Fingertip Representation in the Human Somatosensory Cortex: An fMRI Study

Eight right-handed adult humans underwent functional magnetic resonance imaging (fMRI) of their brain while a vibratory stimulus was applied to an individual digit tip (digit 1, 2, or 5) on the right hand. Multislice echoplanar imaging techniques were utilized during digit stimulation to investigate the organization of the human primary somatosensory (SI) cortex, cortical regions located on the upper bank of the Sylvian fissure (SII region), insula, and posterior parietal cortices. Thettest and cluster size analyses were performed to produce cortical activation maps, which exhibited significant regions of interest (ROIs) in all four cortical regions investigated. The frequency of significant ROIs was much higher in SI and the SII region than in the insula and posterior parietal region. Multiple digit representations were observed in the primary somatosensory cortex, corresponding to the four anatomic subdivisions of this cortex (areas 3a, 3b, 1, and 2), suggesting that the organization of the human somatosensory cortex resembles that described in other primates. Overall, there was no simple medial to lateral somatotopic representation in individual subject activity maps. However, the spatial distance between digit 1 and digit 5 cortical representations was the greatest in both SI and the SII region within the group. Statistical analyses of multiple activity parameters showed significant differences between cortical regions and between digits, indicating that vibrotactile activations of the cortex are dependent on both the stimulated digit and cortical region investigated.