The paired-pulse technique has also been used to explore network connectivity and interregional interactions, particularly in the motor system (see Rothwell, 2010 for a recent review). For example, studies have shown that a conditioning stimulus applied to one motor cortex inhibits the response to a subsequent test stimulus delivered to the contralateral motor cortex (Ferbert et al., 1992; Chen et al., 2003). Similarly, TMS of motor cortex suppresses voluntary contraction of the ipsilateral hand for a short period of time (Ferbert et al., 1992; Meyer et al., 1995; Chen et al., 2003). In patients with agenesis of the corpus callosum, no such inhibition was seen (Meyer et al., 1995). Similarly, other studies have demonstrated that a conditioning TMS pulse applied to the right dorsal premotor cortex affected the MEP produced by stimulation of the contralateral primary motor cortex (Mochizuki et al., 2004), helping to confirm that the premotor cortex and motor cortex are functionally connected. In another paired-pulse study exploring cortical processing, Pascual-Leone and Walsh (2001) utilized paired-pulse protocols to demonstrate that backprojections from area V5 to V1 are important in the perception and awareness of visual motion.
