Comparison of Block vs. Event Design in fMRI of the Somatosensory System

Cortical activation after electrical direct median nerve stimulation either in standard block designs or in an event-related design can be found by fMRI. Commonly used stimulation frequencies are in the range from about 100Hz down to less than one per second. A frequency dependence of the cortical response can be observed. Higher stimulation rates improve the statistical significance of the result, which may be a result of stronger activations. As we know from MEG or EEG studies, higher stimulation rates increase habituation effects yielding a loss of signal change. There were indications of differences in the location and/or weighting of the activated clusters between the block and event designs in our first studies. In this study, different event designs were compared to the standard block design with respect to the localizations of the activated clusters aiming at a robust event design activating most of the somatosensory regions known from MEG and EEG studies. We used an optimized sequence of electrical direct nerve stimulation of the median nerve. In both the event design as well as the standard block design significant activations could be found in the primary and secondary somatosensory areas in area 3b, area 1 and the parietal operculum as well as in the frontoparietal operculum and the medial cortex of the insula. In some subjects activations could be found in PPC, the thalamus and the cingulate cortex. Overall, the random effects model of the group analysis showed 9 activated clusters in the standard block design and 12 clusters in the event-related design. Thus, a robust paradigm for event-related fMRI studies of the somatosensory system could be established. This design is more suitable for studies of cognitive tasks of the somatosensory system.