Cortical networks show characteristic recruitment patterns after somatosensory stimulation by pneumatically evoked repetitive hand movements in newborn infants.

Controlled assessment of functional cortical networks is an unmet need in the clinical research of noncooperative subjects, such as infants. We developed an automated, pneumatic stimulation method to actuate naturalistic movements of an infant's hand, as well as an analysis pipeline for assessing the elicited electroencephalography (EEG) responses and related cortical networks. Twenty newborn infants with perinatal asphyxia were recruited, including 7 with mild-to-moderate hypoxic-ischemic encephalopathy (HIE). Statistically significant corticokinematic coherence (CKC) was observed between repetitive hand movements and EEG in all infants, peaking near the contralateral sensorimotor cortex. CKC was robust to common sources of recording artifacts and to changes in vigilance state. A wide recruitment of cortical networks was observed with directed phase transfer entropy, also including areas ipsilateral to the stimulation. The extent of such recruited cortical networks was quantified using a novel metric, Spreading Index, which showed a decrease in 4 (57%) of the infants with HIE. CKC measurement is noninvasive and easy to perform, even in noncooperative subjects. The stimulation and analysis pipeline can be fully automated, including the statistical evaluation of the cortical responses. Therefore, the CKC paradigm holds great promise as a scientific and clinical tool for controlled assessment of functional cortical networks.

Temporally stable beta sensorimotor oscillations and corticomuscular coupling underlie force steadiness.

As humans, we seamlessly hold objects in our hands, and may even lose consciousness of these objects. This phenomenon raises the unsettled question of the involvement of the cerebral cortex, the core area for voluntary motor control, in dynamically maintaining steady muscle force. To address this issue, we measured magnetoencephalographic brain activity from healthy adults who maintained a steady pinch grip. Using a novel analysis approach, we uncovered fine-grained temporal modulations in the beta sensorimotor brain rhythm and its coupling with muscle activity, with respect to several aspects of muscle force (rate of increase/decrease or plateauing high/low). These modulations preceded changes in force features by ∼40 ms and possessed behavioral relevance, as less salient or absent modulation predicted a more stable force output. These findings have consequences for the existing theories regarding the functional role of cortico-muscular coupling, and suggest that steady muscle contractions are characterized by a stable rather than fluttering involvement of the sensorimotor cortex.

Reproducibility of Rolandic beta rhythm modulation in MEG and EEG.

The Rolandic beta rhythm, at ∼20 Hz, is generated in the somatosensory and motor cortices and is modulated by motor activity and sensory stimuli, causing a short lasting suppression that is followed by a rebound of the beta rhythm. The rebound reflects inhibitory changes in the primary sensorimotor (SMI) cortex, and thus it has been used as a biomarker to follow the recovery of patients with acute stroke. The longitudinal stability of beta rhythm modulation is a prerequisite for its use in long-term follow-ups. We quantified the reproducibility of beta rhythm modulation in healthy subjects in a 1-year-longitudinal study both for MEG and EEG at T0, 1 month (T1-month, n = 8) and 1 year (T1-year, n = 19). The beta rhythm (13-25 Hz) was modulated by fixed tactile and proprioceptive stimulations of the index fingers. The relative peak strengths of beta suppression and rebound did not differ significantly between the sessions, and intersession reproducibility was good or excellent according to intraclass correlation-coefficient values (0.70-0.96) both in MEG and EEG. Our results indicate that the beta rhythm modulation to tactile and proprioceptive stimulation is well reproducible within 1 year. These results support the use of beta modulation as a biomarker in long-term follow-up studies, e.g., to quantify the functional state of the SMI cortex during rehabilitation and drug interventions in various neurological impairments.NEW & NOTEWORTHY The present study demonstrates that beta rhythm modulation is highly reproducible in a group of healthy subjects within a year. Hence, it can be reliably used as a biomarker in longitudinal follow-up studies in different neurological patient groups to reflect changes in the functional state of the sensorimotor cortex.

Neuronal correlates of the subjective experience of attention.

