Activation of a cortical column by a thalamocortical impulse.

Thalamocortical (TC) impulses potently influence the sensory neocortex, but the functional impact of individual TC neurons throughout the layers of the cortex has proved difficult to assess. Here we examine, in awake rabbits, the vertical distribution of monosynaptic currents generated in a somatosensory cortical "barrel" column by spontaneous impulses of single, topographically aligned TC neurons. We show that closely neighboring TC neurons generate widely differing patterns of monosynaptic activation within layers 4 and 6 of their aligned column. Moreover, synaptic currents generated by TC impulses with long preceding interspike intervals are greatly enhanced in both of these layers. The degree of this enhancement differs reliably among neighboring TC neurons but, for a given neuron, is very similar in layers 4 and 6. Our results indicate that in the awake state, TC synapses throughout the depth of the cortex serve as powerful filters of sensory information that reflect individual characteristics of their parent TC neuron.