Facilitation of somatosensory evoked potentials by exploratory finger movements.

Modification of somatosensory processing depending on the behavioral setting was studied. Active alternating movements of the fingers, passive tactile stimuli to the hand, and active exploration of objects were performed during recording of somatosensory evoked potentials (SEPs). SEPs were elicited by compound electrical median nerve stimulation and electrical stimulation at detection threshold of cutaneous median nerve fascicles identified by microneurography. Electrical stimulation was not time-locked to the studied condition. In comparison with SEPs at rest there was attenuation of early cortical potentials up to 25 ms post-trigger in all nonresting conditions. In stimulation of the compound median nerve as well as of isolated cutaneous fascicles of a hand actively exploring an object there was an additional increased negativity, peaking at 28 ms. This facilitory effect was independent of attentional focusing and was absent during exploration using the ipsilateral, non-electrically stimulated hand. In patients with parietal lesions the facilitatory effect was diminished on the affected side. Spline interpolated brain maps at this latency based on 32-channel recordings in healthy volunteers showed a shift of local contralateral positive maximum from frontal to parietal during exploration, indicating enhancement of a tangential dipole. It is suggested that in conditions involving close sensorimotor interaction such as exploratory hand movements there is preactivation of a cortical area which is located in the central sulcus and receives cutaneous somatosensory inputs.