Neural connectivity as revealed by SEF wave shape: A morphological MEG study

The time varying spatial properties of the evoked cerebral response to external stimuli depicts the characteristics of the excited neural circuitry for the specific sensory involved modality. These spatiotemporal features are reflected in the shape of the brain evoked response, and our aim is to quantitatively characterize individually anatomofunctional features for the somatosensory system (somatosensory evoked fields = SEFs). Suitable tool could be magnetoencephalographic (MEG) signal, being not distorted by the extra-cerebral layers (i.e. the skull and meninges) separating neural currents from the recording apparatus: this allows the waveform of the evoked response to directly describe the spatio-temporal cerebral activation. The morphology of SEFs following median nerve stimulation has been described as two initial peaks of opposite polarity (M20, M30, around 20 and 30 ms from the stimulus onset, [1, 2, 3, 4]), followed by a third component between 50 and 100 ms from the stimulus onset. Despite this pattern, the shape of the response is quite variable from subject to subject. In fact, great inter-individual variability has been previously described recognizing three morphology categories for the latter component ([4], for our data, Fig1): in some subjects it has the same polarity as the preceding one (Fig. 1, S1, 61% of our cases), in others it shows the opposite (Fig. 1, S2, 29% of cases) or is even absent (Fig. 1, S3, 10% of cases). Combined with this inter-individual variability of SEF morphology, a great intra-subjective, interhemispheric similarity of SEF pattern is evident (Fig. 1). Moreover, SEF morphology is very stable when tested in successive independent sessions in the same subject. Our study attempts to define a parameter that describes intra-subject interhemispheric similarity, quantitatively evaluates it in a healthy population and calculates its normative limits. Figure 1: Superimposition of all recording channels in left/right parietal regions (respectively left/right columns) in the 100 ms post-stimulus period. SEFs of three paradigmatic subjects are presented, to show different SEF morphologies (compare S1, S2, S3 in one hemisphere) both in the time intervals before 45 ms and between 45 and 100 ms from the stimulus onset. By comparing SEF from left vs right hemisphere for each subject, the intra-subject interhemispheric shape similarity is appreciable. On the right side, their parameters values are reported (IHC_SI, IHC_par).