Marc Op De Beeck

Functional motor-cortex mapping using corticokinematic coherence.

We present a novel method, corticokinematic coherence (CKC), for functional mapping of the motor cortex by computing coherence between cortical magnetoencephalographic (MEG) signals and the kinematics of voluntary movements. Ten subjects performed self-paced flexion-extensions of the right-hand fingers at about 3 Hz, with a three-axis accelerometer attached to the index finger. Cross-correlogram and coherence spectra were computed between 306 MEG channels and the accelerometer signals. In all subjects, accelerometer and coherence spectra showed peaks around 3-5 Hz and 6-10 Hz, corresponding to the movement frequencies. The coherence was statistically significant (P<0.05) in all subjects, with sources at the hand area of the primary motor cortex contralateral to the movement. CKC appears to be a promising and robust method for reliable and convenient functional mapping of the human motor cortex.

The pace of prosodic phrasing couples the listener's cortex to the reader's voice.

We studied online coupling between a readers voice and a listeners cortical activity using a novel, ecologically valid continuous listening paradigm. Whole‐scalp magnetoencephalographic (MEG) signals were recorded from 10 right‐handed, native French‐speaking listeners in four conditions: a female (Exp1f) and a male (Exp1m) reading the same text in French; a male reading a text in Finnish (Exp 2), a language incomprehensible for the subjects, and a male humming Exp1 text (Exp 3). The fundamental frequency (f0) of the readers voice was recorded with an accelerometer attached to the throat, and coherence was computed between f0 time‐course and listeners MEG. Similar levels of right‐hemisphere‐predominant coherence were found at ˜0.5 Hz in Exps 1–3. Dynamic imaging of coherent sources revealed that the most coherent brain regions were located in the right posterior superior temporal sulcus (pSTS) and posterior superior temporal gyrus (pSTG) in Exps 1–2 and in the right supratemporal auditory cortex in Exp 3. Comparison between speech rhythm and phrasing suggested a connection of the observed coherence to pauses at the sentence level both in the spoken and hummed text. These results demonstrate significant coupling at ∼0.5 Hz between readers voice and listeners cortical signals during listening to natural continuous voice. The observed coupling suggests that voice envelope fluctuations, due to prosodic rhythmicity at the phrasal and sentence levels, are reflected in the listeners cortex as rhythmicity of about 2‐s cycles. The predominance of the coherence in the right pSTS and pSTG suggests hemispherical asymmetry in processing of speech sounds at subsentence time scales. Hum Brain Mapp, 2013.

Recording epileptic activity with MEG in a light-weight magnetic shield

Ten patients with focal epilepsy were studied with magnetoencephalography (MEG) to determine if a new light-weight magnetically shielded room (lMSR) provides sufficient attenuation of magnetic interference to detect and localize the magnetic correlates of epileptic activity. Interictal MEG epileptic events co-localizing with the presumed location of the epileptogenic zone were found in all patients. MEG measurements performed in the lMSR provide an adequate signal-to-noise ratio for non-invasive localization of epileptic foci.

Neuronal network coherent with hand kinematics during fast repetitive hand movements

We quantified the coupling between magnetoencephalographic (MEG) cortical signals and the kinematics of fast repetitive voluntary hand movements monitored by a 3-axis accelerometer. Ten healthy right-handed adults performed self-paced flexion-extension movements of right-hand fingers at ~3Hz with either touching the thumb during flexions (TOUCH) or not (noTOUCH). At the sensor level, we found in all subjects and conditions significant coherence at the movement frequency (F0) and its first harmonic (F1). Coherence values were significantly higher in TOUCH compared to noTOUCH. At the group level, dynamic imaging of coherent sources localized the main source of coherent activity at the left primary motor (M1) hand area, except at F0 TOUCH were the main source was localized at the left primary sensory (S1) hand area. Other coherent brain areas were also identified at right S1-M1 cortices (F0), left dorsolateral prefrontal cortex (F1), left posterior parietal cortex (F0 TOUCH and F1 noTOUCH) and left medial S1-M1 areas (TOUCH). This study highlights the prominent role of rhythmic neuronal activity phase-locked to movements for the encoding and the integration of key sensori-motor features of limb kinematics. This study also suggests that somatosensory afferences play a key role to sustain a high synchronization level between the neuronal activity in coherent brain areas and hand acceleration. Some coherent brain regions differed between F0 and F1 in both conditions, suggesting that distinct cortical areas are involved in different features of hand kinematics.

