fMRI reflects functional connectivity of human somatosensory cortex

Unilateral sensory stimulation reliably elicits contralateral somatotopic activation of primary (St) and secondary (Sit) somatosensory cortex. There is an ongoing debate about the occurrence and nature of concomitant ipsilateral SI and SII activation. Here we used functional magnetic resonance imaging (fMRI) in healthy human subjects with unilateral tactile stimulation of fingers and lips, to compare somatosensory activation patterns from distal and proximal body parts. We hypothesized that fMR.I in humans should reflect the functional connectivity of somatosensory cortex as predicted by animal studies. We show that both unilateral finger and lip stimulations activate contraand ipsilateral SI and SH cortices with high detection frequency. Correlations of BOLD-signals to the applied hemodynamic reference function were significantly higher in contralateral as compared to ipsilateral SI and SII cortices for both finger and lip stimulation, reflecting strong contribution of contralateral thalamocortical input. Furthermore, BOLD-signal correlations were higher in SI than in SH activations on the contralateral but not on the ipsilateral side. While these asymmetries within and across hemispheres were consistent for ringer and lip stimulations, indicating analogous underlying organizing principles, they were less prominent for lip stimulation. Somatotopic organization was detected in S1 but not in SII representations of fingers and lips. These results qualitatively and quantitatively support the prevalent concepts of anatomical and functional connectivity in the somatosensory system and therefore may allow interpretation ofsensory evoked fMRI signals in terms of normal human brain function. Thus, the assessment of human somatosensory function with fMRI may permit in the future investigations of pathological conditions. (c) 2007 Elsevier Inc. All rights reserved.