FC between the sensory core thalamus (i.e., VPL) and layers IV and VI of the primary somatosensory cortex, were higher than other layers. This follows from the fact that C-type thalamic neurons in VPL primarily target layer IV in the primary somatosensory cortex, and then corticothalamic pyramidal neurons in layer VI project back to C-type thalamic neurons in VPL [65, 66]. To a large extent, the results confirmed our hypotheses. The cortico-cortical hypothesis is that inter-hemispheric cortico-cortical FC in superficial layers (layers I–III) must be higher compared to deep layers (layers V–VI) following evidence in rodents that 80% of the cell bodies of those callosal projecting neurons are distributed in layer II and layer III, with only 20% in layers V and VI [67–71]. Our results suggested that the inter-hemispheric FC was significantly higher in superficial layers than deeper layers. To our knowledge, this is the very first study to investigate the sensitivity of resting state fMRI connectivity at sub-millimeter spatial scale to the connectional architecture at the laminar level.
