on functional connectivity with the other elements of the default mode network. Following low-frequency (1 Hz) stimulation, functional connectivity between the targeted region and the bilateral hippocampal formations was significantly increased. However, following high-frequency stimulation, functional connectivity with the hippocampal formation was essentially unchanged, while functional connectivity with the medial prefrontal cortex, posterior cingulate cortex and contralateral inferior parietal lobule was decreased. Thus, while traditional fMRI resting state analysis suggested that the default-mode network is comprised of an integrated system encompassing all of these regions, the perturbational approach enabled by TMS demonstrated that the default mode network may actually be comprised of two distinct subsystems. Given the abnormal resting state networks identified in psychiatric diseases such as major depression (Greicius et al., 2007; Sheline et al., 2009), and the clinical utility of rTMS as a treatment for depression (see above), this study has important theoretical and practical implications.
