Completing the corticofugal loop: a visual role for the corticogeniculate type 1 metabotropic glutamate receptor.

The way in which the brain deals with sensory information relies not only on feedforward processing of signals from the periphery but also on feedback inputs. This is the case of the massive projection back from layer 6 in the visual cortex to the thalamus, for which, despite being the greatest single source of synaptic contacts, the functional role still remains unclear. In the cat lateral geniculate nucleus, part of this cortical feedback is mediated by type 1 metabotropic glutamate receptors (mGluR1s), which are exclusively located on distal segments of the relay-cell dendrites. Here we show that in adult cats the cortex uses a synaptic drive mediated by these receptors (mGluR1) specifically to enhance the excitatory center of the thalamic receptive field. Moreover the effect is maximum in response to those stimuli that effectively drive cortical cells, and importantly, it does not affect the spatiotemporal structure of the thalamic receptive field. Therefore, cortex, by closing this corticofugal "loop," is able to increase the gain of its thalamic input within a focal spatial window, selecting key features of the incoming signal.