Multiplicity in the high-frequency signals during the short-latency somatosensory evoked cortical activity in humans

Modulation of somatosensation by transcranial magnetic stimulation over somatosensory cortex: a systematic review

Over the last three decades, transcranial magnetic stimulation (TMS) has gained popularity as a tool to modulate human somatosensation. However, the effects of different stimulation types on the multiple distinct subdomains of somatosensation (e.g., tactile perception, proprioception and pain) have not been systematically compared. This is especially notable in the case of newer theta-burst stimulation protocols now in widespread use. Here, we aimed to systematically and critically review the existing TMS literature and provide a complete picture of current knowledge regarding the role of TMS in modulating human somatosensation across stimulation protocols and somatosensory domains. Following the PRISMA guidelines, fifty-four studies were included in the current review and were compared based on their methodologies and results. Overall, findings from these studies provide evidence that different types of somatosensation can be both disrupted and enhanced by targeted stimulation of specific somatosensory areas. Some mixed results, however, were reported in the literature. We discussed possible reasons for these mixed results, methodological limitations of existing investigations, and potential avenues for future research.

Evidence for prefrontal cortex hypofunctioning in schizophrenia through somatosensory evoked potentials

Patients with schizophrenia (SCZ) exhibit a variety of symptoms related to altered processing of somatosensory information. Little is known, however, about the neural substrates underlying somatosensory impairments in SCZ. This study endeavored to evaluate somatosensory processing in patients with SCZ compared to healthy individuals by generating somatosensory evoked potentials through stimulation of the right median nerve. The median nerve was stimulated by a peripheral nerve stimulator in 34 SCZ and 33 healthy control (HC) participants. The peripheral nerve stimulus (PNS) intensity was adjusted to 300 percent of sensory threshold and delivered at 0.1 Hz. The EEG data were acquired through 64-channels per 10-20 montage. We collected and averaged 100 trials and the recording electrodes of interest were the F3/F5 electrodes representing the dorsolateral prefrontal cortex (DLPFC) and C3/CP3 representing the somatosensory cortex (S1). In response to PNS, SCZ participants experienced over the DLPFC N30 amplitude that was significantly smaller than that of HC participants. By contrast, S1 N20 was of similar amplitude between the two groups. In addition, we found an association between N20 and N30 amplitudes in SCZ but not in HC participants. Our findings suggest that patients with SCZ demonstrate aberrant processing of somatosensory activation by the DLPFC locally and not due to a connectivity disruption between S1 and DLPFC. These results could help to develop a model through which to DLPFC hypofunctioning could be studied. Our findings may also help to identify a potential biological target to treat somatosensory information processing related deficits in SCZ.

Primary somatosensory contextual modulation is encoded by oscillation frequency change

OBJECTIVE

This study characterized thalamo-cortical communication by assessing the effect of context-dependent modulation on the very early somatosensory evoked high-frequency oscillations (HF oscillations).

METHODS

We applied electrical stimuli to the median nerve together with an auditory oddball paradigm, presenting standard and deviant target tones representing differential cognitive contexts to the constantly repeated electrical stimulation. Median nerve stimulation without auditory stimulation served as unimodal control.

RESULTS

A model consisting of one subcortical (near thalamus) and two cortical (Brodmann areas 1 and 3b) dipolar sources explained the measured HF oscillations. Both at subcortical and the cortical levels HF oscillations were significantly smaller during bimodal (somatosensory plus auditory) than unimodal (somatosensory only) stimulation. A delay differential equation model was developed to investigate interactions within the 3-node thalamo-cortical network. Importantly, a significant change in the eigenfrequency of Brodmann area 3b was related to the context-dependent modulation, while there was no change in the network coupling.

CONCLUSION

This model strongly suggests cortico-thalamic feedback from both cortical Brodmann areas 1 and 3b to the thalamus. With the 3-node network model, thalamo-cortical feedback could be described.

SIGNIFICANCE

Frequency encoding plays an important role in contextual modulation in the somatosensory thalamo-cortical network.

Detecting millisecond-range coupling delays between brainwaves in terms of power correlations by magnetoencephalography

BACKGROUND

The spatiotemporal coupling of brainwaves is commonly quantified using the amplitude or phase of signals measured by electro- or magnetoencephalography (EEG/MEG). To enhance the temporal resolution for coupling delays down to millisecond level, a new power correlation (PC) method is proposed and tested.

NEW METHOD

The cross-correlations of any two brainwave powers at two locations are calculated sequentially through a measurement using the convolution theorem. For noise suppression, the cross-correlation series is moving-average filtered, preserving the millisecond resolution in the cross-correlations, but with reduced noise. The coupling delays are determined from the delays of the cross-correlation peaks.

RESULTS

Simulations showed that the new method detects reliably power cross-correlations with millisecond accuracy. Moreover, in MEG measurements on three healthy volunteers, the method showed average alpha-alpha coupling delays of around 0-20 ms between the occipital areas of two hemispheres. Lower-frequency brainwaves vs. alpha waves tended to have a larger lag; higher-frequency waves vs. alpha waves showed delays with large deviations.

COMPARISON WITH EXISTING METHODS

The use of signal power instead of its square root (amplitude) in the cross-correlations improves noise cancellation. Compared to signal phase, the signal power analysis time delays do not have periodic ambiguity. In addition, the novel method allows fast calculation of cross-correlations.

CONCLUSIONS

The PC method conveys novel information about brainwave dynamics. The method may be extended from sensor-space to source-space analysis, and can be applied also for electroencephalography (EEG) and local field potentials (LFP).

Recording human cortical population spikes non-invasively--An EEG tutorial

BACKGROUND

Non-invasively recorded somatosensory high-frequency oscillations (sHFOs) evoked by electric nerve stimulation are markers of human cortical population spikes. Previously, their analysis was based on massive averaging of EEG responses. Advanced neurotechnology and optimized off-line analysis can enhance the signal-to-noise ratio of sHFOs, eventually enabling single-trial analysis.

