Statistical probability mapping reveals high-frequency magnetoencephalographic activity in supplementary motor area during self-paced finger movements

Probability waves: Adaptive cluster-based correction by convolution of p-value series from mass univariate analysis

BACKGROUND

Methods for p-value correction are criticized for either increasing Type II error or improperly reducing Type I error in large exploratory data analysis. This text considers patterns in probability vectors resulting from mass univariate analysis to correct p-values, where clusters of significant p-values may indicate true H0 rejection.

NEW METHOD

We used ERP experimental data from control and ADHD boys to test the method. The Log10 of p-vector was convolved with a Gaussian window whose length was set as the shortest lag above which autocorrelation of each ERP wave may be assumed to have vanished. We realized Monte-Carlo simulations (MC) to (1) evaluate confidence intervals of rejected and non-rejected areas of our data, (2) to evaluate differences between corrected and uncorrected p-vectors or simulated ones in terms of distribution of significant p-values, and (3) to empirically verify the type-I error rate (comparing 10,000 pairs of mixed samples whit control and ADHD subjects).

RESULTS

The differences between simulation or raw p-vector and corrected p-vectors were, respectively, minimal and maximal for window length set by autocorrelation in p-vector convolution.

COMPARISON WITH EXISTING METHODS

Our method was less conservative while FDR methods rejected basically all significant p-values.The MC simulations presented 2.78 ± 4.83% of difference (20 channels) from corrected p-vector, while difference from raw p-vector was 596 ± 5.00% (p = 0.0003).

CONCLUSION

As a cluster-based correction, the present new method seems to be biological and statistically suitable to correct p-values in mass univariate analysis of ERP waves, which adopts adaptive parameters to correction.

Non-linear Relationship between BOLD Activation and Amplitude of Beta Oscillations in the Supplementary Motor Area during Rhythmic Finger Tapping and Internal Timing

Functional imaging studies using BOLD contrasts have consistently reported activation of the supplementary motor area (SMA) both during motor and internal timing tasks. Opposing findings, however, have been shown for the modulation of beta oscillations in the SMA. While movement suppresses beta oscillations in the SMA, motor and non-motor tasks that rely on internal timing increase the amplitude of beta oscillations in the SMA. These independent observations suggest that the relationship between beta oscillations and BOLD activation is more complex than previously thought. Here we set out to investigate this rapport by examining beta oscillations in the SMA during movement with varying degrees of internal timing demands. In a simultaneous EEG-fMRI experiment, 20 healthy right-handed subjects performed an auditory-paced finger-tapping task. Internal timing was operationalized by including conditions with taps on every fourth auditory beat, which necessitates generation of a slow internal rhythm, while tapping to every auditory beat reflected simple auditory-motor synchronization. In the SMA, BOLD activity increased and power in both the low and the high beta band decreased expectedly during each condition compared to baseline. Internal timing was associated with a reduced desynchronization of low beta oscillations compared to conditions without internal timing demands. In parallel with this relative beta power increase, internal timing activated the SMA more strongly in terms of BOLD. This documents a task-dependent non-linear relationship between BOLD and beta-oscillations in the SMA. We discuss different roles of beta synchronization and desynchronization in active processing within the same cortical region.

Cue-related Temporal Factors Modulate Movement-related Beta Oscillatory Activity in the Human Motor Circuit

In humans, there is a strong beta (15-30 Hz) event-related desynchronization (ERD) that begins before movement, which has been tentatively linked to motor planning operations. The dynamics of this response are strongly modulated by whether a pending movement is cued and the inherent parameters of the cue. However, previous studies have focused on the information content of cues and not on parameters such as the timing of the cue relative to other events. Variations in such timing are critical, as they directly impact the amount of time that participants have to plan pending movements. In this study, participants performed finger-tapping sequences during magnetoencephalography, and we manipulated the amount of time (i.e., "long" vs. "short") between the presentation of the to-be-executed sequence and the cue to initiate the sequence. We found that the beta ERD was stronger immediately after the cue to move in the contralateral postcentral gyrus and bilateral parietal cortices during the short compared with long planning time condition. During movement execution, the beta ERD was stronger in the premotor cortex and the SMA in the short relative to long condition. Finally, peak latency in the SMA significantly correlated with RT, such that the closer the peak beta ERD was to the cue to move, the quicker the participant responded. The results of this study establish that peri-movement beta ERD activity across the cortical motor circuit is highly sensitive to cue-related temporal factors, with a direct link to motor performance.