Primary motor cortex and cerebellum are coupled with the kinematics of observed hand movements.

To find out in which detail the kinematics of observed movements is represented in the viewers brain, we searched for brain areas displaying coherent magnetoencephalographic (MEG) activity with observed repetitive hand movements. Whole-scalp MEG signals were recorded from 10 right-handed young adults who observed repetitive 3-Hz right-hand flexion-extension movements performed by the experimenter. The coherence between the subjects MEG signals and the experimenters index-finger acceleration was computed to index the level of actor-observer coupling. The underlying cortical activity was identified with Dynamic Imaging of Coherent Sources. In all subjects, coherence spectra showed statistically significant peaks at movement frequency (F0) and its first harmonic (F1), strongest at visual areas. At F0, additional significant local coherence maxima, clearly distinct from the coherent visual areas, occurred in the primary motor (M1) cortices of both hemispheres and in the cerebellum (posterior vermis and hemispheres). Our results highlight the time-sensitive involvement of the M1 cortices and cerebellum in the kinematic representation of observed repetitive, non-goal directed motor actions.

A geometric correction scheme for spatial leakage effects in MEG/EEG seed-based functional connectivity mapping

Spatial leakage effects are particularly confounding for seed‐based investigations of brain networks using source‐level electroencephalography (EEG) or magnetoencephalography (MEG). Various methods designed to avoid this issue have been introduced but are limited to particular assumptions about its temporal characteristics. Here, we investigate the usefulness of a model‐based geometric correction scheme (GCS) to suppress spatial leakage emanating from the seed location. We analyze its properties theoretically and then assess potential advantages and limitations with simulated and experimental MEG data (resting state and auditory‐motor task). To do so, we apply Minimum Norm Estimation (MNE) for source reconstruction and use variation of error parameters, statistical gauging of spatial leakage correction and comparison with signal orthogonalization. Results show that the GCS has a local (i.e., near the seed) effect only, in line with the geometry of MNE spatial leakage, and is able to map spatially all types of brain interactions, including linear correlations eliminated after signal orthogonalization. Furthermore, it is robust against the introduction of forward model errors. On the other hand, the GCS can be affected by local overcorrection effects and seed mislocation. These issues arise with signal orthogonalization too, although significantly less extensively, so the two approaches complement each other. The GCS thus appears to be a valuable addition to the spatial leakage correction toolkits for seed‐based FC analyses in source‐projected MEG/EEG data. Hum Brain Mapp 36:4604–4621, 2015.

About the electrophysiological basis of resting state networks

Matthew Jon Brookes, Mark Woolrich, Henry Luckhoo, Darren Price, Joanne Hale, Mary Stephenson, Gareth Barnes, Stephen Smith, and Peter Morris Sir Peter Mansfield Magnetic Resonance Centre, University of Nottingham, Nottingham, Nottinghamshire, United Kingdom, Oxford centre for human brain activity, University of Oxford, Oxford, Wellcome trust centre for neuroimaging, University College London, London, Oxford Centre for functional MRI of the brain, University of Oxford, Oxford

Left Superior Temporal Gyrus Is Coupled to Attended Speech in a Cocktail-Party Auditory Scene.