METHODS

The rationale for developing dedicated low-noise EEG technology for sHFOs is unfolded. Detailed recording procedures and tailored analysis principles are explained step-by-step. Source codes in Matlab and Python are provided as supplementary material online.

RESULTS

Combining synergistic hardware and analysis improvements, evoked sHFOs at around 600 Hz ('σ-bursts') can be studied in single-trials. Additionally, optimized spatial filters increase the signal-to-noise ratio of components at about 1 kHz ('κ-bursts') enabling their detection in non-invasive surface EEG.

CONCLUSIONS

sHFOs offer a unique possibility to record evoked human cortical population spikes non-invasively. The experimental approaches and algorithms presented here enable also non-specialized EEG laboratories to combine measurements of conventional low-frequency EEG with the analysis of concomitant cortical population spike responses.

Influence of spiking activity on cortical local field potentials

The intra-cortical local field potential (LFP) reflects a variety of electrophysiological processes including synaptic inputs to neurons and their spiking activity. It is still a common assumption that removing high frequencies, often above 300 Hz, is sufficient to exclude spiking activity from LFP activity prior to analysis. Conclusions based on such supposedly spike-free LFPs can result in false interpretations of neurophysiological processes and erroneous correlations between LFPs and behaviour or spiking activity. Such findings might simply arise from spike contamination rather than from genuine changes in synaptic input activity. Although the subject of recent studies, the extent of LFP contamination by spikes is unclear, and the fundamental problem remains. Using spikes recorded in the motor cortex of the awake monkey, we investigated how different factors, including spike amplitude, duration and firing rate, together with the noise statistic, can determine the extent to which spikes contaminate intra-cortical LFPs. We demonstrate that such contamination is realistic for LFPs with a frequency down to ∼10 Hz. For LFP activity below ∼10 Hz, such as movement-related potential, contamination is theoretically possible but unlikely in real situations. Importantly, LFP frequencies up to the (high-) gamma band can remain unaffected. This study shows that spike–LFP crosstalk in intra-cortical recordings should be assessed for each individual dataset to ensure that conclusions based on LFP analysis are valid. To this end, we introduce a method to detect and to visualise spike contamination, and provide a systematic guide to assess spike contamination of intra-cortical LFPs.

Towards non-invasive multi-unit spike recordings: mapping 1kHz EEG signals over human somatosensory cortex

OBJECTIVE

Scalp-derived human somatosensory evoked potentials (SEPs) contain high-frequency oscillations (600 Hz; 'sigma-burst') reflecting concomitant bursts of spike responses in primary somatosensory cortex that repeat regularly at 600 Hz. Notably, recent human intracranial SEP have revealed also 1 kHz responses ('kappa-burst'), possibly reflecting non-rhythmic spiking summed over multiple cells (MUA: multi-unit activity). However, the non-invasive detection of EEG signals at 1 kHz typical for spikes has always been limited by noise contributions from both, amplifier and body/electrode interface. Accordingly, we developed a low-noise recording set-up optimised to map non-invasively 1 kHz SEP components.

METHODS

SEP were recorded upon 4 Hz left median nerve stimulation in 6 healthy human subjects. Scalp potentials were acquired inside an electrically and magnetically shielded room using low-noise custom-made amplifiers. Furthermore, in order to reduce thermal Johnson noise contributions from the sensor/skin interface, electrode impedances were adjusted to ≤ 1 kΩ. Responses averaged after repeated presentation of the stimulus (n=4000 trials) were evaluated by spatio-temporal pattern analyses in complementary spectral bands.

RESULTS

Three distinct spectral components were identified: N20 (<100 Hz), sigma-burst (450-750 Hz), and kappa-burst (850-1200 Hz). The two high-frequency bursts (sigma, kappa) exhibited distinct and partially independent spatiotemporal evolutions, indicating subcortical as well as several cortical generators.

CONCLUSIONS

Using a dedicated low-noise set-up, human SEP 'kappa-bursts' at 1 kHz can be non-invasively detected and their scalp distribution be mapped. Their topographies indicate a set of subcortical/cortical generators, at least partially distinct from the topography of the 600 Hz sigma-bursts described previously.

SIGNIFICANCE

The non-invasive detection and surface mapping of 1 kHz EEG signals presented here provides an essential step towards non-invasive monitoring of multi-unit spike activity.

High frequency oscillations evoked by peripheral magnetic stimulation

The analysis of somatosensory evoked potentials (SEP) and / or fields (SEF) is a well-established and important tool for investigating the functioning of the peripheral and central human nervous system. A standard technique to evoke SEPs / SEFs is the stimulation of the median nerve by using a bipolar electrical stimulus. We aim at an alternative stimulation technique enabling stimulation of deep nerve structures while reducing patient stress and error susceptibility. In the current study, we apply a commercial transcranial magnetic stimulation system for peripheral magnetic stimulation of the median nerve. We compare the results of simultaneously recorded EEG signals to prove applicability of our technique to evoke SEPs including low frequency components (LFC) as well as high frequency oscillations (HFO). Therefore, we compare amplitude, latency and time-frequency characteristics of the SEP of 14 healthy volunteers after electric and magnetic stimulation. Both low frequency components and high frequency oscillations were detected. The HFOs were superimposed onto the primary cortical response N20. Statistical analysis revealed significantly lower amplitudes and increased latencies for LFC and HFO components after magnetic stimulation. The differences indicate the inability of magnetic stimulation to elicit supramaximal responses. A psycho-perceptual evaluation showed that magnetic stimulation was less unpleasant for 12 out of the 14 volunteers. In conclusion, we showed that LFC and HFO components related to median nerve stimulation can be evoked by peripheral magnetic stimulation.