Somatosensory system deficits in schizophrenia revealed by MEG during a median-nerve oddball task

Although impairments related to somatosensory perception are common in schizophrenia, they have rarely been examined in functional imaging studies. In the present study, magnetoencephalography (MEG) was used to identify neural networks that support attention to somatosensory stimuli in healthy adults and abnormalities in these networks in patient with schizophrenia. A median-nerve oddball task was used to probe attention to somatosensory stimuli, and an advanced, high-resolution MEG source-imaging method was applied to assess activity throughout the brain. In nineteen healthy subjects, attention-related activation was seen in a sensorimotor network involving primary somatosensory (S1), secondary somatosensory (S2), primary motor (M1), pre-motor (PMA), and paracentral lobule (PCL) areas. A frontal-parietal-temporal "attention network", containing dorsal- and ventral-lateral prefrontal cortex (DLPFC and VLPFC), orbitofrontal cortex (OFC), anterior cingulate cortex (ACC), superior parietal lobule (SPL), inferior parietal lobule (IPL)/supramarginal gyrus (SMG), and temporal lobe areas, was also activated. Seventeen individuals with schizophrenia showed early attention-related hyperactivations in S1 and M1 but hypo-activation in S1, S2, M1, and PMA at later latency in the sensorimotor network. Within this attention network, hypoactivation was found in SPL, DLPFC, orbitofrontal cortex, and the dorsal aspect of ACC. Hyperactivation was seen in SMG/IPL, frontal pole, and the ventral aspect of ACC in patients. These findings link attention-related somatosensory deficits to dysfunction in both sensorimotor and frontal-parietal-temporal networks in schizophrenia.

Motor imagery influences the execution of repetitive finger opposition movements

Motor imagery (MI) is the ability to imagine performing a movement without executing it. In literature, there have been numerous reports on the influence of MI on motor practice and the beneficial effects of "mental practice" on the physical performance has been suggested to rely to the close temporal association between motor rehearsal and actual performance. In the present study, we aimed to evaluate whether the addition of a period of motor imagery between two motor practice trials could modify movement execution in a repetitive finger opposition motor task performed at maximal speed and whether the effect of motor imagery on motor practice is dependant on the complexity of movement. We observed that the addition of motor imagery to the sole motor practice was able to influence the performance of repetitive finger opposition movements inducing an increase of the velocity of movement greater than that observed with the motor practice alone. Further the addition of motor imagery was able to induce a modification in the motor strategy in terms of duration of the main phases of movements. This was more evident when subjects executed a finger sequential task with respect to a simple finger tapping task. We assume that mental rehearsal facilitates the brain network involved in sensorimotor control, particularly acting on those neural structures involved in the motor program.

Post-stroke tactile allodynia and its modulation by vestibular stimulation: a MEG case study

BACKGROUND

There is behavioural evidence that caloric vestibular stimulation (CVS) can alleviate central pain. Several such patients have also noted that it reduces tactile allodynia, an especially ill-understood phenomenon in these patients.

AIMS OF THE STUDY

The first aim is to use magnetoencephalography (MEG) to study neural activity associated with tactile allodynia in central post-stroke pain (CPSP). The second is to assess how this would be affected, if at all, by CVS. The third is to assess the ability of the VESTAL solution for MEG to detect anterior cingulate activation.

METHODS

A 58-year-old woman with CPSP, and marked unilateral tactile allodynia, participated in a MEG study with imaging pre- and post-CVS.

RESULTS

Tactile simulation within the patient's allodynic area resulted in contralateral activation of the primary motor and anterior cingulate cortices, which had normalized 24 h post-CVS.

CONCLUSIONS

We suggest that the unexpected primary motor cortex activation in response to light touch in the allodynic area arises from inappropriate activation of a normal mechanism, which may occur when a threat to homeostasis is present, to lower motor thresholds and allow for more rapid performance of corrective actions. We propose this may be mediated by the interoceptive cortex in the dorsal posterior insula.

