Mapping the somatosensory face representation by automated tactile stimulation during fMRI

Background: A standardized sensory stimulation of the face during fMRI is challenging due to the narrow geometry of the head coil. An automated application of stimuli is advantageous compared to a manual application as it does not introduce an examiner-dependent variance of stimulation and allows for an event-related study design as well as group comparisons.

Methods: A new MR-compatible stimulation device delivered physiologic tactile stimuli (von Frey-filaments of normalized intensity) to the forehead, upper lip and dorsum of the hand on both sides during an event-related fMRI design. Whole brain functional images were aquired from 15 right-handed healthy volunteers on a 1.5 T-Siemens symphony scanner and statistically evaluated using SPM2.

Results: A random effects analysis of the whole group yielded activation of the contralateral primary and bilateral secondary somatosensory cortex as well as associated somatosensory areas such as the posterior insula, thalamus and inferior parietal lobe during tactile stimulation compared to rest. The punctate tactile stimuli used in this study allowed to distinguish the cortical representation of the forehead, upper lip and hand in the primary somatosensory cortex (S1).

Conclusion: Purpose-built devices allow a rather convenient mapping of somatosensory representations. The anatomical locations of the face and hand representations in S1 found in this study corroborate earlier data on the somatotopy of S1. The use of standardized physiologic stimuli which were applied automatically with a high spatiotemporal accuracy yields a more accurate mapping than a manual stimulus application.