We present a novel method for the extraction of neuronal components showing cross-frequency phase synchronization.
In general the method can be applied for the detection of phase interactions between components with frequencies f1 and f2, where f2 ≈ rf1 and r is some integer. We refer to the method as cross-frequency decomposition (CFD), which consists of the following steps: (a) extraction of f1-oscillations with the spatio-spectral decomposition algorithm (SSD); (b) frequency modification of the f1-oscillations obtained with SSD; and (c) finding f2-oscillations synchronous with f1-oscillations using least-squares estimation.
Our simulations showed that CFD was capable of recovering interacting components even when the signal-to-noise ratio was as low as 0.01. An application of CFD to the real EEG data demonstrated that cross-frequency phase synchronization between alpha and beta oscillations can originate from the same or remote neuronal populations.
CFD allows a compact representation of the sets of interacting components. The application of CFD to EEG data allows differentiating cross-frequency synchronization arising due to genuine neurophysiological interactions from interactions occurring due to quasi-sinusoidal waveform of neuronal oscillations.
CFD is a method capable of extracting cross-frequency coupled neuronal oscillations even in the presence of strong noise.
Copyright © 2011 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
The performance of the three methods depends on the amount of averaged trials. Moreover, differences are found on both amplitude and latency of ERP components recorded in two environments (0 T vs 3 T). We showed that, while ERPs can be extracted from simultaneous EEG–fMRI data at 3 T, the static magnetic field might affect the physiological processes under investigation.The reproducibility of the ERPs in different recording environments (0 T vs 3 T) is a relevant issue that deserves further investigation to clarify the equivalence of cognitive processes in both behavioral and imaging studies.
Electrocorticography (ECoG) is a standard procedure for the localization of the epileptogeniczone during the surgical treatment of symptomatic epilepsy. The purpose of this study was to evaluate the diagnostic efficacy of intraoperative pre- and post-resective ECoG for the localization of the epileptogenic zone in patients with symptomatic epilepsy associated with supratentorial brain tumors. 1. In the surgical treatment of symptomatic epilepsy associated with intracerebral neoplasm, intraoperative ECG remains a relatively effective method. 2. The effectiveness of intraoperative pre- and post-resection ECoG is affected by factors associated with the performance of neurosurgical surgery (the influence of general anesthetics, mechanical effects on the brain, repeated electrocoagulation), which significantly alter the index of epileptiform activity.
Objective Removal of brain tissue generating pathological high-frequency oscillations (pHFOs) has been related to better seizure outcome than resection of seizure onset zone. However, there is still a lack of understanding what oscillations are to be considered pathological. Methods A female patient (age 53) with 10 year duration of temporal lobe tumor-related epilepsy was admitted to Polenov’s Neurosurgery Institute for tumor resection. The patient underwent a two-staged surgery with subdural implantation of a grid electrode (4 × 5) over the temporal lobe to identify the epileptogenic zone (EZ). During the second stage wideband intraoperative electrocorticography (iECoG) was recorded (up to 500 Hz, sampling frequency 2000 Hz, Mitsar-EEG 202 amplifier). Results Electrocorticographic monitoring data were subjected to visual analysis in traditional frequency range (0.5–70 Hz). Six of 20 electrodes were marked as EZ electrodes. The distance between tumor margin and EZ electrodes reached 1–2.5 cm. Subpial resection of this zone was arranged. During the surgery iECoG data in 0.5–70 Hz frequency band were uninformative, while in 80–500 Hz range bursts of fast ripples (250–500 Hz, 100 μV, extended up to 3 s) were recorded over the marked EZ electrodes. The tumor and EZ were completely resected. Discussion Observed data demonstrate that HFOs coincide with EZ marked during long-term monitoring. The patient is seizure-free for 5 months at the moment, though a more prolonged follow-up is required. Conclusion Wideband iECoG recordings might give us more essential information in case of tumor-related epilepsy. As is shown, fast ripples may be a valid marker of EZ. Significance Pathological HFOs show promise for optimising epilepsy surgery in tumor-related epilepsy.