BA3b and BA1 activate in a serial fashion after median nerve stimulation: direct evidence from combining source analysis of evoked fields and cytoarchitectonic probabilistic maps

This study combines source analysis imaging data for early somatosensory processing and the probabilistic cytoarchitectonic maps (PCMs). Human somatosensory evoked fields (SEFs) were recorded by stimulating left and right median nerves. Filtering the recorded responses in different frequency ranges identified the most responsive frequency band. The short-latency averaged SEFs were analyzed using a single equivalent current dipole (ECD) model and magnetic field tomography (MFT). The identified foci of activity were superimposed with PCMs. Two major components of opposite polarity were prominent around 21 and 31 ms. A weak component around 25 ms was also identified. For the most responsive frequency band (50-150 Hz) ECD and MFT revealed one focal source at the contralateral Brodmann area 3b (BA3b) at the peak of N20. The component ~25 ms was localised in Brodmann area 1 (BA1) in 50-150 Hz. By using ECD, focal generators around 28-30 ms located initially in BA3b and 2 ms later to BA1. MFT also revealed two focal sources - one in BA3b and one in BA1 for these latencies. Our results provide direct evidence that the earliest cortical response after median nerve stimulation is generated within the contralateral BA3b. BA1 activation few milliseconds later indicates a serial mode of somatosensory processing within cytoarchitectonic SI subdivisions. Analysis of non-invasive magnetoencephalography (MEG) data and the use of PCMs allow unambiguous and quantitative (probabilistic) interpretation of cytoarchitectonic identity of activated areas following median nerve stimulation, even with the simple ECD model, but only when the model fits the data extremely well.

Very high-frequency oscillations (over 1000 Hz) of somatosensory-evoked potentials directly recorded from the human brain

The aims of this study were to record high-frequency oscillations (HFOs) associated with somatosensory-evoked potentials from subdural electrodes and to investigate their generators and clinical significance. Six patients who underwent long-term subdural electrode monitoring were studied. Somatosensory-evoked potentials were recorded directly from the subdural electrode after stimulation of the median nerve. Bandpass filter was 10 to 10,000 Hz for conventional somatosensory-evoked potential and 500 to 10,000 Hz for HFO. Three types of HFO were recorded. The first component was early HFO (407-926 Hz), which occurred before N20 peak. The second component was late HFO (408-909 Hz), which occurred after N20 peak. In addition, a novel component was recorded with a range from 1,235 to 2,632 Hz, and this component was termed very HFO. Early and late HFOs were recorded from relatively wide areas centering around the primary motor and primary sensory areas, whereas very HFO was localized around the primary sensory areas. In this study, at least three components of HFO could be identified. Only very HFO was localized around primary sensory areas, suggesting a possibility that very HFO may provide an effective method of identifying the central sulcus.

The direct relationship between inhibitory currents and local field potentials

The frequency profiles of various extracellular field oscillations are known to reflect functional brain states, yet we lack detailed explanations of how these brain oscillations arise. Of particular clinical relevance are the high-frequency oscillations (HFOs) associated with interictal events and the onset of seizures. These time periods are also when pyramidal firing appears to be vetoed by high-frequency volleys of inhibitory synaptic currents, thereby providing an inhibitory restraint that opposes epileptiform spread (Trevelyan et al., 2006, 2007). The pattern and timing of this inhibitory volley is suggestive of a causal relationship between the restraint and HFOs. I show that at these times, isolated inhibitory currents from single pyramidal cells have a similarity to the extracellular signal that significantly exceeds chance. The ability to extrapolate from discrete currents in single cells to the extracellular signal arises because these inhibitory currents are synchronized in local populations of pyramidal cells. The visibility of these inhibitory currents in the field recordings is greatest when local pyramidal activity is suppressed: the correlation between the inhibitory currents and the field signal becomes worse when local activity increases, suggestive of a switch from one source of HFO to another as the restraint starts to fail. This association suggests that a significant component of HFOs reflects the last act of defiance in the face of an advancing ictal event.

MEG's ability to localise accurately weak transient neural sources

OBJECTIVE

To investigate the accurate localisation of weak, transient, neural sources under conditions of varying difficulty.

METHODS

Multiple dipolar sources placed within a head-shaped phantom at superficial and deep locations were driven separately or simultaneously by a short-lasting current with varied amplitudes. Artificial MEG signals that were very similar to the human High Frequency Oscillations (HFO) were produced. MEG signals of HFO were also recorded from median nerve stimulation. Different inverse techniques were used to localise the phantom dipoles and the human HFO generators.

RESULTS

The human HFO were measured around 200 and 600Hz by using only 120 trials. The 200Hz HFO were localised to BA3b. The superficial phantom's source was localised with an accuracy of 2-3mm by all inverse techniques (120 trials). The 'subcortical' source was localised with an error of approximately 5mm. Localisation of deeper 'thalamic' sources required more trials.

CONCLUSION

MEG can detect and localise weak transient activations and the human HFO with an accuracy of a few mm at cortical and subcortical regions even when a small number of trials are used.

SIGNIFICANCE

Localizing HFO to specific anatomical structures has high clinical utility, for example in epilepsy, where discrete HFO appears to be generated just before focal epileptic activity.

