Parametrizing the Conditionally Gaussian Prior Model for Source Localization with Reference to the P20/N20 Component of Median Nerve SEP/SEF

In this article, we focused on developing the conditionally Gaussian hierarchical Bayesian model (CG-HBM), which forms a superclass of several inversion methods for source localization of brain activity using somatosensory evoked potential (SEP) and field (SEF) measurements. The goal of this proof-of-concept study was to improve the applicability of the CG-HBM as a superclass by proposing a robust approach for the parametrization of focal source scenarios. We aimed at a parametrization that is invariant with respect to altering the noise level and the source space size. The posterior difference between the gamma and inverse gamma hyperprior was minimized by optimizing the shape parameter, while a suitable range for the scale parameter can be obtained via the prior-over-measurement signal-to-noise ratio, which we introduce as a new concept in this study. In the source localization experiments, the primary generator of the P20/N20 component was detected in the Brodmann area 3b using the CG-HBM approach and a parameter range derived from the existing knowledge of the Tikhonov-regularized minimum norm estimate, i.e., the classical Gaussian prior model. Moreover, it seems that the detection of deep thalamic activity simultaneously with the P20/N20 component with the gamma hyperprior can be enhanced while using a close-to-optimal shape parameter value.

The effect of stimulation type, head modeling, and combined EEG and MEG on the source reconstruction of the somatosensory P20/N20 component

Modeling and experimental parameters influence the Electro- (EEG) and Magnetoencephalography (MEG) source analysis of the somatosensory P20/N20 component. In a sensitivity group study, we compare P20/N20 source analysis due to different stimulation type (Electric-Wrist [EW], Braille-Tactile [BT], or Pneumato-Tactile [PT]), measurement modality (combined EEG/MEG - EMEG, EEG, or MEG) and head model (standard or individually skull-conductivity calibrated including brain anisotropic conductivity). Considerable differences between pairs of stimulation types occurred (EW-BT: 8.7 ± 3.3 mm/27.1° ± 16.4°, BT-PT: 9 ± 5 mm/29.9° ± 17.3°, and EW-PT: 9.8 ± 7.4 mm/15.9° ± 16.5° and 75% strength reduction of BT or PT when compared to EW) regardless of the head model used. EMEG has nearly no localization differences to MEG, but large ones to EEG (16.1 ± 4.9 mm), while source orientation differences are non-negligible to both EEG (14° ± 3.7°) and MEG (12.5° ± 10.9°). Our calibration results show a considerable inter-subject variability (3.1-14 mS/m) for skull conductivity. The comparison due to different head model show localization differences smaller for EMEG (EW: 3.4 ± 2.4 mm, BT: 3.7 ± 3.4 mm, and PT: 5.9 ± 6.8 mm) than for EEG (EW: 8.6 ± 8.3 mm, BT: 11.8 ± 6.2 mm, and PT: 10.5 ± 5.3 mm), while source orientation differences for EMEG (EW: 15.4° ± 6.3°, BT: 25.7° ± 15.2° and PT: 14° ± 11.5°) and EEG (EW: 14.6° ± 9.5°, BT: 16.3° ± 11.1° and PT: 12.9° ± 8.9°) are in the same range. Our results show that stimulation type, modality and head modeling all have a non-negligible influence on the source reconstruction of the P20/N20 component. The complementary information of both modalities in EMEG can be exploited on the basis of detailed and individualized head models.

Effects of observing normal and abnormal goal-directed hand movements on somatosensory cortical activation

Existing evidence indicates the importance of observing correct, normal actions on the motor cortical activities. However, the exact neurophysiological mechanisms, particularly in the somatosensory system, remain unclear. This study aimed to elucidate the effects of observing normal and abnormal hand movements on the contralateral primary somatosensory (cSI), contralateral (cSII) and ipsilateral (iSII) secondary somatosensory activities. Experiment I was designed to investigate the effects of motor outputs on the somatosensory processing, in which subjects were instructed to relax or manipulate a small cube. Experiment II was tailored to examine the somatosensory responses to the observation of normal (Normal) and abnormal (Abnormal) hand movements. The subjects received electrical stimulation to right median nerve and magnetoencephalography (MEG) recordings during the whole experimental period. Regional cortical activation and functional connectivity were analyzed. Compared to the resting condition, a reduction in cSI and an enhancement of SII activation was found when subjects manipulated a cube, suggesting the motor outputs have an influence on the somatosensory responses. Further investigation of the effects of observing different hand movements showed that cSII activity was significantly stronger in the Normal than Abnormal condition. Moreover, compared with Abnormal condition, a higher cortical coherence of cSI-iSII at theta bands and cSII-iSII at beta bands was found in Normal condition. Conclusively, the present results suggest stronger activation and enhanced functional connectivity within the somatosensory system during the observation of normal than abnormal hand movements. These findings also highlight the importance of viewing normal, correct hands movements in the stroke rehabilitation.