The effect of top-down attention on stimulus-evoked responses and alpha oscillations and the association between arousal and pupil diameter are well established. However, the relationship between these indices, and their contribution to the subjective experience of attention, remains largely unknown. Participants performed a sustained (10-30 s) attention task in which rare (10%) targets were detected within continuous tactile stimulation (16 Hz). Trials were followed by attention ratings on an 8-point visual scale. Attention ratings correlated negatively with contralateral somatosensory alpha power and positively with pupil diameter. The effect of pupil diameter on attention ratings extended into the following trial, reflecting a sustained aspect of attention related to vigilance. The effect of alpha power did not carry over to the next trial and furthermore mediated the association between pupil diameter and attention ratings. Variations in steady-state amplitude reflected stimulus processing under the influence of alpha oscillations but were only weakly related to subjective ratings of attention. Together, our results show that both alpha power and pupil diameter are reflected in the subjective experience of attention, albeit on different time spans, while continuous stimulus processing might not contribute to the experience of attention.

Cortical Tracking of Speech-in-Noise Develops from Childhood to Adulthood.

In multitalker backgrounds, the auditory cortex of adult humans tracks the attended speech stream rather than the global auditory scene. Still, it is unknown whether such preferential tracking also occurs in children whose speech-in-noise (SiN) abilities are typically lower compared with adults. We used magnetoencephalography (MEG) to investigate the frequency-specific cortical tracking of different elements of a cocktail party auditory scene in 20 children (age range, 6-9 years; 8 females) and 20 adults (age range, 21-40 years; 10 females). During MEG recordings, subjects attended to four different 5 min stories, mixed with different levels of multitalker background at four signal-to-noise ratios (SNRs; noiseless, +5, 0, and -5 dB). Coherence analysis quantified the coupling between the time courses of the MEG activity and attended speech stream, multitalker background, or global auditory scene, respectively. In adults, statistically significant coherence was observed between MEG signals originating from the auditory system and the attended stream at <1, 1-4, and 4-8 Hz in all SNR conditions. Children displayed similar coupling at <1 and 1-4 Hz, but increasing noise impaired the coupling more strongly than in adults. Also, children displayed drastically lower coherence at 4-8 Hz in all SNR conditions. These results suggest that children's difficulties to understand speech in noisy conditions are related to an immature selective cortical tracking of the attended speech streams. Our results also provide unprecedented evidence for an acquired cortical tracking of speech at syllable rate and argue for a progressive development of SiN abilities in humans.SIGNIFICANCE STATEMENT Behaviorally, children are less proficient than adults at understanding speech-in-noise. Here, neuromagnetic signals were recorded while healthy adults and typically developing 6- to 9-year-old children attended to a speech stream embedded in a multitalker background noise with varying intensity. Results demonstrate that auditory cortices of both children and adults selectively track the attended speaker's voice rather than the global acoustic input at phrasal and word rates. However, increments of noise compromised the tracking significantly more in children than in adults. Unexpectedly, children displayed limited tracking of both the attended voice and the global acoustic input at the 4-8 Hz syllable rhythm. Thus, both speech-in-noise abilities and cortical tracking of speech syllable repetition rate seem to mature later in adolescence.

Comparing MEG and EEG in detecting the ~20-Hz rhythm modulation to tactile and proprioceptive stimulation.

Modulation of the ~20-Hz brain rhythm has been used to evaluate the functional state of the sensorimotor cortex both in healthy subjects and patients, such as stroke patients. The ~20-Hz brain rhythm can be detected by both magnetoencephalography (MEG) and electroencephalography (EEG), but the comparability of these methods has not been evaluated. Here, we compare these two methods in the evaluating of ~20-Hz activity modulation to somatosensory stimuli. Rhythmic ~20-Hz activity during separate tactile and proprioceptive stimulation of the right and left index finger was recorded simultaneously with MEG and EEG in twenty-four healthy participants. Both tactile and proprioceptive stimulus produced a clear suppression at 300-350 â€‹ms followed by a subsequent rebound at 700-900 â€‹ms after stimulus onset, detected at similar latencies both with MEG and EEG. The relative amplitudes of suppression and rebound correlated strongly between MEG and EEG recordings. However, the relative strength of suppression and rebound in the contralateral hemisphere (with respect to the stimulated hand) was significantly stronger in MEG than in EEG recordings. Our results indicate that MEG recordings produced signals with higher signal-to-noise ratio than EEG, favoring MEG as an optimal tool for studies evaluating sensorimotor cortical functions. However, the strong correlation between MEG and EEG results encourages the use of EEG when translating studies to clinical practice. The clear advantage of EEG is the availability of the method in hospitals and bed-side measurements at the acute phase.

