TMS enables the assessment of dynamical changes in the interactions between cortical regions. One of the earliest uses of TMS involved producing a “virtual lesion” to assess the temporal relationship of involvement of different cortical regions in specific cognitive functions (Walsh & Pascual-Leone, 2005). For example, Amassian et al demonstrated that TMS to the occipital pole was effective in abolishing visual perception of a letter if the pulse was administered between 80 and 100ms after stimulus onset; pulses administered significantly before or after this interval had no such effect (Amassian et al., 1989). Such studies can reveal surprising results. For example, Chambers et al (2004) demonstrated that the right angular gyrus is involved in the reorienting of spatial attention at two distinctly different time points (between 90 and 120 ms after stimulus onset, and again between 210 and 240 ms after stimulus onset), suggesting that the same cortical region can be involved at different time points during a single task (Chambers et al., 2004). Furthermore, experiments with TMS can delineate the time-course of interactions between different cortical regions. As an