Spatial characterization of interictal high frequency oscillations in epileptic neocortex

Interictal high frequency oscillations (HFOs), in particular those with frequency components in excess of 200 Hz, have been proposed as important biomarkers of epileptic cortex as well as the genesis of seizures. We investigated the spatial extent, classification and distribution of HFOs using a dense 4 × 4 mm² two dimensional microelectrode array implanted in the neocortex of four patients undergoing epilepsy surgery. The majority (97%) of oscillations detected included fast ripples and were concentrated in relatively few recording sites. While most HFOs were limited to single channels, ∼10% occurred on a larger spatial scale with simultaneous but morphologically distinct detections in multiple channels. Eighty per cent of these large-scale events were associated with interictal epileptiform discharges. We propose that large-scale HFOs, rather than the more frequent highly focal events, are the substrates of the HFOs detected by clinical depth electrodes. This feature was prominent in three patients but rarely seen in only one patient recorded outside epileptogenic cortex. Additionally, we found that HFOs were commonly associated with widespread interictal epileptiform discharges but not with locally generated ‘microdischarges’. Our observations raise the possibility that, rather than being initiators of epileptiform activity, fast ripples may be markers of a secondary local response.

Modelling of cortical and thalamic 600 Hz activity by means of oscillatory networks

The purpose of this study is to investigate information processing in the primary somatosensory system with the help of oscillatory network modelling. Specifically, we consider interactions in the oscillatory 600Hz activity between the thalamus and the cortical Brodmann areas 3b and 1. This type of cortical activity occurs after electrical stimulation of peripheral nerves such as the median nerve. Our measurements consist of simultaneous 31-channel MEG and 32-channel EEG recordings and individual 3D MRI data. We perform source localization by means of a multi-dipole model. The dipole activation time courses are then modelled by a set of coupled oscillators, described by linear second-order ordinary delay differential equations (DDEs). In particular, a new model for the thalamic activity is included in the oscillatory network. The parameters of the DDE system are successfully fitted to the data by a nonlinear evolutionary optimization method. To activate the oscillatory network, an individual input function is used, based on measurements of the propagated stimulation signal at the biceps. A significant feedback from the cortex to the thalamus could be detected by comparing the network modelling with and without feedback connections. Our finding in humans is supported by earlier animal studies. We conclude that this type of rhythmic brain activity can be modelled by oscillatory networks in order to disentangle feed forward and feedback information transfer.

Nonlinear interactions of high-frequency oscillations in the human somatosensory system

OBJECTIVE

The source of somatosensory evoked high-frequency activity at about 600 Hz is still not completely clear. Hence, we aimed to study the influence of double stimulation on the human somatosensory system by analyzing both the low-frequency activity and the high-frequency oscillations (HFOs) at about 600 Hz.

METHODS

We used median nerve stimulation at seven interstimuli intervals (ISIs) with a high time resolution between 2.4 and 4.8 ms to investigate the N15, N20 and superimposed HFOs. Simultaneously, the electroencephalogram and the magnetoencephalogram of 12 healthy participants were recorded. Subsequently, the source analysis of precortical and cortical dipoles was performed.

RESULTS

The difference computations of precortical dipole activation curves showed in both the low- and high-frequency range a correlation between the ISI and the latency of the second stimulus response. The cortical low-frequency response showed a similar behavior. Contrarily, in the second response of cortical HFOs this latency shift could not be confirmed. We found amplitude fluctuations that were dependent on the ISI in the low-frequency activity and the HFOs. These nonlinear interactions occurred at ISIs, which differ by one full HFO period (1.6 ms).

CONCLUSIONS

Low-frequency activity and HFOs originate from different generators. Precortical and cortical HFOs are independently generated. The amplitude fluctuations dependent on ISI indicate nonlinear interference between successive stimuli.

SIGNIFICANCE

Information processing in human somatosensory system includes nonlinearity.

Exploring the variability of single trials in somatosensory evoked responses using constrained source extraction and RMT

This paper describes the theoretical background of a new data-driven approach to encephalographic single-trial (ST) data analysis. Temporal constrained source extraction using sparse decomposition identifies signal topographies that closely match the shape characteristics of a reference signal, one response for each ST. The correlations between these ST topographies are computed for formal Correlation Matrix Analysis (CMA) based on Random Matrix Theory (RMT). The RMT-CMA provides clusters of similar ST topologies in a completely unsupervised manner. These patterns are then classified into deterministic set and noise using well established RMT results. The efficacy of the method is applied to EEG and MEG data of somatosensory evoked responses (SERs). The results demonstrate that the method can recover brain signals with time course resembling the reference signal and follow changes in strength and/or topography in time by simply stepping the reference signal through time.

Modulation of high-frequency (600 Hz) somatosensory-evoked potentials after rTMS of the primary sensory cortex

Somatosensory inputs to the primary sensory cortex (S1) after median nerve stimulation include temporally overlapping parallel processing, as reflected by standard low-frequency somatosensory-evoked potentials (LF-SEPs) and high-frequency SEPs (HF-SEPs), the latter being more sensitive to arousal and to other rapid adaptive changes. Experimental data suggest that cortical HF-SEPs are formed by two successive pre- and postsynaptic components, respectively, generated in the terminal part of thalamo-cortical radiation (early burst) and in specialized neuronal pools within S1 (later burst). In eight healthy subjects, slow (1 Hz) or rapid (10 Hz) repetitive transcranial magnetic stimulations (rTMS), which are known to induce opposite changes on cortical excitability, applied on S1 did not modify LF-SEPs, while HF-SEPs showed a series of dissociate changes in the early and later high-frequency burst, moreover occurring with a different time-course. Slow rTMS caused an immediate and lasting decrease of the later burst activity, coupled with an immediate increase of the earlier part of the burst, suggesting that inhibition of cortical excitability triggered opposite, compensatory effects at subcortical levels; rapid rTMS induced a delayed increase of later HF-SEPs, leaving unaltered the earlier subcortical burst. Findings causally demonstrate that LF- and HF-SEPs reflect two distinct functional pathways for somatosensory input processing, and that early and late high-frequency burst do actually reflect the activity of different generators, as suggested by experimental data. Possible underlying neurophysiological phenomena are discussed.