Neocortical activity tracks the hierarchical linguistic structures of self-produced speech during reading aloud.

How the human brain uses self-generated auditory information during speech production is rather unsettled. Current theories of language production consider a feedback monitoring system that monitors the auditory consequences of speech output and an internal monitoring system, which makes predictions about the auditory consequences of speech before its production. To gain novel insights into underlying neural processes, we investigated the coupling between neuromagnetic activity and the temporal envelope of the heard speech sounds (i.e., cortical tracking of speech) in a group of adults who 1) read a text aloud, 2) listened to a recording of their own speech (i.e., playback), and 3) listened to another speech recording. Reading aloud was here used as a particular form of speech production that shares various processes with natural speech. During reading aloud, the reader's brain tracked the slow temporal fluctuations of the speech output. Specifically, auditory cortices tracked phrases (<1 â€‹Hz) but to a lesser extent than during the two speech listening conditions. Also, the tracking of words (2-4 â€‹Hz) and syllables (4-8 â€‹Hz) occurred at parietal opercula during reading aloud and at auditory cortices during listening. Directionality analyses were then used to get insights into the monitoring systems involved in the processing of self-generated auditory information. Analyses revealed that the cortical tracking of speech at <1 â€‹Hz, 2-4 â€‹Hz and 4-8 â€‹Hz is dominated by speech-to-brain directional coupling during both reading aloud and listening, i.e., the cortical tracking of speech during reading aloud mainly entails auditory feedback processing. Nevertheless, brain-to-speech directional coupling at 4-8 â€‹Hz was enhanced during reading aloud compared with listening, likely reflecting the establishment of predictions about the auditory consequences of speech before production. These data bring novel insights into how auditory verbal information is tracked by the human brain during perception and self-generation of connected speech.

Feasibility and reproducibility of electroencephalography-based corticokinematic coherence.

Corticokinematic coherence (CKC) is the phase coupling between limb kinematics and cortical neurophysiological signals, reflecting cortical processing of proprioceptive afference, and it is reproducible when estimated with magnetoencephalography (MEG). However, feasibility and reproducibility of CKC based on electroencephalography (EEG) is still unclear and is the primary object of the present report. Thirteen healthy right-handed volunteers (seven females, 21.7 ± 4.3 yr) participated in two combined MEG/EEG sessions 12.6 ± 1.3 mo apart. Participants' dominant and nondominant index finger was continuously moved at 3 Hz for 4 min separately using a pneumatic-movement actuator. Coherence was computed between finger acceleration and three derivations of EEG signals: 1) average reference, 2) bipolar derivations, and 3) surface Laplacian. CKC strength was defined as the peak coherence value at movement frequency. Intraclass-correlation coefficient values (0.74-0.93) indicated excellent intersession reproducibility for CKC strength for all derivations and moved fingers. CKC strength obtained with EEG was approximately two times lower compared with MEG, but the values were positively correlated across the participants. CKC strength was significantly (P < 0.01) higher for bipolar (session 1: 0.19 ± 0.09; session 2: 0.20 ± 0.10) and surface Laplacian (session 1: 0.22 ± 0.09; session 2: 0.21 ± 0.09) derivations than for the average reference (session 1: 0.10 ± 0.04; session 2: 0.11 ± 0.05). We demonstrated that CKC is a feasible and reproducible tool to monitor proprioception using EEG recordings, although the strength of CKC was twice lower for EEG compared with MEG. Laplacian and bipolar (CP3-C1/CP3-C3 and CP4-C2/C4-FC2) EEG derivation(s) are recommended for future research and clinical use of CKC method. NEW & NOTEWORTHY The most important message in this report is that the corticokinematic coherence (CKC) method is a feasible and reproducible tool to quantify, map, and follow cortical proprioceptive ("the movement sense") processing using EEG that is more widely available for CKC recordings than previously used magnetoencephalography designs, in basic research, but especially in clinical environments. We provide useful recommendations for optimal EEG derivations for cost-effective experimental designs, making it possible to scale up in sample size in future studies.

