Functional identification of primate lateral geniculate nucleus projections to visual cortex using optogenetics and electrical stimulation

The lateral geniculate nucleus (LGN) of macaque monkeys relays signals from the retina to the primary visual cortex (V1) via three anatomically segregated projection streams. Axons of the magno- and parvo-cellular system terminated mainly in the granular layer 4 of V1 while konio cells project to the superficial layers 1,2 and 3 of V1. Further among all LGN cells konio cells are the only ones that express the calcium binding protein CamKIIα. Because of these clear anatomical and biochemical distinctions the LGN-V1 circuit is an ideal candidate to test circuit mapping approaches. Here we used electrical microstimulation and konio-cell specific optogenetics to functionally test the feedforward connectivity between the LGN and V1. Selective activation of the LGN konio layers with either optogenetics or electrical mircrostimulation caused an electrical current inflow in the V1 supra-granular layers following the anatomical predictions of the konio pathway. Microstimulation of LGN parvo layers caused an initial sink in the granular layer 4 of V1 and an additional sink in the supra-granular layers, closely resembling the visually evoked pattern of cortical activity. Histological analysis confirmed the predominant expression of the optogenetic construct in cells of the konio cellular system but also an unexpected retrograde traveling mechanism that resulted in labelling of V1 layer 6 cortico-thalamic cells and retinal ganglion cells. Taken together, these findings indicate comparable capacities of both stimulation methods to isolate and identify thalamo-cortical circuit mechanisms of the primate brain