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

Objective: To investigate the effect of interstimulus interval (ISI) on attentional enhancement of cortical activity in human somatosensory cortices. Methods: 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. Results: Attention to the tactile stimulation amplified SEFs in contralateral secondary (SII) 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. Conclusions: 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.