Tactile stimulation of the face and hand studied by functional MRI

Background: Tactile stimuli are physiological somatosensory stimuli which predominantly activate touch-encoding afferents. In contrast to electrical stimuli, they can be applied to the face during fMRI. As somatosensory perception depends on experimental factors such as the subject's attention, we compared two different experimental designs for tactile stimulation during fMRI.

Methods: Eight right-handed volunteers (four females, four males) were studied with a Siemens Symphony 1.5T scanner during two runs of a blocked and two runs of an event-related design (165 images/run). The intensity of tactile stimuli was adjusted for each subject and the stimuli were unilaterally applied to the ipsilateral upper lip and thenar. Single stimuli with a jittered interstimulus interval were used during the event-related design while periods of continuous stimulation were alternated with periods of rest during the blocked design. The order of runs and the side of stimulation were balanced across the group.

Results: All subjects experienced the feeling of „light touch“ during the experiment but no unpleasent, itchy or painful sensation. Tactile stimulation activated distinct face and hand representation areas in the primary somatosensory cortex (S1) contralaterally in six, ipsilaterally in one and bilaterally in another subject. Activation of the bilateral parietal operculum (S2) was found in all subjects. Although there was no trend in favour of one paradigm regarding extent (cluster size) and Z-scores of activation, S1 was only found significantly activated in two subjects during the event-related paradigm. Posterior insula and thalamus were activated sporadically.

Conclusion: Tactile stimulation during functional MRI could distinguish the face and hand representation area of S1 in all subjects and activated a somatosensory network including the bilateral secondary somatosensory cortex, insula and thalamus. The event-related design appeared to be superior to the blocked design despite the reduced number of stimuli as it better controls for experimental factors such as anticipation, attention and habituation of subjects. The significance of cortical activation during „light touch“ is remarkable compared to electric or pain excitation since only tactile afferences were stimulated.