Most of the studies to date on human brain functions rely on brain mapping techniques with which one examines how regional activation changes depending on various cognitive demands. Activation in a given region, however, is the result of inputs from other regions, and is also the cause of changes in activation in other regions. Therefore, in order to understand how the brain works, it is essential to examine neural signal transmission across regions. To this end, we have recently developed a technique of concurrent TMS and EEG and examined the efficacy of signal transmission across cortical regions. We found that the efficacy of signal transmission from the prefrontal cortex is i) modulated in task-dependent manners, ii) modulated in history-dependent manners, iii) associated with behavioral performance, iv) distinct from the pattern of regional activation. We also found that the signal transmission v) is modulated in a direction-specific manner between prefrontal and parietal regions, and vi) changes in a temporally dynamic manner during cognitive processes. Using this technique, we show that visual selective attention is mediated by a tract-specific change in the efficacy of signal transmission. Specifically, we show that trial-by-trial fluctuations in attention performance and previous trial effects are associated with changes in signal transmission from the frontal eye field to specific posterior cortical regions.