Event-Related Potential Study of Executive Dysfunctions in a Speeded Reaction Task in Cocaine Addiction

This study used a flanker task with NoGo elements to investigate frontal executive function deficits in 19 cocaine abusers. The executive functions of interest in this study were cortical inhibition or ability to withhold motor response, the ability to select an appropriate response among several competing ones, the ability to inhibit inappropriate responses, and the ability to detect error and exercise corrective control. These processes were evaluated with specific frontal and parietal event-related potentials (ERP) registered during performance on this speeded reaction time task with conflicting motor response demands. Specifically we used behavioral response measures, stimulus-locked anterior (frontal N200, N450) ERP markers of conflict detection, response inhibition (NoGo-N2 and NoGo-P3), and response-locked error-related negativity (ERN) that represent different time points of signal classification, motor response conflict detection, response inhibition, and error monitoring processes. The results revealed that the higher-level executive motor control attributed to the prefrontal cortex is hypoactive in cocaine abusers, and therefore is incapable to effectively resolve response conflicts arising between the competing motor response alternatives. It was also demonstrated that the mesial frontal structures, such as the anterior cingulate cortex, implicated in motor response conflict detection and error monitoring functions were also compromised in addicts. It is reasonable to propose that a 'hypofunctional' prefrontal and midfrontal processing results in a diminished ability to effectively override strong habitual automated response tendencies controlled by the lower-level neural mechanisms triggered by the external stimuli. The results propose a neurobiological basis for the understanding why cocaine abusers are facing difficulties in controlling their drug-seeking and drug-taking behaviors, and why their drug-related habitual behavior is so vulnerable to be triggered by external (e.g., drug-related items and environment) cues.

Nonparametric statistical testing of coherence differences

Many important questions in neuroscience are about interactions between neurons or neuronal groups. These interactions are often quantified by coherence, which is a frequency-indexed measure that quantifies the extent to which two signals exhibit a consistent phase relation. In this paper, we consider the statistical testing of the difference between coherence values observed in two experimental conditions. We pay special attention to problems induced by (1) unequal sample sizes and (2) the fact that coherence is typically evaluated at a large number of frequency bins and between large numbers of pairs of neurons or neuronal groups (the multiple comparisons problem). We show that nonparametric statistical tests provide convincing and elegant solutions for both problems. We also show that these tests allow to incorporate biophysically motivated constraints in the test statistic, which may drastically increase the sensitivity of the test. Finally, we explain why the nonparametric test is formally correct. This means that we formulate a null hypothesis (identical probability distribution in the different experimental conditions) and show that the nonparametric test controls the false alarm rate under this null hypothesis. The proposed methodology is illustrated by analyses of data collected in a study on cortico-spinal coherence [Schoffelen JM, Oostenveld R, Fries P. Neuronal coherence as a mechanism of effective corticospinal interaction. Science 2005;308(5718):111-3].

Nonparametric statistical testing of EEG- and MEG-data

In this paper, we show how ElectroEncephaloGraphic (EEG) and MagnetoEncephaloGraphic (MEG) data can be analyzed statistically using nonparametric techniques. Nonparametric statistical tests offer complete freedom to the user with respect to the test statistic by means of which the experimental conditions are compared. This freedom provides a straightforward way to solve the multiple comparisons problem (MCP) and it allows to incorporate biophysically motivated constraints in the test statistic, which may drastically increase the sensitivity of the statistical test. The paper is written for two audiences: (1) empirical neuroscientists looking for the most appropriate data analysis method, and (2) methodologists interested in the theoretical concepts behind nonparametric statistical tests. For the empirical neuroscientist, a large part of the paper is written in a tutorial-like fashion, enabling neuroscientists to construct their own statistical test, maximizing the sensitivity to the expected effect. And for the methodologist, it is explained why the nonparametric test is formally correct. This means that we formulate a null hypothesis (identical probability distribution in the different experimental conditions) and show that the nonparametric test controls the false alarm rate under this null hypothesis.

