Proprioception acts as the main source of input in human S-I activation experiments: a functional MRI study

During tactile exploration cells in human somatosensory cortex S-I receive input from skin receptors and from proprioceptive feedback. To study the extent to which these sources contribute to cell activation we used functional magnetic resonance imaging (fMRI) in order to visualize the spatial extent and amplitude of activation in S-I during active finger movement and passive stimulation of finger tips. In all subjects (n = 6) we measured activation elicited by unilateral single finger tapping (active task) and mechanical stimulation of the palm of the index finger (passive task). In the finger tapping condition all subjects showed a strict contralateral activation of somatosensory cortex S-I and motor cortex M-I. In the passive stimulation experiment we found activation of the contralateral somatosensory cortex S-I only. Although subjects were trained to perform the finger movement with the same frequency and pressure in comparison to the passive stimulation, the activation within S-I induced by finger movements was always significantly larger than that induced by passive stimulation. This result implies that activation of somatosensory cortex originates to a large extent from proprioception while tactile input plays a minor role in S-I excitation.