Identifying mutual information transfer in the brain with differential-algebraic modeling: Evidence for fast oscillatory coupling between cortical somatosensory areas 3b and 1

Understanding information transfer in the brain is a major challenge in today's neurosciences. Commonly, information transfer between brain areas is analyzed with the help of correlation measures for electrophysiological data. However, such approaches cannot distinguish between mutual coupling and other mechanisms of creating correlations between responses, such as common input from other sources. Functional coupling is mandatory for information transfer. Here we propose to analyze coupling between active brain areas with the help of models described by a system of differential-algebraic equations. Comparing models with various degrees of coupling, we show that mutual information transfer can be distinguished from one-way information transfer for activated cortical areas estimated by source localization techniques. We exemplify the technique with fast oscillatory activity found in both cortical areas 3b and 1 after peripheral nerve stimulation.

Human high frequency somatosensory evoked potential components are refractory to circadian modulations of tonic alertness

The impact of vigilance states, such as sleep or arousal changes, on the high-frequency (600 Hz) components (HFOs) of somatosensory evoked potentials (SEPs) is known. The present study sought to characterize the effects of circadian fluctuations of tonic alertness on HFOs in awake humans. Median nerve SEPs were recorded at four times during a 24-hour waking period. In parallel to the SEP recordings, a reaction-time (RT) task was performed to assess tonic alertness. Additionally, the spontaneous EEG was monitored. The low-frequency SEP component N20 and the early and late HFO parts did not change across the measurement sessions. In contrast, RTs were clearly prolonged at night and on the second morning. EEG also showed increased delta power at night. HFOs are sensitive to pronounced vigilance changes, such as sleep, but are refractory to fluctuations of tonic alertness. Tonic alertness is regarded to be the top-down cognitive control mechanism of wakefulness, whereas sleep is mediated by overwhelming bottom-up regulation, which seems apparently more relevant for, at least in part, subcortically triggered high-frequency burst generation in the ascending somatosensory system.

Magnetoencephalography as a research tool in neuroscience: state of the art

Magnetoencephalography (MEG) is a noninvasive neuroimaging method for detecting, analyzing, and interpreting the magnetic field generated by the electrical activity in the brain. Modern hardware can capture the MEG signal at hundreds of points around the head in a snapshot lasting only a fraction of a millisecond. The sensitivity of modern hardware is high enough to permit the extraction of a clean signal generated by the brain well above the noise level of the MEG hardware. It is possible to identify signatures of superficial and often deep generators in the raw MEG signal, even in snapshots of data. In a more quantitative way, tomographic images of the electrical current density in the brain can be extracted from each snapshot of MEG signal, providing a direct correlate of coherent collective neuronal activity. A number of recent studies have scrutinized brain function in the new spatiotemporal window that real-time tomographic analysis of MEG signals has opened. The results have allowed the variability in a single area to be seen in the context of activity in other areas and background rhythmic activity. In this view, normal brain function is seen as a cascade of extremely fast events and the unfolding of specialized processes, segregated in space and time and organized into well-defined stages of processing.

High frequency oscillations in the somatosensory evoked potentials (SSEP's) are mainly due to phase-resetting phenomena

A small series of high frequency oscillations (HFOs) overlapping the earliest part of the N20 wave can be observed in the somatosensory evoked potentials (SSEPs) of normal subjects. We tried to elucidate whether these high frequency components are mainly due to phase-resetting phenomena, to the emergence of new oscillations related to the stimuli, or to a combination of both. Averaged median-nerve SSEPs from seven healthy subjects were studied by means of time-frequency analysis. The presence of new oscillatory activities was evaluated by averaging the energy of the single-trial time-frequency transforms in the HFOs range (400-1000 Hz). To study phase-resetting phenomena, we measured inter-trial coherence (ITC) in the same frequency range. A marked inter-trial coherence related to the HFOs was found, whereas energy changes (related to the emergence of new oscillations) were minimal. The combination of these three different approaches suggests that the HFOs are mainly due to resettings of the ongoing EEG activity originated in response to the stimuli. The emergence of new activities does not seem to be a relevant mechanism in the formation of these components.

High-Frequency Oscillations in the Somatosensory Evoked Potentials of Patients With Cortical Myoclonus: Pathophysiologic Implications

SUMMARY

A small series of high-frequency wavelets overlapping the earliest part of the N20 wave (high-frequency oscillations, HFOs) can be observed in the somatosensory evoked potentials (SSEPs) of normal subjects after filtering then with a high-pass filter (>500 Hz). These HFOs have been related to interneuronal activity in the primary somatosensory cortex. In patients with cortical myoclonus there is a sensorimotor cortical hyperexcitability, expressed neurophysiologically as high-amplitude waves in the SSEPs (giant SSEPs). There have been contradicting reports in the literature on the changes in the HFOs in these patients. The authors studied HFOs in a group of 20 patients with cortical myoclonus of different origins and in a control group by means of time-frequency transforms, comparing the results obtained with the amplitude and latency of the classical SSEP waves. All controls had normal HFOs, with two components. Nine patients had no HFOs, nine patients had low-amplitude and/or delayed HFOs, and the remaining two patients, the only without ataxia, had high-amplitude HFOs with a long latency. These results suggest heterogeneity in the pathophysiology of cortical myoclonus, which might be related to the different systems affected.

