Differences in projection patterns between large and small corticothalamic terminals.

We injected tracer into wide regions of visual cortex in the cat to produce retrograde and orthograde labeling in the thalamus, chiefly in the lateral geniculate nucleus and lateral posterior-pulvinar complex (LP-Pulvinar). We used the electron microscope to measure the sizes of orthogradely labeled terminals in thalamus and used these measurements to help determine whether the terminals were "RL" (large, presumed excitatory) or "RS" (small, presumed excitatory). We also distinguished reciprocal regions, which were zones of corticothalamic feedback defined by the presence of many retrogradely labeled cell bodies and orthogradely labeled terminals, from nonreciprocal regions, which were zones of feedforward corticothalamic projections defined by the presence of orthogradely labeled terminals alone. The lateral geniculate nucleus, a reciprocal region, had retrogradely labeled cell bodies as well as labeled RS terminals. Likewise, reciprocal regions in LP-Pulvinar were dominated by labeled RS terminals. In contrast, nonreciprocal regions were dominated by labeled RL terminals. Based on other evidence of corticothalamic projections that RL and RS terminals derive, respectively, from layer 5 and layer 6, we suggest the same relationship here, leading to the conclusion that the corticothalamic input from layer 6 is largely feedback, whereas that from layer 5 is largely feedforward. This finding lends credence to a recent hypothesis that layer 5 corticothalamic axons represent the afferent limb of a cortico-thalamo-cortical pathway that is critical for corticocortical communication.