  Functional connectivity (FC) between brain regions tracks symptom severity in
many neuropsychiatric disorders. Transcranial magnetic stimulation (TMS)
directly alters regional activity and indirectly alters FC. Predicting how FC
will change following TMS is difficult, but would allow novel therapies that
target FC to improve symptoms. We address this challenge by proposing a
predictive model that explains how TMS-induced activation can change the
strength of FC. Here, we focus on the FC of the frontoparietal (FPS) and
default mode (DMS) systems given the importance of their FC in executive
function and affective disorders. We fit this model to neuroimaging data in 29
individuals who received TMS to the frontal cortex and evaluated the FC between
the FPS and DMS. For each individual, we measured the TMS-induced change in FC
between the FPS and DMS (the FC network), and the structural coupling between
the stimulated area and the FPS and DMS (the structural context network (SCN)).
We find that TMS-induced FC changes are best predicted when the model accounts
for white matter fibers from the stimulated area to the two systems. We find
that the correlation between these two networks (structure-function coupling) -
and therefore the predictability of the TMS-induced modulation - was highest
when the SCN contained a dense core of intraconnected regions, indicating that
the stimulated area had ample access to an anatomical module. Further, we found
that when the core of the SCN overlapped with the FPS and DMS, we observed the
greatest change in the strength of their FC. Broadly, our findings explain how
the structural connectivity of a stimulated region modulates TMS-induced
changes in the brain's functional network. Efforts to account for such
structural connections could improve predictions of TMS response, further
informing the development of TMS protocols for clinical translation.
