Relationship between responses to contra- and ipsilateral stimuli in the human second somatosensory cortex SII

We studied the interaction between responses to contra- and ipsilateral stimuli in the human second somatosensory cortex SII by recording somatosensory evoked magnetic fields (SEFs) from 8 healthy subjects with a 122-channel whole-scalp SQUID magnetometer. Right (R) and left (L) median nerves were electrically stimulated at the wrists at intensities exceeding the motor threshold. In each stimulus sequence, the four equiprobable pairs (L-L, R-R, L-R, R-L) were presented in a random, order once every 2 s, with a 300-ms interstimulus interval within the pair. The responses were modelled with a four-dipole model, with current dipoles located in the SI and Sn: cortices of both hemispheres. The SII responses peaked around 85-120 ms and responses to the Ist (2nd) stimulus on. the pair were on average 2 (12) ms earlier and about 3 (2.5) times stronger for contralateral than ipsilateral stimuli. Independently of the condition, the 2nd response always peaked later than the Ist; the mean delay was 16 ms. The responses to the 2nd stimulus depended only slightly on the type of the Ist: the latency increased more and the amplitude decreased less after different than identical Ist stimuli. These results suggest that neuronal activations due to contra- and ipsilateral stimuli overlap strongly in the human SII cortex. (C) 1999 Academic Press.