places. These rhythms were found to be suppressed while subjects scanned memory for musical tones in a Sternberg paradigm. For 3 subjects, both the MEG suppression time (ST) and reaction time (RT) increased linearly with memory set size. The correlation between ST measured over the left hemisphere and set size was significant for two subjects but not significant for the third, and the slopes of the regression lines relating ST to set size were too shallow to be related to the time required to scan memory. However, the correlation between ST of the right hemisphere and set size was highly significant for all subjects, and the slopes of the regression lines were comparable to those relating RT to set size. The electroencephalogram (EEG) recorded with midline electrodes failed to reveal a significant relationship between suppression time and set size for 2 of the subjects, thus ruling out global alpha blockage and generalized arousal as the basis for the task-related suppression duration. The electric N100, measured at Cz, decreased significantly in amplitude with set size for 2 subjects, but it increased significantly in amplitude for the third subject. In contrast, RT increased with set size for all subjects. N100m measured over the right hemisphere was similar to the behavior of N100, while N100m measured over the left hemisphere showed little change in amplitude with set size, thus establishing an asymmetry in N100 between the hemispheres. Since N100 amplitude is normally larger when attention is paid to auditory stimuli, differential attention alone cannot account for the relation between ST and set size. Furthermore, the processing negativity, which may be superimposed on N100 in selective attention tasks, was not discernible for any set size. It was also found that ST prior to the button press was not correlated with RT. Hence, the covariation of set size with ST is not attributable to preparation for a motor response.(ABSTRACT TRUNCATED AT 400 WORDS)

On cortical folds and neuromagnetic fields

A folded cortical source of neuromagnetic fields, similar in configuration to the visual cortex, was simulated. Cortical activity was modelled by different distributions of independent current dipoles. The map of the summed fields of the dipoles of this cruciform model changed, depending upon the statistical distribution of the electrical activity of the dipoles and its geometry. Arrays of dipoles of random orientations and strengths produced field patterns that could be interpreted as due to moving neural currents, although the geometry of the neural tissue remained unchanged and the average activity remained approximately constant. The field topography at any instant was apparently unrelated to the depth or orientation of the underlying structure, thus raising questions about how to interpret topographic MEG and EEG displays. Furthermore, asynchronous activity (defined as independent directions and magnitudes of activity of the dipoles) did not result in less field power than when the dipoles were synchronized, i.e., when the direction of current flow was correlated across all of the dipoles within the cruciform structure. Therefore, in this model 'alpha blockage' cannot be mimicked by desynchronization. More generally, for the cruciform or any other symmetrically folded and active cortical sheet, 'blockage' cannot be attributed to desynchronization. The same is true for the EEG except that smooth unfolded sheets of radially oriented dipoles would result in enhancement of voltage due to synchronization. Such radial dipoles do not contribute to the MEG. Blockage was simulated by reducing the amount of activity within different portions of the synchronized cruciform model. This resulted in a dramatic increase in the net field because attenuation broke the symmetry of the synchronized cruciform structure. With asynchronous dipoles populating the structure, the attenuation of the same portion of the structure had no easily discerned effect on the net field. However, maps of average field power were consistently related to the position of the region of attenuated activity. The locations of regions of attenuated activity were determined by taking the difference between the mean square field pattern obtained when all portions of the cruciform structure were active and the pattern obtained when a portion of the structure was relatively inactive. When activity of the same portions were incremented rather than attenuated, the resulting plot of average power was essentially the same as that of the attenuated portion derived by taking these differences between power distributions. The major conclusions are that the concepts of synchronization and desynchronization have no explanatory power unless the physical conditions under which they occur are specified precisely.(ABSTRACT TRUNCATED AT 400 WORDS)