Paired-pulse transcranial magnetic stimulation (TMS) allows investigating inhibitory and excitatory interactions in the human motor cortex noninvasively. Short-interval intracortical inhibition (SICI) and facilitation (SICF) are used to measure cortico–cortical excitability in patients with, e.g., stroke, dystonia, and Parkinson’s disease. However, the role of the induced electric field (E-field) orientation remains partly unclear. Posterior–anterior (PA)-oriented E-field elicits motor evoked potentials (MEPs) with the lowest stimulus intensities due to the recruitment of corticospinal neurons, indirectly via excitatory synaptic inputs to corticospinal axons (indirect (I-) waves). Stimulation in the lateral–medial (LM) orientation directly activates corticospinal axons, which leads to the generation of both direct (D-) and I-waves. Conditioning stimulus (CS) with an intensity between 50% and 90% of resting motor threshold (RMT) induces activation of GABAA inhibitory mechanisms observed as the SICI (inhibitory) effect on MEP amplitude. In contrast, if the CS intensity is above RMT, the SICF (excitatory) phenomenon can be present due to the superposition of D- and I-waves. Our aim was to investigate the dependence of inhibitory and facilitatory mechanisms on the orientation of the induced E-field of CS and TS. We developed a multi-locus TMS (mTMS) transducer, which allowed us to control the E-field orientation independently for CS and TS at a millisecond inter-pair interval (IPI). Eight healthy subjects (five males; mean age 29, range 21–35 years) participated in the study. mTMS was applied to the hotspot of the abductor pollicis brevis (APB) muscle in the left primary motor cortex. The stimulus intensities were based on the individual RMT of APB for PA and LM orientations. TS and single pulses were administered at 110% RMT. Twenty single pulses were applied for each TS orientation and for each of the 32 paired-pulse conditions. CS and TS stimuli were applied in every combination of the PA and LM orientations with four CS intensities (50, 70, 90, and 110% RMT) and two IPIs (1.5 and 2.7 ms) in a random order. Interaction between CS orientation, IPI, and CS intensity significantly affected TS MEP amplitudes. We observed no statistically significant difference between the responses induced by PA- and LM-oriented TS. CS at 70% RMT for SICI and at 110% RMT for SICF induced similar effects regardless of the TS orientation. We established that LM-oriented CS at 90% RMT produced a greater inhibition than stimuli at the same intensity in the PA orientation. Our results emphasize the minimal influence of the CS E-field dorientation on the test pulse. Additionally, we demonstrate the pivotal role of the stimulus intensity for any CS orientation. SICI and SICF evoked using perpendicular CS and TS directions indicate that we stimulated overlapping neuronal populations with both pulses.
Trial-to-trial variability of the motor evoked potentials (MEP) to transcranial magnetic stimulation (TMS) is a well-known phenomenon. However, the relationship between the fluctuations of the different types of the motor output and other motor system parameters such as corticospinal excitability, interhemispheric inhibition (IHI) and their interhemispheric asymmetry have not yet been fully investigated. We studied 20 young healthy right-handed volunteers. Four TMS sessions were performed (two single-pulse TMS and two paired-coil TMS with IHI paradigm sessions for each hemisphere), 70 stimuli were delivered during every session. Coefficient of quartile variation (CQV) was used to quantify trail-to-trail variability of MEPs amplitude. Resting motor threshold values were correlated between hemispheres (r = .842, p < .001). IHI phenomenon from the left hemisphere was obtained in 18 out of 20 volunteers, while IHI phenomena from the right hemisphere was shown in 16 out of 20. A strong correlation between the variability of MEP‘s amplitudes during IHI paradigm and the degree of IHI was found for the left hand (r = −.718, p < .001). We also observed a strong correlation between CQV of MEPs from both hands to single-pulse TMS (r = .632, p = .004). A side-specific correlation between the variability of the responses to single-pulse and paired-coil TMS was found for the dominant hemisphere (r = .524, p = .021). Our preliminary results demonstrate the importance of the trial-to-trial variability of the MEPs and its interhemispheric specificity as a defining characteristics of the motor system. This study was partially supported by ofi-m RFBR grant 17-29-02518, by HSE Basic Research Program and Russian Academic Excellence Project ‘5-100’.