Prefrontal gamma-band activity distinguishes between sound durations

The present study used magnetoencephalography to assess the cortical representation of brief sound durations during a short-term memory task. Twelve subjects were instructed to memorize sounds S1 with durations of either 100 or 200 ms during an 800-ms delay phase. Subsequently, they had to judge whether the duration of a probe sound S2 matched the memorized stimulus. Statistical probability mapping of oscillatory signals revealed several components of gamma-band activity (GBA) over prefrontal cortex. A first component with a center frequency of 40 Hz responded more strongly to longer than shorter sounds during the encoding of S1. During the subsequent delay phase, shorter and longer durations were associated with topographically and spectrally distinct GBA enhancements at 71 and 80 Hz, respectively. S2 was again associated with stronger oscillatory activation for longer than shorter sounds at approximately 72 Hz. Non matching compared with matching S1-S2 pairs elicited an additional approximately 66 Hz GBA component peaking at approximately 200 ms after the offset of S2. The analysis of magnetoencephalographic GBA thus served to identify prefrontal network components underlying the representation of different sound durations during the various phases of a delayed matching-to-sample task.

Effects of memory load on cortical oscillatory activity during auditory pattern working memory

The present magnetoencephalography study investigated memory load-dependent changes in cortical oscillatory activity during a modified auditory version of the Sternberg paradigm. Twelve subjects were presented with test stimulus sets of 1-3 syllables spoken in a natural female voice. After an 800-ms delay period, a probe syllable was presented and subjects had to judge whether the probe had been included in the preceding test set. Compared to a non-memory-control task, memory trials elicited an increase of beta activity over right temporal regions and an increase of alpha activity over right middle prefrontal cortex at the end of the delay phase. Monotonic increases in spectral amplitude as a function of memory load were revealed for the beta band over right temporal sensors and the alpha band over right frontal sensors during the delay period. The results demonstrate the relevance of both beta and alpha oscillations for the memorization of multiple stimuli. The former may be associated with the representation of task-relevant stimulus features, while the latter may reflect the top-down control of these representations.

Gamma-band activity over early sensory areas predicts detection of changes in audiovisual speech stimuli

Oscillatory activity in the gamma-band range in human magneto- and electroencephalogram is thought to reflect the oscillatory synchronization of cortical networks. Findings of enhanced gamma-band activity (GBA) during cognitive processes like gestalt perception, attention and memory have led to the notion that GBA may reflect the activation of internal object representations. However, there is little direct evidence suggesting that GBA is related to subjective perceptual experience. In the present study, magnetoencephalogram was recorded during an audiovisual oddball paradigm with infrequent visual (auditory /ta/ + visual /pa/) or acoustic deviants (auditory /pa/ + visual /ta/) interspersed in a sequence of frequent audiovisual standard stimuli (auditory /ta/ + visual /ta/). Sixteen human subjects had to respond to perceived acoustic changes which could be produced either by real acoustic or illusory (visual) deviants. Statistical probability mapping served to identify correlations between oscillatory activity in response to visual and acoustic deviants, respectively, and the detection rates for either type of deviant. The perception of illusory acoustic changes induced by visual deviants was closely associated with gamma-band amplitude at approximately 80 Hz between 250 and 350 ms over midline occipital cortex. In contrast, the detection of real acoustic deviants correlated positively with induced GBA at approximately 42 Hz between 200 and 300 ms over left superior temporal cortex and negatively with evoked gamma responses at approximately 41 Hz between 220 and 240 ms over occipital areas. These findings support the relevance of high-frequency oscillatory activity over early sensory areas for perceptual experience.

Gamma-band activity dissociates between matching and nonmatching stimulus pairs in an auditory delayed matching-to-sample task

Electro- and magnetoencephalography studies have suggested that increased gamma-band activity (GBA) is a correlate of activated neural stimulus representations. In this study, a delayed matching-to-sample paradigm for auditory spatial information was employed to investigate the role of magnetoencephalographic gamma-band activity in the differentiation between matching and nonmatching stimulus pairs. Twelve subjects made same-different judgments about the lateralization angle of pairs of filtered noise stimuli (S1 and S2) presented with 0.8-s delays. One half of the subjects had to respond to matching stimulus pairs, the other half to nonmatching stimulus pairs. Cortical oscillatory activity in the memory task was compared to a control task requiring the detection of background noise intensity changes. Memory-related GBA increases were revealed over midline parietal areas in the middle of the delay phase and during the presentation of S2 and over frontocentral areas at the end of the delay phase. This replicated previous findings. In addition, nonmatching trials were associated with increased GBA over right parietal areas in response to S2. The midline parietal GBA increase during S2 in the memory condition may have reflected the representation of S1 needed for a comparison between S1 and S2. When S1 and S2 were identical, no further representation was required. In contrast, for nonmatching pairs, a second representation was activated over right parietal areas.