Coupling between human brain activity and body movements: Insights from non-invasive electromagnetic recordings.

Electroencephalographic and magnetoencephalographic data have characterized two types of brain-body interactions observed during various types of motor actions, "corticokinematic" and "corticomuscular" coupling. Here, we review the literature on these interactions in healthy individuals, discuss several open debates, and outline current limitations and directions for future research. Corticokinematic coupling (commonly referred to as corticokinematic coherence) probes the relationship between activity of sensorimotor network nodes and various movement-related signals (e.g., speed, velocity, acceleration). It is mainly driven by movement rhythmicity during active, passive, and observed dynamic motor actions. It typically predominates at the primary sensorimotor cortex contralateral to the moving limb, occurs at movement frequency and its harmonics, and predominantly reflects the cortical processing of proprioceptive feedback driven by movement rhythmicity in a broad range of dynamic motor actions. Corticomuscular coupling (commonly referred to as corticomuscular coherence) probes the interaction between sensorimotor cortical rhythms and electromyographic (EMG) activity that mainly occurs during steady isometric muscle contraction. We will here focus on the ~20-Hz coupling that is observed during weak isometric contraction and is linked to the modulation of the descending motor command by the ~20-Hz sensorimotor rhythm. This review argues that corticokinematic and corticomuscular couplings have different neural bases. Corticokinematic coupling is mainly driven by afferent signals, while corticomuscular coupling is mainly (but not solely) driven by efferent signals. This distinction should be considered when investigating interactions between brain and body movements.

Spatio-temporal profile of brain activity during gentle touch investigated with magnetoencephalography.

Positive affective touch plays a central role in social and inter-personal interactions. Low-threshold mechanoreceptive afferents, including slowly-conducting C-tactile (CT) afferents found in hairy skin, transmit such signals from gentle touch to the brain. Tactile signals are processed, in part, by the posterior insula, where it is the thought to be the primary target for CTs. We used magnetoencephalography (MEG) to assess brain activity evoked by gentle, naturalistic stroking touch on the arm delivered by a new MEG-compatible brush robot. We aimed to use high temporal resolution MEG to allow us to distinguish between brain responses from fast-conducting Aß and slowly-conducting CT afferents. Brush strokes were delivered to the left upper arm and left forearm of 15 healthy participants. We hypothesized that late brain responses, due to slow CT afference, would appear with a time shift between the two different locations on the arm. Our results show that gentle touch rapidly activated somatosensory, motor, and cingulate regions within the first 100 ms of skin contact, which was driven by fast-conducting mechanoreceptive afference, and that these responses were sustained during touch. Peak latencies in the posterior insula were shifted as a function of stimulus location and temporally-separate posterior insula activations were induced by Aß and CT afference that may modulate the emotional processing of gentle touch on hairy skin. We conclude that the detailed information regarding temporal and spatial brain activity from MEG provides new insights into the central processing of gentle, naturalistic touch, which is thought to underpin affective tactile interactions.

MEG Insight into the Spectral Dynamics Underlying Steady Isometric Muscle Contraction.

To gain fundamental knowledge on how the brain controls motor actions, we studied in detail the interplay between MEG signals from the primary sensorimotor (SM1) cortex and the contraction force of 17 healthy adult humans (7 females, 10 males). SM1 activity was coherent at ∼20 Hz with surface electromyogram (as already extensively reported) but also with contraction force. In both cases, the effective coupling was dominant in the efferent direction. Across subjects, the level of ∼20 Hz coherence between cortex and periphery positively correlated with the "burstiness" of ∼20 Hz SM1 (Pearson r ≈ 0.65) and peripheral fluctuations (r ≈ 0.9). Thus, ∼20 Hz coherence between cortex and periphery is tightly linked to the presence of ∼20 Hz bursts in SM1 and peripheral activity. However, the very high correlation with peripheral fluctuations suggests that the periphery is the limiting factor. At frequencies <3 Hz, both SM1 signals and ∼20 Hz SM1 envelope were coherent with both force and its absolute change rate. The effective coupling dominated in the efferent direction between (1) force and the ∼20 Hz SM1 envelope and (2) the absolute change rate of the force and SM1 signals. Together, our data favor the view that ∼20 Hz coherence between cortex and periphery during isometric contraction builds on the presence of ∼20 Hz SM1 oscillations and needs not rely on feedback from the periphery. They also suggest that effective cortical proprioceptive processing operates at <3 Hz frequencies, even during steady isometric contractions.SIGNIFICANCE STATEMENT Accurate motor actions are made possible by continuous communication between the cortex and spinal motoneurons, but the neurophysiological basis of this communication is poorly understood. Using MEG recordings in humans maintaining steady isometric muscle contractions, we found evidence that the cortex sends population-level motor commands that tend to structure according to the ∼20 Hz sensorimotor rhythm, and that it dynamically adapts these commands based on the <3 Hz fluctuations of proprioceptive feedback. To our knowledge, this is the first report to give a comprehensive account of how the human brain dynamically handles the flow of proprioceptive information and converts it into appropriate motor command to keep the contraction force steady.

