Characteristics of the human contra- versus ipsilateral SII cortex

Objectives: In order to study the interaction between left- and right-sided stimuli on the activation of cortical somatosensory areas, we recorded som atosensory evoked magnetic fields (SEFs) from 8 healthy subjects with a 122 channel whole-scalp SQUID gradiometer. Methods: Right and left median nerves were stimulated either alternately wi thin the same run, with interstimulus intervals (ISIs) of 1.5 and 3 s, or s eparately in different runs with a 3 s ISI. In all conditions 4 cortical so urce areas were activated: the contralateral primary somatosensory cortex ( SI), the contra- and ipsilateral secondary somatosensory cortices (SII) and the contralateral posterior parietal cortex (PPC). Results: The earliest activity starting at 20 ms was generated solely in th e SI cortex, whereas longer-latency activity was detected from all 4 source areas. The mean peak latencies for SII responses were 86-96 ms for contral ateral and 94-97 ms for ipsilateral stimuli. However, the activation of rig ht and left SII areas started at 61 +/- 3 and 62 +/- 3 ms to contralateral stimuli and at 66 +/- 2 and 63 +/- 2 ms to ipsilateral stimuli, suggesting a simultaneous commencing of activation of the SII areas. PPC sources were activated between 70 and 110 ms in different subjects. The 1.5 a TSI altern ating stimuli elicited smaller SII responses than the 3 s ISI non-alternati ng stimuli, suggesting that a considerable part of the neural population in SII responds both to contra- and ipsilateral stimuli. The earliest SI resp onses did not differ between the two conditions. There were no significant differences in source locations of SII responses to ipsi- and contralateral stimuli in either hemisphere. Subaverages of the responses in sets of 30 r esponses revealed that amplitudes of the SII responses gradually attenuated during repetitive stimulation, whereas the amplitudes of the SI responses were not changed. Conclusions: The present results implicate that ipsi- and contralateral SII receive simultaneous input, and that a large part of Sn neurons responds b oth to contra- and ipsilateral stimulation. The present data also highlight the different behavior of ST and SII cortices to repetitive stimuli. (C) 2 000 Elsevier Science Ireland Ltd. All rights reserved.