Paradoxical fMRI overconnectivity upon chemogenetic silencing of the prefrontal cortex

While shaped and constrained by axonal connections, fMRI-based functional connectivity can reorganize in response to varying interareal input or pathological perturbations. However, the causal contribution of regional brain activity to whole-brain fMRI network organization remains unclear. Here we combine neural silencing, resting-state fMRI and in vivo electrophysiology to causally probe how inactivation of a cortical node affects brain-wide fMRI coupling in the mouse. We find that chronic suppression of the medial prefrontal cortex (PFC) via overexpression of a potassium channel paradoxically increases fMRI connectivity between the silenced area and its direct thalamo-cortical terminals. Acute chemogenetic inactivation of the PFC reproduces analogous patterns of fMRI overconnectivity, with increased fMRI coupling between polymodal thalamic regions and widespread cortical areas. Using multielectrode electrophysiological recordings, we further show that chemogenetic inactivation of the PFC results in enhanced slow (0.1 - 4 Hz) oscillatory coupling between fMRI overconnected areas, and that changes in δ band coherence are linearly correlated with corresponding increases in fMRI connectivity. These results provide causal evidence that cortical inactivation does not necessarily lead to reduced inter-areal coupling, but can counterintuitively increase fMRI connectivity via enhanced, less-localized slow oscillatory processes, with important implications for modelling and understanding fMRI overconnectivity in pathological states.