Bilateral cortical representation of the trunk midline in human first somatic sensory area.

The cortical representation of the trunk zone in the human first somatosensory area was studied with functional magnetic resonance imaging (fMRI) to establish whether the cutaneous regions close to the midline are represented in this area of both hemispheres. Cortical activation foci evoked by unilateral tactile stimulation of ventral trunk regions were detected in the postcentral gyrus of the contralateral hemisphere slightly medial to or just behind the omega-shaped region of the central sulcus and in the anterior bank of the postcentral sulcus. These regions probably correspond to the trunk ventral midline representation zones of areas 3a-3b and 1-2, respectively. Stimulation of cutaneous regions adjacent to the midline evoked activation foci also in the ipsilateral postcentral gyrus in regions symmetrical to those activated in the contralateral hemisphere. These data demonstrate that in humans, as in nonhuman primates, the cutaneous regions adjacent to the trunk midline are represented bilaterally in the first somatic sensory cortex. Whether the ipsilateral activation depends on callosal or extracallosal inputs remains to be elucidated.