Effects of somatosensory stimulation and attention on human somatosensory cortex: an fMRI study

It is well known that primary and non-primary areas of human somatosensory cortex are involved in the processing of adequate deviant/rare stimuli and omission of frequent stimuli. However, the relative weight and interaction of these variables is poorly known. This functional magnetic resonance imaging (fMRI) study tested the hypothesis that somatosensory stimulus processing and attention especially interact in non-primary somatosensory areas including secondary somatosensory cortex (SII) and insula. To test this hypothesis, responses of somatosensory cortex were mapped during four conditions of an oddball paradigm: DELIVERED COUNT and IGNORE (count or ignore deviant/rare electrical stimuli, respectively); OMITTED COUNT and IGNORE (count or ignore the rare omission of frequent electrical stimuli, respectively). The deviant/rare and frequent electrical stimuli were delivered to median and ulnar nerve, respectively. It was observed that contralateral (left) primary somatosensory responses were not markedly modulated by the mentioned deviant/rare events. Furthermore, contralateral SII and insula responded to all but not OMITTED IGNORE (purely attentive) condition, whereas ipsilateral (right) SII responded to all conditions. Finally, ipsilateral insula responded to the COUNT (attentive) conditions, regardless of the physical presence of the deviant/rare stimuli. The results suggest that in somatosensory modality, bilateral SII and left (contralateral) insula reflect complex integrative processes of stimulus elaboration and attention, whereas right (ipsilateral) insula mainly sub-serves active attention to deviance within a sequence of somatosensory stimuli.