Spatio-temporal dynamics of feedforward and feedback processing in the visual thalamo-cortico-thalamic loop

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In textbooks, the visual system is often portrayed consisting of processing stages that are arranged in a hierarchy and connected via feedforward projections. Considering the anatomy of the visual system in more detail, however, it becomes clear that feedforward projections are often grossly outnumbered by their feedback counterparts. This general rule also applies to the dorsolateral geniculate nucleus (dLGN) of the thalamus, where ~30% of synaptic inputs to relay cells are made by corticothalamic (CT) pyramidal cells, while only ~10% constitute feedforward inputs from retinal ganglion cells. How this anatomically massive feedback projection to one of the earliest visual processing stages influences representations in dLGN and impacts computations in the thalamocortical loop has so far remained incompletely understood.The proposed research is a follow-up project that continues to be centered around the impact of corticothalamic feedback on information processing in the early visual system. While our research in the previous funding period provided insights into the retinotopic organization of CT feedback circuits in thalamus and the dual nature of CT feedback effects in shaping spatial processing, i.e. facilitation and suppression, our central aim for this follow-up proposal is to elucidate the spatio-temporal dynamics of feedforward and feedback processing in thalamo-cortico-thalamic interactions. In particular, we will (1) isolate the temporal impact of CT feedback in dLGN and visTRN, we will (2) investigate the role of CT feedback in shaping spatio-temporal selectivity by using pathway-specific, optogenetic suppression of CT feedback terminals and simultaneous recordings from thalamus and cortex, and we will (3) probe the role of CT feedback in thalamocortical rhythms and communication.Together, this research promises to contribute to answering the fundamental question of how the direct excitatory and indirect inhibitory CT feedback dynamically interact to modulate thalamo-cortical visual processing.