Reproducibility of corticokinematic coherence.

Corticokinematic coherence (CKC) between limb kinematics and magnetoencephalographic (MEG) signals reflects cortical processing of proprioceptive afference. However, it is unclear whether strength of CKC is reproducible across measurement sessions. We thus examined reproducibility of CKC in a follow-up study. Thirteen healthy right-handed volunteers (7 females, 21.7 ±â€¯4.3 yrs) were measured using MEG in two separate sessions 12.6 ±â€¯1.3 months apart. The participant was seated and relaxed while his/her dominant or non-dominant index finger was continuously moved at 3 Hz (4 min for each hand) using a pneumatic movement actuator. Finger kinematics were recorded with a 3-axis accelerometer. Coherence was computed between finger acceleration and MEG signals. CKC strength was defined as the peak coherence value at 3 Hz form a single sensor among 40 pre-selected Rolandic gradiometers contralateral to the movement. Pneumatic movement actuator provided stable proprioceptive stimuli and significant CKC responses peaking at the contralateral Rolandic sensors. In the group level, CKC strength did not differ between the sessions in dominant (Day-1 0.40 ±â€¯0.19 vs. Day-2 0.41 ±â€¯0.17) or non-dominant (0.35 ±â€¯0.16 vs. 0.36 ±â€¯0.17) hand, nor between the hands. Intraclass-correlation coefficient (ICC) values indicated excellent inter-session reproducibility for CKC strength for both dominant (0.86) and non-dominant (0.97) hand. However, some participants showed pronounced inter-session variability in CKC strength, but only for the dominant hand. CKC is a promising tool to study proprioception in long-term longitudinal studies in the group level to follow, e.g., integrity of cortical proprioceptive processing with motor functions after stroke.

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 listener's 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 listener's 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.

Effect of interstimulus interval on cortical proprioceptive responses to passive finger movements.

Shortening of the interstimulus interval (ISI) generally leads to attenuation of cortical sensory responses. For proprioception, however, this ISI effect is still poorly known. Our aim was to characterize the ISI dependence of movement-evoked proprioceptive cortical responses and to find the optimum ISI for proprioceptive stimulation. We measured, from 15 healthy adults, magnetoencephalographic responses to passive flexion and extension movements of the right index finger. The movements were generated by a movement actuator at fixed ISIs of 0.5, 1, 2, 4, 8, and 16 s, in separate blocks. The responses peaked at ~ 70 ms (extension) and ~ 90 ms (flexion) in the contralateral primary somatosensory cortex. The strength of the cortical source increased with the ISI, plateauing at the 8-s ISI. Modeling the ISI dependence with an exponential saturation function revealed response lifetimes of 1.3 s (extension) and 2.2 s (flexion), implying that the maximum signal-to-noise ratio (SNR) in a given measurement time is achieved with ISIs of 1.7 s and 2.8 s respectively. We conclude that ISIs of 1.5-3 s should be used to maximize SNR in recordings of proprioceptive cortical responses to passive finger movements. Our findings can benefit the assessment of proprioceptive afference in both clinical and research settings.

MEG-compatible pneumatic stimulator to elicit passive finger and toe movements.