Stabilization of bimanual coordination due to active interhemispheric inhibition: a dynamical account

Based on recent brain-imaging data and congruent theoretical insights, a dynamical model is derived to account for the patterns of brain activity observed during stable performance of bimanual multifrequency patterns, as well as during behavioral instabilities in the form of phase transitions between such patterns. The model incorporates four dynamical processes, defined over both motor and premotor cortices, which are coupled through inhibitory and excitatory inter- and intrahemispheric connections. In particular, the model underscores the crucial role of interhemispheric inhibition in reducing the interference between disparate frequencies during stable performance, as well as the failure of this reduction during behavioral transitions. As an aside, the model also accounts for in- and antiphase preferences during isofrequency movements. The viability of the proposed model is illustrated by magnetoencephalographic signals that were recorded from an experienced subject performing a polyrhythmic tapping task that was designed to induce transitions between multifrequency patterns. Consistent with the models dynamics, contra- and ipsilateral cortical areas of activation were frequency- and phase-locked, while their activation strength changed markedly in the vicinity of transitions in coordination.

Magnetoencephalographic gamma-band responses to illusory triangles in humans

Electroencephalography studies have suggested that the perception of illusory figures is associated with increases in gamma-band activity putatively reflecting the formation of synchronously firing neuronal assemblies. Here we assessed magnetoencephalographic gamma-band activity, which has been shown to be topographically more focal than in electroencephalogram. In line with functional brain imaging findings, we hypothesized gamma-band activity over ventral visual stream areas. In addition, we expected that the analysis of oscillatory activity would provide information on the time courses and connectivity patterns of these activations. Following a paradigm previously assessed with electroencephalography, 16 adults were presented four types of stimuli at equal probabilities: illusory (Kanizsa) triangles, real triangles, no-triangle stimuli with rotated inducer disks, and curved illusory triangles serving as targets that subjects had to respond to. Induced oscillatory responses were compared between illusory triangles and no-triangle stimuli and between illusory and real triangles using a statistical probability mapping method. Illusory triangles were distinguished from no-triangles by increased activity at around 70 Hz over midline occipital cortex peaking at 240 ms after stimulus onset. This was followed by activations over bilateral lateral occipital areas at 430 ms. Illusory triangles differed from real triangles by increased spectral activity at 90 Hz over posterior parietal cortex between 100 and 450 ms after stimulus onset, suggesting an involvement of visual dorsal stream regions. Coherence analysis showed increased connectivity between posterior parietal and lateral occipital cortex. These findings suggest that illusory triangles are encoded in parallel by networks along the visual ventral and dorsal streams.

Hearing lips: gamma-band activity during audiovisual speech perception

Auditory pattern changes have been shown to elicit increases in magnetoencephalographic gamma-band activity (GBA) over left inferior frontal cortex, forming part of the putative auditory ventral "what" processing stream. The present study employed a McGurk-type paradigm to assess whether GBA would be associated with subjectively perceived changes even when auditory stimuli remain unchanged. Magnetoencephalograms were recorded in 16 human subjects during audiovisual mismatch perception. Both infrequent visual (auditory /ta/ + visual /pa/) and acoustic deviants (auditory/pa/ + visual /ta/) were compared with frequent audiovisual standards (auditory /ta/ and visual /ta/). Statistical probability mapping revealed spectral amplitude increases at approximately 75 and approximately 78 Hz to visual deviants. GBA to visual deviants peaked 160 ms after auditory stimulus onset over posterior parietal cortex, at 270 ms over occipital areas and at 320 ms over left inferior frontal cortex. The latter GBA enhancement was consistent with the increase observed previously to pure acoustic mismatch, supporting a role of left inferior frontal cortex for the representation of perceived auditory pattern change. The preceding gamma-band changes over posterior areas may reflect processing of incongruent lip movements in visual motion areas and back-projections to earlier visual cortex.

