Probing modifications of cortical excitability during stroke recovery with navigated transcranial magnetic stimulation.

OBJECTIVE: To follow cortical excitability changes during recovery from stroke with navigated transcranial magnetic stimulation (nTMS), in particular, to characterize changes of short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF), to correlate them with recovery of upper extremity function, and to detect possible shifts of cortical hand representations. METHODS: Single and paired pulse nTMS were delivered to the hemisphere with infarction and to the hemisphere without infarction in 14 first-ever stroke patients at 1 (T1) and 3 months (T2) after stroke. Electromyographic responses to nTMS stimulation were recorded from the first dorsal interosseus muscles. nTMS was used to ensure an accurate coil repositioning in repeated measurements. Hand function recovery was clinically evaluated using the Action Research Arm Test (ARAT) and 9-hole peg test (9-HPT). RESULTS: SICI and ICF were modulated in both hemispheres during recovery. Inhibition in the hemisphere without infarction correlated significantly with the affected hand performance at T2; stronger disinhibition (poor inhibition) was associated with worse hand performance. Location of hand muscle representations was shifted in 3 well-recovered patients out of 14 patients at T2. CONCLUSIONS: In line with earlier studies, disinhibition in the hemisphere without infarction may be related to poor recovery of the affected hand. Usage of the affected hand during stroke recovery seems to influence these cortical excitability changes. nTMS is a valuable tool for tracking muscle cortical representation changes during brain reorganization.

Fine tuning the correlation limit of spatio-temporal signal space separation for magnetoencephalography.

Head, jaw and tongue movements contribute to speech artifacts in magnetoencephalography (MEG). Their sources lay close to MEG sensors, therefore, the spatio-temporal signal space separation method (tSSS), specifically suppressing nearby artifacts, can be used for speech artifact suppression. After data reconstruction by signal space separation (referred as SSS), tSSS identifies artifacts by their correlated temporal behavior inside and outside the sensor helmet. The artifacts to be eliminated are thresholded by the quantitative level of this correlation determined by correlation limit (CL). Unnecessarily high CL value may result in suboptimal interference suppression. We evaluated the performance of tSSS with different CLs on MEG data containing speech artifacts. MEG was recorded with 204 planar gradiometers and 102 magnetometers in two subjects counting aloud. The recorded data were processed by tSSS using CLs 0.98, 0.8 and 0.6, and traces were compared. The speech artifact was increasingly suppressed with decreasing CL, but sufficient suppression was achieved at different CL in each subject. Alpha rhythm was not suppressed with CL 0.98 or 0.8; some amplitude reduction with CL 0.6 occurred in one subject. The tSSS is a robust tool suppressing MEG artifacts. It can be fine tuned for challenging artifacts which, after insufficient rejection might resemble brain signals.

Electrophysiological correlates of short-latency afferent inhibition: a combined EEG and TMS study.

Cutaneous stimulation produces short-latency afferent inhibition (SAI) of motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS). Since the demonstration of SAI is primarily based on the attenuation of MEPs, its cortical origin is not yet fully understood. In the present study we combined TMS with concurrent electroencephalography (EEG) in order to obtain direct cortical correlates of SAI. TMS-evoked EEG responses and MEPs were analysed with and without preceding electrical stimulation of the index finger cutaneous afferents in ten healthy volunteers. We show that the attenuation of MEPs by cutaneous stimulation has its counterpart in the attenuation of the N100 EEG response. Moreover, the attenuation of the cortical N100 component correlated positively with the strength of SAI, indicating that the transient changes in cortical excitability can be reflected in the amplitude dynamics of MEPs. We hypothesize that the hyperpolarization of the pyramidal cells due to SAI lowers the capacity of TMS to induce the inhibitory current needed to elicit N100, thus leading to its attenuation. We suggest that the observed interaction of two inhibitory processes, SAI and N100, provides further evidence for the cortical origin of SAI.

Validation of head movement correction and spatiotemporal signal space separation in magnetoencephalography.

OBJECTIVE: Our aim was to assess the effectiveness and reliability of spatiotemporal signal space separation (tSSS) and movement correction (MC) in magnetoencephalography (MEG) recordings disturbed by head movements and magnetized material on the head. METHODS: We recorded MEG from 20 healthy adults in stationary (reference) head position and during controlled head movements. Nearby magnetic interference sources were simulated by attaching magnetized particles on the subject's head. Auditory and somatosensory stimuli were presented. MC, tSSS and averaging were performed to obtain auditory (AEF) and somatosensory (SEF) evoked fields. Neuronal sources were modeled as equivalent current dipoles. MC was also validated by reconstructing signals generated by current dipoles in a phantom. RESULTS: After MC, the AEF and SEF responses recorded during intermittent head movements were similar in amplitude to the reference recordings and differed by 5-7mm in source location. The tSSS method removed artifacts due to the attached magnetized particles but did not affect the reference data. CONCLUSIONS: The methods are able to reliably recover MEG responses contaminated by movements and magnetic artifacts on the head. SIGNIFICANCE: The combination of tSSS and MC methods is especially useful in clinical measurements, where movements and magnetic disturbances are commonly present.