Abnormal Somatosensory Synchronization in Patients With Paroxysmal Kinesigenic Dyskinesia: A Magnetoencephalographic Study

Paroxysmal kinesigenic dyskinesia (PKD) is a rare group of hyperkinetic movement disorders characterized by brief attacks of choreoathetosis or dystonia. To clarify the alterations of the functional connectivity within the somatosensory network in PKD patients, magnetoencephalographic (MEG) responses to paired median-nerve electrical stimulation were recorded in 10 PKD patients treated by carbamazepine or oxcarbamazepine and 22 age-matched controls. In patients, MEG recordings were obtained during drug-on and -off periods. Source-based functional connectivity analysis was performed between contralateral primary (cSI) and secondary (cSII), and ipsilateral secondary (iSII) somatosensory areas. During drug-off periods, patients with PKD demonstrated decreased cSI-iSII and increased cSII-iSII somatosensory connectivity at theta band. Drug-on periods lowered the functional connectivity in cSI-cSII at alpha and beta bands and in cSII-iSII at theta band compared with the drug-off periods. We suggest that altered theta functional connectivity in cSI-iSII and cSII-iSII could be the neurophysiological signatures in PKD.

Magnetoencephalographic Imaging of Auditory and Somatosensory Cortical Responses in Children with Autism and Sensory Processing Dysfunction

This study compared magnetoencephalographic (MEG) imaging-derived indices of auditory and somatosensory cortical processing in children aged 8–12 years with autism spectrum disorder (ASD; N = 18), those with sensory processing dysfunction (SPD; N = 13) who do not meet ASD criteria, and typically developing control (TDC; N = 19) participants. The magnitude of responses to both auditory and tactile stimulation was comparable across all three groups; however, the M200 latency response from the left auditory cortex was significantly delayed in the ASD group relative to both the TDC and SPD groups, whereas the somatosensory response of the ASD group was only delayed relative to TDC participants. The SPD group did not significantly differ from either group in terms of somatosensory latency, suggesting that participants with SPD may have an intermediate phenotype between ASD and TDC with regard to somatosensory processing. For the ASD group, correlation analyses indicated that the left M200 latency delay was significantly associated with performance on the WISC-IV Verbal Comprehension Index as well as the DSTP Acoustic-Linguistic index. Further, these cortical auditory response delays were not associated with somatosensory cortical response delays or cognitive processing speed in the ASD group, suggesting that auditory delays in ASD are domain specific rather than associated with generalized processing delays. The specificity of these auditory delays to the ASD group, in addition to their correlation with verbal abilities, suggests that auditory sensory dysfunction may be implicated in communication symptoms in ASD, motivating further research aimed at understanding the impact of sensory dysfunction on the developing brain.

Cortical Mechanisms of Tongue Sensorimotor Functions in Humans: A Review of the Magnetoencephalography Approach

The tongue plays important roles in a variety of critical human oral functions, including speech production, swallowing, mastication and respiration. These sophisticated tongue movements are in part finely regulated by cortical entrainment. Many studies have examined sensorimotor processing in the limbs using magnetoencephalography (MEG), which has high spatiotemporal resolution. Such studies have employed multiple methods of analysis, including somatosensory evoked fields (SEFs), movement-related cortical fields (MRCFs), event-related desynchronization/synchronization (ERD/ERS) associated with somatosensory stimulation or movement and cortico-muscular coherence (CMC) during sustained movement. However, the cortical mechanisms underlying the sensorimotor functions of the tongue remain unclear, as contamination artifacts induced by stimulation and/or muscle activity within the orofacial region complicates MEG analysis in the oral region. Recently, several studies have obtained MEG recordings from the tongue region using improved stimulation methods and movement tasks. In the present review, we provide a detailed overview of tongue sensorimotor processing in humans, based on the findings of recent MEG studies. In addition, we review the clinical applications of MEG for sensory disturbances of the tongue caused by damage to the lingual nerve. Increased knowledge of the physiological and pathophysiological mechanisms underlying tongue sensorimotor processing may improve our understanding of the cortical entrainment of human oral functions.

Magnetoencephalography study of the effect of attention modulation on somatosensory processing in patients with major depressive disorder

AIMS

Although affective and/or attention modulation of somatosensory processing has been well studied, the biological bases of somatic symptoms in patients with major depressive disorder (MDD) have rarely been examined. To elucidate changes in somatosensory processing underlying somatic symptoms in patients with MDD, we conducted a magnetoencephalography study of patients with MDD and healthy controls.