Widely distributed magnetoencephalography spikes related to the planning and execution of human saccades

With sufficiently fast data sampling, ubiquitous sharp transients appear in magnetoencephalography (MEG) data. Initially, no known collective neuronal activity could explain MEG signal generation well above 100 Hz, so it was assumed that these transients were entirely composed of background electronic noise that could be eliminated by filtering and averaging. Recent studies at the cellular level provided evidence for synchronous synaptic input to dendrites and volleys of near-simultaneous action potentials. MEG studies have also identified high-frequency oscillations well above 200 Hz after averaging large number of somatosensory evoked responses. In this study, we searched for evidence of high-frequency neuronal activity in the raw MEG signal using the highest sampling rate available with our hardware. Two human subjects participated in three experiments using visual cues to define planning, preparation, and execution or inhibition of saccades. Tomographic analysis identified "MEG spikes" that were widely distributed across the cortex, cerebellum, and brainstem during cue presentations and saccades. Here we demonstrate how these MEG spikes can be recorded and localized in real time and show that task demands influence their properties. The MEG spikes were organized into feedforward and corollary discharge sequences that could, when combined with the slower activity-linked processing in discrete brain areas over long periods, lasting hundreds of milliseconds. Preparation for impending saccade began as soon as relevant information became available. Cues providing partial information initiated competing motor programs for as yet undecided future actions that were maintained until cues with new information resolved the uncertainty.

Effects of temporary bilateral ligation of the internal carotid arteries on the low- and high-frequency somatic evoked potentials in the swine

OBJECTIVE

We studied effects of a temporary bilateral ligation of the internal carotid arteries on the subcortical and cortical structures of the somatosensory system by examining the thalamic input and postsynaptic cortical responses contained in the somatic evoked potentials (SEPs) recorded from the primary somatosensory cortex (SI) of the juvenile piglets in vivo. We predicted that the ligation should differentially affect these structures due to differences in blood supply.

METHODS

The SEPs between 1 and 3000 Hz were measured in the SI cortex with a multichannel electrode array before, during and after a 20 min bilateral ligation of the internal carotid arteries in the swine under a barbiturate anesthesia.

RESULTS

The ligation differentially affected the thalamic input and the cortical responses contained in the high-frequency signals (HFSs) between 400 and 2000 Hz. The amplitude of the thalamic input did not change, but the amplitudes of the cortical HFS postsynaptic to the thalamic inputs decreased immediately after start of ligation, recovering over the next 30-90 min. The latency showed a small, but significant increase for several minutes after the start of ligation for both the thalamic input and cortical responses. The ligation increased the latency and reduced the amplitude of the peak of the first cortical response in the wideband SEP corresponding to human N20.

CONCLUSIONS

The HFS is useful for distinguishing selective effects of the temporary ligation on the subcortical and cortical structures of the somatosensory system. Since the porcine N20 starts after the presynaptic HFS, it was not useful in differentiating thalamic and cortical effects.

SIGNIFICANCE

The HFS may open a new window in studying the cortical physiology in humans.

High-frequency signals (> 400 hz): a new window in electrophysiological analysis of the somatosensory system

High-frequency signals (HFSs) between 400-1500 Hz in Magnetoencephalography (MEG) and Electroencephalography (EEG) provide a new window in electrophysiological analysis of the somatosensory system in humans and in other animals. The HFS in the primary somatosensory (SI) cortex precedes the conventional N20. In the swine model, they appear to be due to spiking in thalamocortical axonal terminals and in the soma and dendrites of cortical neurons. These spiking activities seem to activate slower conductances in the pyramidal cells in layers II-III and V, which give rise to N20. The HFS monitoring may be useful for separately evaluating the electrophysiology of the subcortical and cortical components of the somatosensory pathway.

Methods for parameter identification in oscillatory networks and application to cortical and thalamic 600 Hz activity

Directed information transfer in the human brain occurs presumably by oscillations. As of yet, most approaches for the analysis of these oscillations are based on time-frequency or coherence analysis. The present work concerns the modeling of cortical 600 Hz oscillations, localized within the Brodmann Areas 3b and 1 after stimulation of the nervus medianus, by means of coupled differential equations. This approach leads to the so-called parameter identification problem, where based on a given data set, a set of unknown parameters of a system of ordinary differential equations is determined by special optimization procedures. Some suitable algorithms for this task are presented in this paper. Finally an oscillatory network model is optimally fitted to the data taken from ten volunteers.

Time-frequency filtering of MEG signals with matching pursuit

Time-frequency signal analysis based on various decomposition techniques is widely used in biomedical applications. Matching Pursuit is a new adaptive approach for time-frequency decomposition of such biomedical signals. Its advantage is that it creates a concise signal approximation with the help of a small set of Gabor atoms chosen iteratively from a large and redundant set. In this paper, the usage of Matching Pursuit for time-frequency filtering of biomagnetic signals is proposed. The technique was validated on artificial signals and its performance was tested for varying signal-to-noise ratios using both simulated and real MEG somatic evoked magnetic field data.

Origins of the somatic N20 and high-frequency oscillations evoked by trigeminal stimulation in the piglets

OBJECTIVE

In humans, the somatic evoked potentials (SEPs) and magnetic fields (SEFs) elicited by peripheral nerve stimulation contain high-frequency oscillations (HFOs) around 600 Hz superimposed on the initial cortical response N20. Responses elicited by snout stimulation in the swine also contain similar HFOs during the rising phase of the porcine N20. This study examined the generators of the N20 and HFOs in the swine.

METHODS

We recorded intracortical SEPs and multi-unit activities in the sulcal area of the primary somatosensory cortex (SI) simultaneously with SEFs. The laminar profiles of the potential and current-source-density (CSD) were analyzed.

RESULTS

The CSD analysis revealed that the N20 was produced by two dipolar generators, both directed toward the cortical surface. After the arrival of the initial thalamocortical volley in layer IV, the sink of the first generator shifted toward shallower layers II-III with a velocity of 0.109+/-0.038 m/s (mean+/-SD). The sink of the second generator moved to layer V. The initial thalamocortical axonal component of the HFO was produced by repolarizing current with the sink in layer IV. The CSD laminar profile of the postsynaptic component was very similar to the profile of intracortical N20. The current sink within each cycle of HFO propagated upward with a velocity of 0.633+/-0.189 m/s, indicating backpropagation.

