Deep cortical layers are activated directly by Thalamus. Science 340, 1591–1594. doi: 10.1126/science.1236425

The thalamocortical (TC) projection to layer 4 (L4) is thought to be the main route by which sensory organs communicate with cortex. Sensory information is believed to then propagate through the cortical column along the L4→L2/3→L5/6 pathway. Here, we show that sensory-evoked responses of L5/6 neurons in rats derive instead from direct TC synapses. Many L5/6 neurons exhibited sensory-evoked postsynaptic potentials with the same latencies as L4. Paired in vivo recordings from L5/6 neurons and thalamic neurons revealed substantial convergence of direct TC synapses onto diverse types of infragranular neurons, particularly in L5B. Pharmacological inactivation of L4 had no effect on sensory-evoked synaptic input to L5/6 neurons. L4 is thus not an obligatory distribution hub for cortical activity, and thalamus activates two separate, independent “strata” of cortex in parallel. The conventional model of neocortex isthat sensory processing begins in L4,which has been known for a century to be the principal target of thalamic afferents. Cortical layers are believed to transform sen-sory information as excitation spreads serially along the L4→L2/3→L5/6 pathway (1–4). Thi