Magnetoencephalographic (MEG) signals recorded from the primary sensorimotor (SM1) cortex are coherent with kinematics of both active and passive finger movements. The coherence mainly reflects movement-related proprioceptive afference to the cortex. Here we describe a novel MEG-compatible stimulator to generate computer-controlled passive finger and toe movements that can be used as stimuli in functional brain-imaging experiments. The movements are produced by pneumatic artificial muscle (PAM), elastic actuator that shortens with increasing air pressure. To test the applicability of the stimulator to functional brain-imaging, 4-min trains of passive repetitive 5-mm flexion-extension movements of the right and left index finger and the right hallux were produced at 3Hz while the subject's brain activity was measured with whole-scalp MEG and finger or toe kinematics with an accelerometer. In all ten subjects studied, statistically significant coherence (up to 0.78) occurred between the accelerometer and MEG signals at the movement frequency or its first harmonic. Sources of coherent activity were in the contralateral hand or foot SM1 cortices. Movement-evoked fields elicited with intermittent movements of the right index finger (once every 3.2-4.0s; mean±SD peak response latency 88±25ms) were co-located with the respective coherent sources. We further moved the right index finger at 3, 6, and 12Hz (movement ranges 5, 3, and 2mm, respectively), and analyzed the first 1, 2, and 4-min epochs of data. One minute of data was sufficient to locate the left hand area of the SM1 cortex at all movement frequencies. Sound-induced spurious coherence was reliably ruled out in a control experiment. Our novel movement stimulator thus provides a robust and reliable tool to track proprioceptive afference to the cortex and to locate the SM1 cortex.

Corticokinematic coherence mainly reflects movement-induced proprioceptive feedback.

Corticokinematic coherence (CKC) reflects coupling between magnetoencephalographic (MEG) signals and hand kinematics, mainly occurring at hand movement frequency (F0) and its first harmonic (F1). Since CKC can be obtained for both active and passive movements, it has been suggested to mainly reflect proprioceptive feedback to the primary sensorimotor (SM1) cortex. However, the directionality of the brain-kinematics coupling has not been previously assessed and was thus quantified in the present study by means of renormalized partial directed coherence (rPDC). MEG data were obtained from 15 subjects who performed right index-finger movements and whose finger was, in another session, passively moved, with or without tactile input. Four additional subjects underwent the same task with slowly varying movement pace, spanning the 1-5 Hz frequency range. The coupling between SM1 activity recorded with MEG and finger kinematics was assessed with coherence and rPDC. In all conditions, the afferent rPDC spectrum, which resembled the coherence spectrum, displayed higher values than the efferent rPDC spectrum. The afferent rPDC was 37% higher when tactile input was present, and it was at highest at F1 of the passive conditions; the efferent rPDC level did not differ between conditions. The apparent latency for the afferent input, estimated within the framework of the rPDC analysis, was 50-100 ms. The higher directional coupling between hand kinematics and SM1 activity in afferent than efferent direction strongly supports the view that CKC mainly reflects movement-related somatosensory proprioceptive afferent input to the contralateral SM1 cortex.

Cortical kinematic processing of executed and observed goal-directed hand actions.

Motor information conveyed by viewing the kinematics of an agent's action helps to predict how the action will unfold. Still, how observed movement kinematics is processed in the brain remains to be clarified. Here, we used magnetoencephalography (MEG) to determine at which frequency and where in the brain, the neural activity is coupled with the kinematics of executed and observed motor actions. Whole-scalp MEG signals were recorded from 11 right-handed healthy adults while they were executing (Self) or observing (Other) similar goal-directed hand actions performed by an actor placed in front of them. Actions consisted of pinching with the right hand green foam-made pieces mixed in a heap with pieces of other colors placed on a table, and put them in a plastic pot on the right side of the heap. Subjects' and actor's forefinger movements were monitored with an accelerometer. The coherence between movement acceleration and MEG signals was computed at the sensor level. Then, cortical sources coherent with movement acceleration were identified with Dynamic Imaging of Coherent Sources. Statistically significant sensor-level coherence peaked at the movement frequency (F0) and its first harmonic (F1) in both movement conditions. Apart from visual cortices, statistically significant local maxima of coherence were observed in the right posterior superior temporal gyrus (F0), bilateral superior parietal lobule (F0 or F1) and primary sensorimotor cortex (F0 or F1) in both movement conditions. These results suggest that observing others' actions engages the viewer's brain in a similar kinematic-related manner as during own action execution. These findings bring new insights into how human brain activity covaries with essential features of observed movements of others.

