Functional neuroimaging of the somatosensory system with Ultra-high-field fMRI and MEG

Multimodal neuroimaging using a combination of Magnetoencephalography (MEG) and ultra-high-field fMRI are used in order to gain further insight into the neural oscillations and haemodynamic responses in the somatosensory cortex. Single pulse electrical median nerve stimulation (MNS) with regular and jittered intervals is used in MEG. A preliminary study is used to determine acceptable trial number and length, and highlights points to be considered in paradigm optimization. Time-frequency analysis shows that the largest activities are beta event-related desynchronization (ERD) and event-related synchronization (ERS) between 13Hz and 30Hz. No significant difference in both the induced oscillations and evoked responses are found. Paired pulse MNS with varying ISIs are studied using MEG and 7T fMRI. The beta ERD is suggested to have a gating role with a magnitude irrespective of the starting point of stimulus. Non-linearity effects both in beta ERD/ERS and P35m are shown for ISIs of up to 2s, implying that the non-linear neural responses to the stimulus may still contribute to the BOLD non-linearity even when the evoked response has returned to baseline. Multiple pulse MNS with varying pulse train length and frequency are also investigated using MEG and MEG-fMRI. The gating role of beta ERD is further confirmed and the N160m is suggested to be modulated under this role. No accumulative effect is seen in the ERS with increasing pulse number but the amplitude of the ERS is modulated by the frequency. This can be explained by a Cortical Activation Model (CAM). Efforts to spatially separate the beta ERD and ERS are shown for all three studies. Group averaged SAM images suggest a separation of activation areas along the central gyrus. Significant difference are found in the z MNI coordinate between beta ERD and ERS peak locations, suggesting that these two effects could arise from different generators. In the multiple pulse frequency study, by including the temporal signature of beta ERD and ERS as a regressor in BOLD fMRI analysis, delayed BOLD responses are located posterior to the standard BOLD response. However, the exact nature of the relationship between this delayed BOLD response and the ERS effects requires further work

Functional neuroimaging of the somatosensory system with Ultra-high-field fMRI and MEG

Multimodal neuroimaging using a combination of Magnetoencephalography (MEG) and ultra-high-field fMRI are used in order to gain further insight into the neural oscillations and haemodynamic responses in the somatosensory cortex. Single pulse electrical median nerve stimulation (MNS) with regular and jittered intervals is used in MEG. A preliminary study is used to determine acceptable trial number and length, and highlights points to be considered in paradigm optimization. Time-frequency analysis shows that the largest activities are beta event-related desynchronization (ERD) and event-related synchronization (ERS) between 13Hz and 30Hz. No significant difference in both the induced oscillations and evoked responses are found. Paired pulse MNS with varying ISIs are studied using MEG and 7T fMRI. The beta ERD is suggested to have a gating role with a magnitude irrespective of the starting point of stimulus. Non-linearity effects both in beta ERD/ERS and P35m are shown for ISIs of up to 2s, implying that the non-linear neural responses to the stimulus may still contribute to the BOLD non-linearity even when the evoked response has returned to baseline. Multiple pulse MNS with varying pulse train length and frequency are also investigated using MEG and MEG-fMRI. The gating role of beta ERD is further confirmed and the N160m is suggested to be modulated under this role. No accumulative effect is seen in the ERS with increasing pulse number but the amplitude of the ERS is modulated by the frequency. This can be explained by a Cortical Activation Model (CAM). Efforts to spatially separate the beta ERD and ERS are shown for all three studies. Group averaged SAM images suggest a separation of activation areas along the central gyrus. Significant difference are found in the z MNI coordinate between beta ERD and ERS peak locations, suggesting that these two effects could arise from different generators. In the multiple pulse frequency study, by including the temporal signature of beta ERD and ERS as a regressor in BOLD fMRI analysis, delayed BOLD responses are located posterior to the standard BOLD response. However, the exact nature of the relationship between this delayed BOLD response and the ERS effects requires further work

Neuromagnetic characterization of the human secondary somatosensory cortex

This thesis aims to characterize functions of the secondary somatosensory cortex SII in humans by means of neuromagnetic recordings. It starts with a review of literature about methodological considerations concerning the generation of neuromagnetic fields and how they can be measured and modeled. Then follows a brief overview of the somatosensory system, its receptors, ascending pathways, and cortical regions. The main part of the text summarizes the individual publications. Studies I and II show a strong interaction at SII between inputs of the two hands and of different fingers in the same hand, suggesting that SII plays an important role in integrating sensory information from the two body halves. Studies III and IV reveal left hemisphere-dominant SII activation for median nerve stimulation and passive movements of the index finger, suggesting hemispheric specialization for the processing of somatosensory information. Study V analyzes functional connectivity between SI and SII cortices by means of phase-locked activity. Study VI shows that SII activation is distinctly affected by contraction of different muscles. Altogether these studies provide an insight in to the role of SII in somatosensory processing.reviewe