Gating Input to Visual Cortex by Feedback to LGN

Anatomical studies have documented massive back-projections from higher to lower visual cortices and to the lateral geniculate nucleus (LGN). The large number of synapses from these sources suggest that they should have a profound influence on the information carried by feed-forward inputs to these cells. However, the functional role of these connections is unclear. In order to explore the role of the feedback connections, we have recorded spike trains from electrodes placed in LGN in the macaque nwnkcy under sufcnta anesthesia, and have compared LGN cells' activity with and without suppression by cooling of feedback from primary visual cortex (Vl). Normally, ma,gno and parvo LGN cells show a wide range over which their responses are proportional to stimulus contrast. Inactivation of Vl feedback causes LGN cells to become more nonlinear and Jess sensitive to high contrast than during nonna.l conditions. Responses during Vl inactivation have a similar shape to those of retinal ganglion cells. \Vc have also tested the properties of the so-called extended smTOmHl as they relate to cortical activity and to influences on responses to LGN stimulation. A model of this data suggests au interpretation in terms of two fnuctiona.l components of feedback: a contrast-dependent component which dominates at high input contrast, and a constant baseline level of inhibitory feedback. VVc also show that the influence of the extended surround on the classical center mechanism is more complicated than a simple integration model.