Reciprocal modulation of neuromagnetic induced gamma activity by attention in the human visual and auditory cortex

For attentional control of behavior, the brain permanently resolves a competition between the impressions supplied by different senses. Here, using a dual-modality temporal order detection task, we studied attentional modulation of oscillatory neuromagnetic activity in the human cerebral cortex. On each trial, after simultaneous exposure to visual and auditory noise, subjects were presented with an asynchronous pair of a visual and an auditory stimulus. Either of the two stimuli could occur first equally often, their order was not cued. Subjects had to determine the leading stimulus in a pair and attentively monitor it to respond upon its offset. With the attended visual or auditory stimuli, spectral power analysis revealed marked enhancements of induced gamma activity within 250 ms post-stimulus onset over the modality-specific cortices (occipital at 64 Hz, right temporal at 53 Hz). When unattended, however, the stimuli led to a significantly decreased (beneath baseline) gamma response in these cortical regions. The gamma decreases occurred at lower frequencies ( approximately 30 Hz) than did the gamma increases. An increase in the gamma power and frequency for the attended modality and their decrease for the unattended modality suggest that attentional regulation of multisensory processing involves reciprocal changes in synchronization of respective cortical networks. We assume that the gamma decrease reflects an active suppression of the task-irrelevant sensory input. This suppression occurs at lower frequencies, suggesting an involvement of larger scale cell assemblies.

Dynamics of gamma-band activity in human magnetoencephalogram during auditory pattern working memory

Both electrophysiological research in animals and human brain imaging studies have suggested that, similar to the visual system, separate cortical ventral "what" and dorsal "where" processing streams may also exist in the auditory domain. Recently we have shown enhanced gamma-band activity (GBA) over posterior parietal cortex belonging to the putative auditory dorsal pathway during a sound location working memory task. Using a similar methodological approach, the present study assessed whether GBA would be increased over auditory ventral stream areas during an auditory pattern memory task. Whole-head magnetoencephalogram was recorded from N = 12 subjects while they performed a working memory task requiring same-different judgments about pairs of syllables S1 and S2 presented with 0.8-s delays. S1 and S2 could differ either in voice onset time or in formant structure. This was compared with a control task involving the detection of possible spatial displacements in the background sound presented instead of S2. Under the memory condition, induced GBA was enhanced over left inferior frontal/anterior temporal regions during the delay phase and in response to S2 and over prefrontal cortex at the end of the delay period. gamma-Band coherence between left frontotemporal and prefrontal sensors was increased throughout the delay period of the memory task. In summary, the memorization of syllables was associated with synchronously oscillating networks both in frontotemporal cortex, supporting a role of these areas as parts of the putative auditory ventral stream, and in prefrontal, possible executive regions. Moreover, corticocortical connectivity was increased between these structures.

Influence of working memory on patterns of motor related cortico-cortical coupling

Working memory is implicated in various higher-order cognitive operations. We hypothesized that the availability of a temporal representation in working memory would limit the extent of cortico-cortical coupling necessary to undertake a self-paced rhythmic movement. To this end we examined modulations in cortico-cortical interactions as determined by EEG coherence during a delay interval and subsequent movement reproduction. Right hand movement was initially paced by a metronome beat every 0.9 s, followed by a delay interval, after which hand movement was repeated in an unpaced manner. Movement reproduction after a long (22.5 s, corresponding to 25 movement cycles) compared to a short (5.4 s, corresponding to 6 movement cycles) delay interval was associated with an increased degree of functional coupling in the beta frequency band (12-30 Hz) of the left (movement-driving) hemisphere (F3-FC3, F3-C3 and F3-P3 connections) as well as mesial regions (FCz-FC3, FCz-C3 and Cz-FC3 connections) even though overall behavioral characteristics were not influenced. In addition, analysis of the EEG coherence in the delay period revealed a bilateral frontal network (F3-F4, F3-FC4, F4-FC3 and FC3-FC4 connections). Activity in the latter tended to be synchronized in the theta band (4-8 Hz) and was significantly less strong at 22.5 s than 5.4 s. These data suggest that working memory may be partly subserved by synchronization in a bilateral frontal network and may provide an intrinsic contextual influence that shapes the pattern of cortico-cortical interaction during a given task.

