Effect of interstimulus interval on attentional modulation of cortical activities in human somatosensory areas.

Abstract

To investigate the effect of interstimulus interval (ISI) on attentional enhancement of cortical activity in human somatosensory cortices. Somatosensory magnetic fields (SEFs) evoked by electrical stimulation of the right median nerve at ISIs of 0.5, 1, 3 and 5 s were recorded in two separate conditions by using whole head type magnetoencephalography. In the attend-condition: subjects directed attention to the stimuli by watching the stimulated body site and mentally counting the stimuli. In the ignore-condition: subjects ignored the stimuli by reading a book. Attention to the tactile stimulation amplified SEFs in contralateral secondary (SIT) at ISIs of 1, 3 and 5 s and in ipsilateral SII at ISI of 5 s. There was no effect of attention on the SEF in primary. The ratios of the equivalent current dipole moments calculated for the attend-condition compared to ignore-condition were 1.49 +/- 0.14 at ISI of 1 s, 1.47 +/- 0.29 at 3 s and 1.34 +/- 0.23 at 5 s in the contralateral SII and 1.40 +/- 0.24 at 5 s in the ipsilateral SII. Time longer than 0.5 s might be required for active attention genesis and, after it was generated, the cortical activation in the attend condition was proportional to the activity in the ignore condition.