Putative order of functional activity in the somatosensory network as measured by BOLD fMRI. The orders of bottom-up and top-down processing in the somatosensory network were determined using the relative difference in the onset times of averaged BOLD responses between somatosensory regions. The arrows indicate information flow direction, and the difference in relative BOLD onset times between regions (Δt) is also reported. (A) Sensory-evoked signal from forepaw stimulation is first relayed through VPL into the middle layer of S1FL. The thalamocortical inputs next propagate to the upper and lower S1FL, and then the upper S1FL projects to M1 and S2 through the CC pathway, and the lower S1FL projects to the higher-order thalamic nucleus PO. (B) Neural activity in M1 following optogenetic stimulation is relayed into the upper layer of S1FL and then to S2. The signals propagated along the cortical layers are subsequently projected to PO and VPL. Solid line, expected functional pathway; gray dashed line, anatomically possible pathway.