Spatial variability in cortex-muscle coherence investigated with magnetoencephalography and high-density surface electromyography.

Cortex-muscle coherence (CMC) reflects coupling between magnetoencephalography (MEG) and surface electromyography (sEMG), being strongest during isometric contraction but absent, for unknown reasons, in some individuals. We used a novel nonmagnetic high-density sEMG (HD-sEMG) electrode grid (36 mm × 12 mm; 60 electrodes separated by 3 mm) to study effects of sEMG recording site, electrode derivation, and rectification on the strength of CMC. Monopolar sEMG from right thenar and 306-channel whole-scalp MEG were recorded from 14 subjects during 4-min isometric thumb abduction. CMC was computed for 60 monopolar, 55 bipolar, and 32 Laplacian HD-sEMG derivations, and two derivations were computed to mimic "macroscopic" monopolar and bipolar sEMG (electrode diameter 9 mm; interelectrode distance 21 mm). With unrectified sEMG, 12 subjects showed statistically significant CMC in 91-95% of the HD-sEMG channels, with maximum coherence at ∼25 Hz. CMC was about a fifth stronger for monopolar than bipolar and Laplacian derivations. Monopolar derivations resulted in most uniform CMC distributions across the thenar and in tightest cortical source clusters in the left rolandic hand area. CMC was 19-27% stronger for HD-sEMG than for "macroscopic" monopolar or bipolar derivations. EMG rectification reduced the CMC peak by a quarter, resulted in a more uniformly distributed CMC across the thenar, and provided more tightly clustered cortical sources than unrectifed sEMGs. Moreover, it revealed CMC at ∼12 Hz. We conclude that HD-sEMG, especially with monopolar derivation, can facilitate detection of CMC and that individual muscle anatomy cannot explain the high interindividual CMC variability.

Phasic stabilization of motor output after auditory and visual distractors.

To maintain steady motor output, distracting sensory stimuli need to be blocked. To study the effects of brief auditory and visual distractors on the human primary motor (M1) cortex, we monitored magnetoencephalographic (MEG) cortical rhythms, electromyogram (EMG) of finger flexors, and corticomuscular coherence (CMC) during right-hand pinch (force 5-7% of maximum) while 1-kHz tones and checkerboard patterns were presented for 100 ms once every 3.5-5 s. Twenty-one subjects (out of twenty-two) showed statistically significant ∼20-Hz CMC. Both distractors elicited a covert startle-like response evident in changes of force and EMG (∼50% of the background variation) but without any visible movement, followed by ∼1-s enhancement of CMC (auditory on average by 75%, P < 0.001; visual by 33%, P < 0.05) and rolandic ∼20-Hz rhythm (auditory by 14%, P < 0.05; visual by 11%, P < 0.01). Directional coupling of coherence from muscle to the M1 cortex (EMG→MEG) increased for ∼0.5 s at the onset of the CMC enhancement, but only after auditory distractor (by 105%; P < 0.05), likely reflecting startle-related proprioceptive afference. The 20-Hz enhancements occurred in the left M1 cortex and were for the auditory stimuli preceded by an early suppression (by 7%, P < 0.05). Task-unrelated distractors modulated corticospinal coupling at ∼20 Hz. We propose that the distractors triggered covert startle-like responses, resulting in proprioceptive afference to the cortex, and that they also transiently disengaged the subject's attention from the fine-motor task. As a result, the corticospinal output was readjusted to keep the contraction force stable.

Eye Tracking in MEG.

Magnetoencephalography (MEG) can measure brain activity in ms-level temporal resolution. MEG sensors are super sensitive devices for magnetic signals of the brain but are also prone to electromagnetic interferences. The MEG device is located inside the magnetically shielded room (MSR), and any monitoring device used inside the MSR requires special shielding and its location must be carefully selected to suppress electromagnetic interference. Eye-tracker measures eye movements, providing spatial location of the gaze, pupil diameters, and eye blinks. Eye tracking in MEG enables, for example, categorization of the MEG data based on gaze position and interactive stimulus using gaze position. Combining the methods together will require considering the electromagnetic interference for the MEG-that is, additional shielding, positioning of the eye tracker, and subject-specific issues related to make-up and eye-corrective lenses.