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 example, Silvanto et al (2006) studied the effects of single-pulse stimulation to the frontal eye fields (FEF) on the excitability of area V5/MT (as measured by phosphene threshold, the minimum TMS intensity required to produce a phosphene). They demonstrated that FEF stimulation 20 to 40 milliseconds before stimulation of area V5/MT lowered the phosphene threshold significantly. Stimulation of the FEF at other time points had no such effects.