CONCLUSIONS

We propose that the N20 is generated by two sets of excitatory neurons which also produce the HFOs. Although the loci of synaptic inputs are unknown, these neurons appear to fire initially in the soma and produce backpropagating spikes toward distal apical dendrites.

SIGNIFICANCE

These conclusions relate the N20 to the HFO and provide a new explanation of how the current underlying the N20 is invariantly directed toward superficial layers across species.

An integrative MEG-fMRI study of the primary somatosensory cortex using cross-modal correspondence analysis

We develop a novel approach of cross-modal correspondence analysis (CMCA) to address whether brain activities observed in magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) represent a common neuronal subpopulation, and if so, which frequency band obtained by MEG best fits the common brain areas. Fourteen adults were investigated by whole-head MEG using a single equivalent current dipole (ECD) and synthetic aperture magnetometry (SAM) approaches and by fMRI at 1.5 T using linear time-invariant modeling to generate statistical maps. The same somatosensory stimulus sequences consisting of tactile impulses to the right sided: digit 1, digit 4 and lower lip were used in both neuroimaging modalities. To evaluate the reproducibility of MEG and fMRI results, one subject was measured repeatedly. Despite different MEG dipole locations and locations of maximum activation in SAM and fMRI, CMCA revealed a common subpopulation of the primary somatosensory cortex, which displays a clear homuncular organization. MEG activity in the frequency range between 30 and 60 Hz, followed by the ranges of 20-30 and 60-100 Hz, explained best the defined subrepresentation given by both MEG and fMRI. These findings have important implications for improving and understanding of the biophysics underlying both neuroimaging techniques, and for determining the best strategy to combine MEG and fMRI data to study the spatiotemporal nature of brain activity.

Different origins of low- and high-frequency components (600 Hz) of human somatosensory evoked potentials

OBJECTIVE

Human median nerve somatosensory evoked potentials (SEPs) contain a low-amplitude (<500 nV) high-frequency (approximately 600 Hz) burst of repetitive wavelets (HFOs) which are superimposed onto the primary cortical response 'N20.' This study aimed to further clarify the cortical and subcortical structures involved in the generation of the HFOs.

METHODS

128-Channel recordings were obtained to right median nerve stimulation of 10 right-handed healthy human subjects and in 7 of them additional to right ulnar nerve. Data were evaluated by applying principal component analysis and dipole source analysis.

RESULTS

Different source evaluation strategies provided converging evidence for a cortical HFO origin, with two different almost orthogonally oriented generators being active in parallel, but with a phase shift of a quarter of their oscillatory period, while the low-frequency 'N20' is adequately modeled by one tangential dipole source. Median and ulnar derived low-frequency and HFO cortical sources show a somatotopic order. Additionally, generation of the HFOs was localized in subcortical, near-thalamic and subthalamic source sites. The near-thalamic dipole was located at significantly different sites in HFO and low-frequency data.

CONCLUSIONS

The cortical HFO source constellation points to a 'precortical' source in terminals of thalamocortical fibers and a second intracortical HFO origin. Furthermore, HFOs are also generated at subcortical and even subthalamic sites. Near-thalamic, the HFO and low-frequency signals are generated or modulated by different neuron populations involved in the thalamocortical outflow.

Parietal generators of low- and high-frequency MN (median nerve) SEPs: data from intracortical human recordings

OBJECTIVE

To identify low and high-frequency median nerve (MN) somatosensory evoked potential (SEP) generators by means of chronically implanted electrodes in the parietal lobe (SI and neighbouring areas) of two epileptic patients.

METHODS

Wide-pass short-latency and long-latency SEPs to electrical MN stimulation were recorded in two epileptic patients by stereotactically chronically implanted electrodes in the parietal lobe (SI and neighbouring areas). To study high-frequency responses (HFOs) an off-line digital filtering of depth short-latency SEPs was performed (500-800 Hz, 24 dB roll-off). Spectral analysis was performed by fast Fourier transform.

RESULTS

In both patients we recorded a N20/P30 potential followed by a biphasic N50/P70 response. A little negative response in the 100 ms latency range was the last detectable wide-pass SEP in both patients. Two HFOs components (called iP1 and iP2) were detected by mere visual analysis and spectral analysis, and were supposed to be originated within the parietal cortex.

CONCLUSIONS

This was the very first study that recorded wide bandpass and high frequency SEPs by electrodes, exploring both the lateral and the mesial part of the parietal lobe and particularly that of the post-central gyrus.

Interneuron diversity series: inhibitory interneurons and network oscillations in vitro

In vitro models of rhythms of cognitive relevance, such as gamma (30-80 Hz) and theta (5-12 Hz) rhythms in the hippocampus, demonstrate an absolute requirement for phasic inhibitory synaptic transmission. Such rhythms can occur transiently, of approximately 1 s duration, or persistently, lasting for many hours. In the latter case, stable patterns of interneuron output, and their postsynaptic consequences for pyramidal cell membrane potential, occur despite known constraints of synaptic habituation and potentiation. This review concentrates on recent in vitro evidence revealing a division of labour among different subclasses of interneurons with respect to the frequency of persistent rhythms, and the crucial dependence on gap-junction-mediated intercellular communication for the generation and maintenance of these rhythms.

Influence of cholinergic circuitries in generation of high-frequency somatosensory evoked potentials

OBJECTIVE

High-frequency oscillations (HFOs) evoked by upper limb stimulation reflect highly synchronised spikes generated in the somatosensory human system. Since acetylcholine produces differential modulation in subgroups of neurons, we would determine whether cholinergic drive influences HFOs.