Early onset of post-movement beta electroencephalogram synchronization in the supplementary motor area during self-paced finger movement in man

A voluntary finger movement is accompanied by an event-related desynchronization followed by a short burst of beta oscillations or event-related synchronization. These beta bursts are dominant over the contralateral hand representation area, but also appear over the midcentral area overlaying the supplementary motor area (SMA) and the foot representation area. We show that the induced midcentral beta oscillations following movement-offset display not only slightly higher frequency components, but have also a significantly earlier onset. These beta oscillations arise likely from the SMA. Assuming that the short-lasting beta synchronizations at frequencies below 35 Hz after termination of a movement reflect a state of localized cortical inhibition, we propose that the induced midcentral oscillations reflect the inhibition of networks within the SMA. This assumed resetting or inhibitory process within the SMA precedes that of the networks within the primary motor hand area.

Dynamics of gamma-band activity during an audiospatial working memory task in humans

The representation of visual objects in short-term memory has been shown to be related to increased gamma-band activity in the electroencephalogram. Using a similar paradigm, we investigated oscillatory magnetoencephalographic activity in human subjects during a delayed matching-to-sample task requiring working memory of auditory spatial information. The memory task involved same-different judgments about the lateralization angle of pairs of filtered noise stimuli (S1 and S2) separated by 800 msec delays of background noise. This was compared with a control condition requiring the detection of a possible change in the background noise volume appearing instead of S2 (volume task). Statistical probability mapping revealed increased spectral activity at 59 Hz over left parietal cortex during the delay phase of the memory condition. In addition, 59 Hz coherence was enhanced between left parietal and right frontal sensors. During the end of the delay and during the presentation of S2, enhanced gamma-band activity at 67 Hz was observed over right frontal and later over midline parietal areas. In contrast, the volume task was characterized by increased left inferior frontotemporal 59 Hz spectral amplitude after S1. Apparently representation of the spatial position of a sound source is associated both with synchronization of networks in parietal areas involved in the auditory dorsal stream and with increased coupling between networks serving representation of audiospatial information and frontal executive systems. The comparison with S2 seemed to activate frontal and parietal neuronal ensembles. Gamma-band activity during the volume task may reflect auditory pattern encoding in auditory ventral stream areas.

Medium-range oscillatory network and the 20-Hz sensorimotor induced potential

Although synchronously oscillating neuronal assemblies have been the subject of many studies, a clear identification of the spatiotemporal characteristics of a medium-range oscillatory network is still lacking. Herein, we present a method for the extraction of a new waveform, namely the mean induced potential (IP), which allows the identification of the spatiotemporal characteristics of induced EEG responses. The IP calculation was applied to the 20-Hz component of the sensorimotor rhythm in order to obtain a 20-Hz sensorimotor induced potential (20-Hz SIP). The spatiotemporal characteristics of the 20-Hz bursts seen after median nerve stimulation and self-paced finger movements were extracted by means of current source density reconstruction and synchronization analysis. A cortical network including the controlateral primary motor cortex, the supplementary motor area, and the controlateral supramarginalis gyrus was found to generate the 20-Hz bursts, and the various activated areas were found to be highly synchronized. Our results demonstrate for the first time the existence of a medium-range cortical network in the human sensorimotor region whose constituents oscillate synchronously.

Dynamics of gamma-band activity induced by auditory pattern changes in humans

Increasing evidence suggests separate auditory pattern and space processing streams. The present paper describes two magnetoencephalogram studies examining gamma-band activity to changes in auditory patterns using consonant-vowel syllables (experiment 1), animal vocalizations and artificial noises (experiment 2). Two samples of each sound type were presented to passively listening subjects in separate oddball paradigms with 80% standards and 20% deviants differing in their spectral composition. Evoked magnetic mismatch fields peaking approximately 190 ms poststimulus showed a trend for a left-hemisphere advantage for syllables, but no hemispheric differences for the other sounds. Frequency analysis and statistical probability mapping of the differences between deviants and standards revealed increased gamma-band activity above 60 Hz over left anterior temporal/ventrolateral prefrontal cortex for all three types of stimuli. This activity peaked simultaneously with the mismatch responses for animal sounds (180 ms) but was delayed for noises (260 ms) and syllables (320 ms). Our results support the hypothesized role of anterior temporal/ventral prefrontal regions in the processing of auditory pattern change. They extend earlier findings of gamma-band activity over posterior parieto-temporal cortex during auditory spatial processing that supported the putative auditory dorsal stream. Furthermore, earlier gamma-band responses to animal vocalizations may suggest faster processing of fear-relevant information.