Using a continuous listening task, we evaluated the coupling between the listeners cortical activity and the temporal envelopes of different sounds in a multitalker auditory scene using magnetoencephalography and corticovocal coherence analysis. Neuromagnetic signals were recorded from 20 right-handed healthy adult humans who listened to five different recorded stories (attended speech streams), one without any multitalker background (No noise) and four mixed with a “cocktail party” multitalker background noise at four signal-to-noise ratios (5, 0, −5, and −10 dB) to produce speech-in-noise mixtures, here referred to as Global scene. Coherence analysis revealed that the modulations of the attended speech stream, presented without multitalker background, were coupled at ∼0.5 Hz to the activity of both superior temporal gyri, whereas the modulations at 4–8 Hz were coupled to the activity of the right supratemporal auditory cortex. In cocktail party conditions, with the multitalker background noise, the coupling was at both frequencies stronger for the attended speech stream than for the unattended Multitalker background. The coupling strengths decreased as the Multitalker background increased. During the cocktail party conditions, the ∼0.5 Hz coupling became left-hemisphere dominant, compared with bilateral coupling without the multitalker background, whereas the 4–8 Hz coupling remained right-hemisphere lateralized in both conditions. The brain activity was not coupled to the multitalker background or to its individual talkers. The results highlight the key role of listeners left superior temporal gyri in extracting the slow ∼0.5 Hz modulations, likely reflecting the attended speech stream within a multitalker auditory scene. SIGNIFICANCE STATEMENT When people listen to one person in a “cocktail party,” their auditory cortex mainly follows the attended speech stream rather than the entire auditory scene. However, how the brain extracts the attended speech stream from the whole auditory scene and how increasing background noise corrupts this process is still debated. In this magnetoencephalography study, subjects had to attend a speech stream with or without multitalker background noise. Results argue for frequency-dependent cortical tracking mechanisms for the attended speech stream. The left superior temporal gyrus tracked the ∼0.5 Hz modulations of the attended speech stream only when the speech was embedded in multitalker background, whereas the right supratemporal auditory cortex tracked 4–8 Hz modulations during both noiseless and cocktail-party conditions.

Spatiotemporal profiles of visual processing with and without primary visual cortex

The spatiotemporal profiles of visual processing are normally distributed in two temporal phases, each lasting about 100 ms. Within each phase, cortical processing begins in V1 and traverses the visual cortical hierarchy. However, the causal role of V1 in starting each of these two phases is unknown. Here we used magnetoencephalography to study the spatiotemporal profiles of visual processing and the causal contribution of V1 in three neurologically intact participants and in a rare patient (GY) with unilateral destruction of V1, in whom residual visual functions mediated by the extra-geniculostriate pathways have been reported. In healthy subjects, visual processing in the first 200 ms post-stimulus onset proceeded in the two usual phases. Normally perceived stimuli in the left hemifield of GY elicited a spatiotemporal profile in the intact right hemisphere that closely matched that of healthy subjects. However, stimuli presented in the cortically blind hemifield produced no detectable response during the first phase of processing, indicating that the responses in extrastriate visual areas during this phase are determined by the feedforward progression of activity initiated in V1. The first responses occurred during the second processing phase, in the ipsilesional high-level visual areas. The activity then spread forward toward higher-level areas and backward toward lower-level areas. However, in contrast to responses in the intact hemisphere, the back-propagated activity in the early visual cortex did not exhibit the classic retinotopic organization and did not have well-defined response peaks.

Neurophysiological activity underlying altered brain metabolism in epileptic encephalopathies with CSWS

We investigated the neurophysiological correlate of altered regional cerebral glucose metabolism observed in children with epileptic encephalopathy with continuous spike-waves during sleep (CSWS) by using a multimodal approach combining time-sensitive magnetic source imaging (MSI) and positron emission tomography with [(18)F]-fluorodeoxyglucose (FDG-PET). Six patients (4 boys and 2 girls, age range: 4-8 years, 3 patients with Landau-Kleffner syndrome (LKS), 3 patients with atypical rolandic epilepsy (ARE)) were investigated by FDG-PET and MSI at the acute phase of CSWS. In all patients, the onset(s) of spike-waves discharges were associated with significant focal hypermetabolism. The propagation of epileptic discharges to other brain areas was associated with focal hypermetabolism (five patients), hypometabolism (one patient) or the absence of any significant metabolic change (one patient). Interestingly, most of the hypometabolic areas were not involved in the epileptic network per se. This study shows that focal hypermetabolism observed at the acute phase of CSWS are related to the onset or propagation sites of spike-wave discharges. Spike-wave discharges propagation can be associated to other types of metabolic changes, suggesting the occurrence of various neurophysiological mechanisms at the cellular level. Most of the hypometabolic areas are not involved in the epileptic network as such and are probably related to a mechanism of remote inhibition. These findings highlight the critical value of combining FDG-PET with time-sensitive functional neuroimaging approaches such as MSI to assess CSWS epileptic network when surgery is considered as a therapeutic approach.