The role of lamina-specific input into cat primary visual cortex

Layer 6 of neocortex (V1) contains neurons that project back to the primary thalamic nuclei. Locally, these same neurons provide the major input to layer 4, which is itself the major thalamorecipient layer. Thus one idea is that layer 6 neurons modulate some of the thalamocortical input. The problem is that we have little idea who excites layer 6 neurons. Within the local circuit the principal input to layer 6 is from layer 5 pyramidal cells, but these provide only a minority of the excitatory synapses. From external sources, the thalamic input is minute. We estimate that in cat V1 (primary visual cortex), at least 70 % of the excitatory synapses in layer 6 have no known origin. Many of these synapses are likely to originate outside V1, so that long-range input may have an important effect on the output of layer 6. Within the local circuit, a number of studies in cat and mouse have suggested that the influence of layer 6 on layer 4 is mainly inhibitory. However, our structural data indicate that in cat V1, nearly half of the excitatory input to (excitatory) layer 4 spiny stellate cells come from layer 6 pyramidal cells. So the burning question is: who talks to layer 6 and what are the corticothalamic cells saying?