Optogenetic investigation of the LGN koniocellular influence on V1 activity

The lateral geniculate nucleus (LGN) of the primate thalamus is organized into parallel parvo-, magno- and konio-cellular projection streams to primary visual cortex (V1). While magno and parvo cells label positive for Parvalbumin and project to layer-4 of V1, konio neurons project to the superficial layers of V1 and are positive for CamKII and Calbindin [1] [2]. Of the three systems, our understanding of the konio pathway and its contribution to vision is still very limited. Here we used optogenetics in anaesthetized macaque monkeys to investigate the influence of koniocellular LGN neurons on V1. To this end we injected the construct AAV5-CamKIIa-ChR2-eYFP into the LGN of two monkeys. Post-mortem histological and immunohistochemical analysis verified that ChR2 expression was predominantly present in the koniocellular system, which is characterized by its expression of CamKII and a focus on the LGN intercalated layers. In earlier experiments optogenetic stimulation that was applied to neurons in the LGN intercalated layers resulted in activation of the superficial layers in V1, but not layer 4, as determined from current-source-density measurements (CSD) from multi-contact laminar recordings in V1. Preliminary analysis of the cortical LFP also showed a power decrease in the beta frequency range (15-30Hz) for the superficial layers during optogenetic stimulation. In additional control experiments in one monkey, we found that electrical micro-stimulation in a parvocellular layer activated layer 4 of V1 similar to visual flicker stimulation. In contrast, electrical microstimulation in the intercalated LGN layers induced activity in superficial layers of V1 similarly to the optogenetic stimulation. In summary, we show for the first time the effective connectivity of the koniocellular LGN projection to V1 and its influence on the LFP. Methodologically, our results demonstrate that both circuit probing approaches, optogenetics and electrical microstimulation, render results with very similar specificity. Reference 1. Hendry, S.H. and T. Yoshioka, A neurochemically distinct third channel in the macaque dorsal lateral geniculate nucleus. Science, 1994. 264(5158): p. 575-577. 2. Casagrande, V.A., et al., The morphology of the koniocellular axon pathway in the macaque monkey. Cereb Cortex, 2007. 17(10): p. 2334-2345