A Functional Near-infrared Spectroscopy Study to Detect Activation of Somatosensory Cortex by Peripheral Nerve Stimulation

Introduction  Multi-channel near-infrared spectroscopy (NIRS) is a method for non-invasively monitoring of relative concentrations of oxygenated, deoxygenated, and total hemoglobin. This technique has found expanding application in brain mapping and functional imaging. The purpose of this study was to investigate whether activation of somatosensory cortex can be detected without the necessity of the patient’s cooperation in performing a task. Methods  Real-time bilateral parietotemporal cerebral oxygenation was monitored in 12 healthy volunteers. The median nerve at the wrist was electrically stimulated repeatedly at an amplitude below the threshold of discomfort. Interstimulus intervals were randomized between 13 and 31 s to minimize synchronization with respiration or other natural oscillations in cerebral oxygenation. Results  In 8 of the 12 subjects, activation over the contralateral primary somatosensory cortex was detected, correlating significantly with the predicted hemodynamic response function. Conclusions  To our knowledge, this is the first time functional NIRS has been used to detect activation of somatosensory cortex with peripheral nerve stimulation. While the sensitivity for detection of the functional hemodynamic response was inadequate for clinical diagnostics, these findings are uniquely important in critical care imaging in that the regional blood flow and oxygenation changes can be detected without the requirement of a volitional task. This advancement potentially expands the capability of this modality to be used in brain mapping and in the evaluation of patients with impaired cognitive or motor function at the bedside.     

Sleep stage dependant changes of the high-frequency part of the somatosensory evoked potentials at the thalamus and cortex.

OBJECTIVES: It is known that the high-frequency oscillations (above 400 Hz) of the somatosensory evoked potentials (SEPs) diminish during sleep while the N20 persists (Neurology 38 (1988) 64; Electroenceph clin Neurophysiol 70 (1988) 126; Electroenceph clin Neurophysiol 100 (1996) 189). We investigated possible differential effects of sleep on the 600 Hz SEPs at the thalamus and cortex. METHODS: SEPs from 10 subjects were recorded using 64 channels following electric stimulation at the wrist during awake state and sleep stages II, IV and REM. Dipole source analysis was applied to separate brain-stem, thalamic and cortical activity in the low-frequency (20-450 Hz) and the high-frequency (450-750 Hz) part of the signal. RESULTS: The low-frequency SEPs showed a non-significant increase of the latency of the N20 and a bifid change of the waveform in 3 subjects. The high-frequency SEPs showed a significant decrease of their amplitude at the level of the thalamus and cortex but not at the brain-stem. This decrease in amplitude at the thalamus and cortex were significantly correlated. There was no effect on the latency of the signal. In addition, at the cortex, differential effects on early and late parts of the 600 Hz oscillations were found by time-frequency analysis using a wavelet transformation. CONCLUSIONS: Sleep dependent decrease of the high-frequency SEPs were first observed at the thalamus pointing to the known function of the reticular thalamic nucleus regulating arousal. The results presented here provide further evidence for a thalamic origin of the 600 Hz oscillations. In addition, on the basis of the differential effects on early (up to the N20 peak) and late (between 20 and 25 ms) parts of the signal, at least one intracortical generator of these oscillations is proposed. In general, the high-frequency SEPs (600 Hz oscillations) are supposed to reflect activity of a somatosensory arousal system.     

Cortical tremor (FCMTE: familial cortical myoclonic tremor with epilepsy)

For 15 years, 50 Japanese and European families with cortical myoclonic tremor and epilepsy were reported in the literature under various names. More recently, the acronym familial cortical myoclonic tremor with epilepsy (FCMTE) has been proposed for this new clinical entity based on both clinical and electrophysiological criteria: irregular postural myoclonic tremor of the distal limbs, familial history of epilepsy, autosomal dominant inheritance, and a rather benign outcome. The diagnosis is confirmed by electrophysiological features favoring cortical reflex myoclonus (enhanced C reflex at rest, giant somatosensory evoked potentials (SEPs), premyoclonus cortical spikes detected by the jerk-locked back-averaging method), and a good response to antiepileptic drugs. The genetic analysis of these families shows heterogeneity with a linkage to chromosome 8q24 for Japanese families, a linkage to chromosome 2p for Italian families, the exclusion of 8q24 locus for a Spanish family, and the exclusion of both loci for a Dutch family. The similarities of this syndrome with the group of myoclonic epilepsy suggest an abnormality of a gene encoding ion channels.     

Effect of Transcranial Static Magnetic Field Stimulation Over the Sensorimotor Cortex on Somatosensory Evoked Potentials in Humans

BackgroundThe motor cortex in the human brain can be modulated by the application of transcranial static magnetic field stimulation (tSMS) through the scalp. However, the effect of tSMS on the excitability of the primary somatosensory cortex (S1) in humans has never been examined.ObjectiveThis study was performed to investigate the possibility of non-invasive modulation of S1 excitability by the application of tSMS in healthy humans.MethodstSMS and sham stimulation over the sensorimotor cortex were applied to 10 subjects for periods of 10 and 15 min. Somatosensory evoked potentials (SEPs) following right median nerve stimulation were recorded before and immediately after, 5 min after, and 10 min after tSMS from sites C3′ and F3 of the international 10-20 system of electrode placement. In another session, SEPs were recorded from 6 of the 10 subjects every 3 min during 15 min of tSMS.ResultsAmplitudes of the N20 component of SEPs at C3′ significantly decreased immediately after 10 and 15 min of tSMS by up to 20%, returning to baseline by 10 min after intervention. tSMS applied while recording SEPs every 3 min and sham stimulation had no effect on SEP.ConclusionstSMS is able to modulate cortical somatosensory processing in humans, and thus might be a useful tool for inducing plasticity in cortical somatosensory processing. Lack of change in the amplitude of SEPs with tSMS implies that use of peripheral nerve stimulation to cause SEPs antagonizes alteration of the function of membrane ion channels during exposure to static magnetic fields.