METHODS

We recorded somatosensory evoked potentials (SEPs) from 31 scalp electrodes in 7 healthy volunteers, before and after single administration of rivastigmine, an inhibitor of central acetylcholinesterase. Right median nerve SEPs have been analysed after digital narrow bandpass filtering (500-700 Hz). Raw data were further submitted to Brain Electrical Source analysis (BESA) to evaluate the respective contribution of lemniscal, thalamic and cortical sources. Lastly, we analysed by Fast Fourier transform spectral changes after drug administration in the 10-30 ms latency range.

RESULTS

Rivastigmine administration caused a significant increase of HFOs in the 18-28 ms latency range. Wavelets occurring before the onset latency of the conventional N20 SEP did not show any significant change. A similar increase concerned the strength of cortical dipolar sources in our BESA model. Lastly, we found a significant power increase of the frequency peak at about 600 Hz in P3-F3 traces after drug intake.

CONCLUSIONS

Our findings demonstrate that the cortical component of HFOs is significantly enhanced by cholinergic activation. Pyramidal chattering cells, which are capable to discharge high-frequency bursts, are mainly modulated by cholinergic inputs; by contrast, acetylcholine does not modify the firing rate of fast-spiking GABAergic interneurons. We thus discuss the hypothesis that cortical HFOs are mainly generated by specialised pyramidal cells.

Stable synchronized high-frequency signals from the main sensory and spinal nuclei of the pig activated by Abeta fibers of the maxillary nerve innervating the snout

The primary somatosensory cortex of various species including man, monkey, pig and rat is capable of producing high-frequency signals in the 600 Hz range and above with very little latency jitter. We have recently observed such cortical signals for the trigeminal system of the swine. This study determined the projection of the maxillary nerve innervating the snout to the sensory trigeminal nuclear complex in the brain stem and stability of outputs of each nucleus receiving the projection. The snout stimulation activated large-caliber Abeta fibers in the trigeminal nerve with a mean velocity of 64.4+/-2.7 m/s (mean+/-1 S.E.M., six animals) comparable in velocity to the tooth pulp Abeta fibers (57.9+/-3.4 m/s) obtained from the same animals. These afferents activated the main sensory nucleus, and subnuclei oralis, interpolaris and caudalis of the spinal nucleus, as judged by evoked field potential maps superimposed on the histological maps of the trigeminal nuclei from the same animals. Inputs from these fast afferents arrived at all the four trigeminal nuclei almost simultaneously within a span of 0.7+/-0.2 ms (mean+/-1 S.D., seven animals). Evoked high-frequency signals were reproducible with a latency jitter of less than 0.2 ms during the first 4 ms of postsynaptic activity for each of main sensory and spinal nuclei. These results indicate that the snout stimulation activates fast-conducting peripheral afferents which project to all the sensory trigeminal nuclei and produces highly reproducible initial responses nearly simultaneously across the multiple trigeminal nuclei. These outputs from the trigeminal nuclei may play an important role in triggering the stable high-frequency signals in the cortex.

Electrical and chemical synapses among parvalbumin fast-spiking GABAergic interneurons in adult mouse neocortex

Networks of gamma-aminobutyric acid (GABA)ergic interneurons connected via electrical and chemical synapses are thought to play an important role in detecting and promoting synchronous activity in the cerebral cortex. Although the properties of electrical and chemical synaptic interactions among inhibitory interneurons are critical for their function as a network, they have only been studied systematically in juvenile animals. Here, we have used transgenic mice expressing the enhanced green fluorescent protein in cells containing parvalbumin (PV) to study the synaptic connectivity among fast-spiking (FS) cells in slices from adult animals (2-7 months old). We have recorded from pairs of PV-FS cells and found that the majority of them were electrically coupled (61%, 14 of 23 pairs). In addition, 78% of the pairs were connected via GABAergic chemical synapses, often reciprocally. The average coupling coefficient for step injections was 1.5% (n = 14), a smaller value than that reported in juvenile animals. GABA-mediated inhibitory postsynaptic currents and potentials decayed with exponential time constants of 2.6 and 5.9 ms, respectively, and exhibited paired-pulse depression (50-ms interval). The inhibitory synaptic responses in the adult were faster than those observed in young animals. Our results indicate that PV-FS cells are highly interconnected in the adult cerebral cortex by both electrical and chemical synapses, establishing networks that can have important implications for coordinating activity in cortical circuits.

Dissociated changes of somatosensory evoked low-frequency scalp responses and 600 Hz bursts after single-dose administration of lorazepam

Electrical stimulation of upper limb nerves allows one to record two types of scalp responses, that is conventional low-frequency somatosensory evoked potentials (SEPs), and bursts of high-frequency (about 600 Hz) wavelets. To further clarify the functional meaning of both types of responses, we investigated whether changes of GABAergic drive could cause significant modifications of conventional as well as high-frequency SEPs. We recorded median nerve SEPs from six healthy volunteers before and after a single oral administration of lorazepam. In order to explain scalp SEP distribution before and after lorazepam administration, we performed the brain electrical source analysis of raw data. After lorazepam administration, conventional scalp SEPs showed a significant amplitude decrease of all cortical components including the primary N20/P20 response, while the subcortical P14 response remained substantially unchanged. Similarly, dipolar analysis showed a significant strength decrease of all cortical dipoles, whereas the strength of both subcortical dipoles (possibly located at the level of the brainstem and thalamus, respectively) remained unchanged. By contrast, no significant changes of high-frequency SEPs were induced by drug intake. Therefore, our findings suggest that the inhibitory effect induced by lorazepam mainly affects intracortical circuitry. Tonic increase of the inhibitory drive, possibly mediated by GABAA receptors, can account for the reduced activity of first order deep spiny neurons generating the primary N20/P20. Conversely, intrinsic firing properties of the cell population generating high-frequency SEP responses are unaffected by the increase of GABAergic drive. This finding lends further substance to the hypothesis that conventional and high-frequency SEPs are generated by different cell populations.