ACTIVATION OF A DISTRIBUTED SOMATOSENSORY CORTICAL NETWORK IN THE HUMAN BRAIN - A DIPOLE MODELING STUDY OF MAGNETIC-FIELDS EVOKED BY MEDIANNERVE-STIMULATION .2. EFFECTS OF STIMULUS RATE, ATTENTION AND STIMULUS DETECTION

In this study we used a repeated measures design and univariate analys is of variance to study the respective effects of ISI, spatial attenti on and stimulus detection on the strengths of the sources previously i dentified by modelling SEFs during the 200 ms following mentally count ed left median nerve stimuli delivered at long and random ISIs (Part I ). We compared the SEF source strengths in response to frequent and ra re stimuli, both in detection and ignoring conditions. This permitted us to establish a hierarchy in the effects of ISI, attention and stimu lus detection on the activation of the cortical network of SEF sources distributed in SI and posterior parietal cortex contralateral to stim ulation, and in the parietal operculum (SII) and premotor frontal cort ex of both hemispheres. In all experimental conditions the SI and pari etal opercular sources were the most active. All sources were more act ive in response to stimuli delivered at long and random ISIs and the f rontal sources were activated only in this condition of stimulation. D riving the subject's attention toward the side stimulated had no detec table effect on the activity of SEF sources at short ISI. At long ISIs mental counting of the stimuli increased the responses of all sources except SI. These results suggest that activation of frontal sources d uring mental counting could reflect a working memory process, and that of posterior parietal sources a spatial attention effect detectable o nly at long ISIs. (C) 1997 Elsevier Science Ireland Ltd.