Magnetic oscillatory responses to lateralization changes of natural and artificial sounds in humans

Oscillatory signals in human magnetoencephalogram were investigated as correlates of cortical network activity in response to sound lateralization changes. Previously, we found lateralized presentations of a monosyllabic word to elicit posterior temporo-parietal gamma-band activity, possibly reflecting synchronization of neuronal assemblies in putative auditory dorsal stream areas. In addition, beta activity was decreased over sensorimotor regions, suggesting the activation of motor networks involved in orientating. The present study investigated responses to lateralization changes of both a barking dog sound and a distorted noise to test whether beta desynchronization would depend on the sound's relevance for orientating. Eighteen adults listened passively to 900 samples of each sound in separate location mismatch paradigms with midline standards and both right- and left-lateralized deviants. Lateralized distorted noises were accompanied by enhanced spectral amplitude at 58-73 Hz over right temporo-parietal cortex. Left-lateralized barking dog sounds elicited right and right-lateralized sounds elicited bilateral temporo-parietal spectral amplitude increases at approximately 77 Hz. This replicated the involvement of posterior temporo-parietal areas in auditory spatial processing. Only barking dog sounds, but not distorted noises, gave rise to 30 Hz desynchronization over contralateral sensorimotor areas, parieto-frontal gamma coherence increases and beta coherence reductions between sensorimotor and prefrontal sensors. Apparently passive listening to lateralized natural sounds with a potential biological relevance led to an activation of motor networks involved in the automatic preparation for orientating. Parieto-frontal coherence increases may reflect enhanced coupling of networks involved in the integration of auditory spatial and motor processes.

Human large-scale oscillatory brain activity during an operant shaping procedure

The present study aimed at examining the oscillatory brain-electric correlates of human operant learning using high-density electroencephalography (EEG). Induced gamma-band activity (GBA) was studied using a fixed-interval reinforcement schedule with a variable limited hold period, which was decreased depending on response accuracy. Thus, participants' behavior was shaped during the course of the learning session. After each response, numbers indicating the money value of that response served as reinforcing stimuli. Random reinforcement and self-paced button pressing without reinforcement were added as control conditions. GBA around 40 Hz was enhanced at posterior electrodes in response to visual feedback stimuli during shaping and random reward compared to the self-paced pressing condition where no visual feedback was provided. Furthermore, shaping was associated with a pronounced left frontal lower gamma (20-30 Hz) increase in response to feedback stimuli, whereas this pattern was not observed in the random reinforcement and self-paced pressing conditions. The present findings are in line with the notion that macroscopic high-frequency dynamics of neuronal cell assemblies may be regarded as a mechanism involved in learning and memory formation.

Right-hemisphere dominance for the processing of sound-source lateralization

Cortical processing of change in direction of a perceived sound source was investigated in 12 human subjects using whole-head magnetoencephalography. The German word "da" was presented either with or without 0.7 msec interaural time delays to create the impression of right- or left-lateralized or midline sources, respectively. Midline stimuli served as standards, and lateralized stimuli served as deviants in a mismatch paradigm. Two symmetrically linked dipoles fitted to the mismatch fields showed stronger moments in the hemisphere contralateral to the side of the deviant. The right dipole displayed equal latencies to both left and right deviants, whereas left dipole latencies were longer for ipsilateral than contralateral deviants. Frequency analysis between 20-70 Hz and statistical probability mapping revealed increased induced gamma-band activity at 53+/-2.5 Hz to both types of deviants. Right deviants elicited spectral amplitude enhancements in this frequency range, peaking at latencies of 160 and 240 msec. These effects were localized bilaterally over the angular gyri and posterior temporal regions. Coherence analysis suggested the existence of two separate interhemispheric networks. For left-lateralized deviants, both spectral amplitude enhancements at 110 and 220 msec and coherence increases were restricted to the right hemisphere. In conclusion, both mismatch dipole latencies at the supratemporal plane and gamma-band activity in posterior parietotemporal areas suggested a right hemisphere engagement in the processing of bidirectional sound-source shifts. In contrast, left-hemisphere regions responded predominantly to contralateral events. These findings may help to elucidate phenomena such as unilateral auditory neglect.