METHODS

After median nerve stimulation, somatosensory evoked fields (SEF) were recorded in 10 patients with MDD and 10 sex-, age-, and height-matched healthy volunteers under somatosensory attending, visually attending, and non-attending conditions. The latencies and magnitudes of N20m and P60m SEF were examined.

RESULTS

In the MDD group, P60m latency was significantly prolonged, irrespective of attention modulation, whereas N20m latency and root mean squares N20m and P60m amplitudes remained unchanged. Prolonged P60m latency negatively correlated with the somatosensory threshold, which was relatively high in the MDD group. Prolonged P60m latency also negatively correlated with a state of anxiety during the examination, but not with depressive symptoms or psychotropic medication.

CONCLUSIONS

These results suggested that patients with MDD experience dysfunction in somatosensory information processing, approximately 60 ms after stimuli, irrespective of attentional conditions.

Reappraisal of field dynamics of motor cortex during self-paced finger movements

BACKGROUND

The exact origin of neuronal responses in the human sensorimotor cortex subserving the generation of voluntary movements remains unclear, despite the presence of characteristic but robust waveforms in the records of electroencephalography or magnetoencephalography (MEG).

AIMS

To clarify this fundamental and important problem, we analyzed MEG in more detail using a multidipole model during pulsatile extension of the index finger, and made some important new findings.

RESULTS

Movement-related cerebral fields (MRCFs) were confirmed over the sensorimotor region contralateral to the movement, consisting of a temporal succession of the first premovement component termed motor field, followed by two or three postmovement components termed movement evoked fields. A source analysis was applied to separately model each of these field components. Equivalent current diploes of all components of MRCFs were estimated to be located in the same precentral motor region, and did not differ with respect to their locations and orientations. The somatosensory evoked fields following median nerve stimulation were used to validate these findings through comparisons of the location and orientation of composite sources with those specified in MRCFs. The sources for the earliest components were evoked in Brodmann's area 3b located lateral to the sources of MRCFs, and those for subsequent components in area 5 and the secondary somatosensory area were located posterior to and inferior to the sources of MRCFs, respectively. Another component peaking at a comparable latency with the area 3b source was identified in the precentral motor region where all sources of MRCFs were located.

CONCLUSION

These results suggest that the MRCF waveform reflects a series of responses originating in the precentral motor area.

Dorsal penile nerve stimulation elicits left-hemisphere dominant activation in the second somatosensory cortex

Activation of peripheral mixed and cutaneous nerves activates a distributed cortical network including the second somatosensory cortex (SII) in the parietal operculum. SII activation has not been previously reported in the stimulation of the dorsal penile nerve (DPN). We recorded somatosensory evoked fields (SEFs) to DPN stimulation from 7 healthy adults with a 122-channel whole-scalp neuromagnetometer. Electrical pulses were applied once every 0.5 or 1.5 sec to the left and right DPN. For comparison, left and right median and tibial nerves were stimulated alternatingly at 1.5-sec intervals. DPN stimuli elicited weak, early responses in the vicinity of responses to tibial nerve stimulation in the primary somatosensory cortex. Strong later responses, peaking at 107-126 msec were evoked in the SII cortices of both hemispheres, with left-hemisphere dominance. In addition to tactile processing, SII could also contribute to mediating emotional effects of DPN stimuli.

Carpal tunnel syndrome modifies sensory hand cortical somatotopy: a MEG study

The adult somatosensory system has shown reorganizational abilities at cortical and subcortical levels after peripheral nerve lesions. In the present study the effects of carpal tunnel syndrome (CTS) are investigated as reflected on the somatotopy of the primary cortical hand representation. Position and intensity of cortical sources activated by the separate electrical stimulation of median nerve and Digits 1, 3, and 5 of both affected and non-affected hands are evaluated by magnetoencephalographic (MEG) technique. Correlation of MEG results with patient-, physician- and neurophysiological-oriented evaluations of CTS was carried out. Patients showed changes in cortical hand somatotopy in strict relationship to self-referred assessment of symptoms and hand disability in daily activities, including: 1) a more extended representation of the affected hand when paresthesias prevailed; and 2) a more restricted representation due to lateral shift of the little finger was observed when pain symptoms dominated the clinical picture. Contralateral to the side of CTS, the cortical sources activated by Digit 5-stimulation appeared significantly enhanced with respect to contralateral ones from non-affected hand. When comparing the amplitude of peripheral sensory nerve action potentials of median and ulnar nerves to that of cortical responses (i.e., ECD strengths of M20 and M30 components after stimulation of Digits 3 and 5), a significant selective amplification of M30 with respect to M20 and sensory nerve action potential (SNAP) appeared during Digit 3 stimulation compared to that observed for Digit 5. This has been interpreted as a central magnification mechanism in brain responsiveness, possibly revealing a safety factor enabling sensory perception despite the small peripheral signal due to nerve trunk dysfunction. Hum.