Somatosensory cortex responses to median nerve stimulation: fMRI effects of current amplitude and selective attention

Objectives: The aim of this study was to localize and to investigate respon se properties of the primary (SI) and the secondary (SII) somatosensory cor tex upon median nerve electrical stimulation. Methods: Functional magnetic resonance imaging (fMRI) was used to quantify brain activation under different paradigms using electrical median nerve st imulation in healthy right-handed volunteers. In total II subjects were stu died using two different stimulus current values in the right hand: at moto r threshold (I-max) and at I-min (1/2 I-max). In 7 of these 11 subjects a p arametric study was then conducted using 4 stimulus intensities (6/6, 5/6, 4/6 and 3/6 I-max). Finally, in 10 subjects an attention paradigm in which they had to perform a counting task during stimulation with I-min was done. Results: SI activation increased with current amplitude. SI did not show si gnificant activation during stimulation at I-min. SII activation did not de pend on current amplitude. Also the posterior parietal cortex appeared to b e activated at I-min. The I-min response in SII significantly increased by selective attention compared to I-min, without attention. At I,, significan t SI activity was observed only in the contralateral hemisphere, the ipsila teral cerebellum while other areas possibly showed bilateral activation. Conclusions: Distributed activation in the human somatosensory cortical sys tem due to median nerve stimulation was observed using fMRI. SI, in contras t to SII, appears to be exclusively activated on the contralateral side of the stimulated hand at I-max, in agreement with the concept of SI's importa nt role in processing of proprioceptive input. Only SII remains significant ly activated in case of lower current values, which are Likely to exclusive ly stimulate the sensible fibres mediating cutaneous receptor input. Select ive attention only enhances SII activity, indicating a higher-order role fo r SII in the processing of somatosensory input. (C) 2000 Elsevier Science I reland Ltd. All rights reserved.