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The Neuroscientist Comments

according to conventional models of the mammalian neocortex, sensory processing begins in layer L4, which is the primary target of thalamic afferents. according to this model, excitation then spreads serially from L4 to L2/L3 and then to L5/L6, with L5 neurons constituting a major output from the cortex and L6 neurons providing feedback to thalamus as well as cortex. It has been known for some time, however, that the thalamocortical axons that project strongly to L4 also send sparser branches to L5/L6. Now Constantinople and Bruno (2013) present evidence suggesting that deep cortical layers are activated directly by the thalamus. Specifically, they show that thalamocortical synapses can directly trigger sensory-evoked responses within L5/L6 neurons, with latencies similar to the responses of L4 neurons. Using paired in vivo recordings from L5/L6 neurons and thalamic neurons, these investigators demonstrate substantial convergence of thalamocortical synapses onto diverse types of neurons within deep cortical layers, particularly L5B. On the basis of these observations, the authors suggest that, in contrast to the conventional serial model in which sensory information is transformed as excitation spreads from thalamus to L4, then to L2/L3, and finally to L5/L6, cortical processing may occur via a bistratified model, whereby the thalamus copies sensory information to both upper neocortical strata (L4 and L2/L3) and a lower stratum (L5/L6), which differ in terms their coding properties and downstream targets.