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Abstract

We rely on visual information to navigate through the natural environment. The extraction of visual features by the nervous system first arises in the retina. The retina sends processed visual information to cortical and sub-cortical brain regions through 20-40 types of retinal ganglion cells (RGCs), each encodes specific aspect of the visual scene. One of the most prominent visual processing in the retina is the computation of motion direction, implemented by retinal direction selective circuit. While our notions about retinal direction circuit have come from the usage of simple parametric stimuli, natural scenes are rarely homogeneous but full of competing signals. This thesis focuses on understanding the underlying mechanisms of noise resilience of retinal direction computation. In chapter 1, we review the recent understanding of the dynamic engagement of circuitry and synaptic mechanisms for robust retinal directional selectivity under various visual conditions. Chapter 2 described the protocol we developed for recording and functional imaging of retinal neurons under two-photon microscopy in the laboratory. In chapter 3, we used synapse-specific genetic manipulation to dissect the role of distinct sets of inhibitory motifs for motion processing. We found that the functional circuitries that process bright versus dark moving objects are not mirrored symmetric. Furthermore, Lateral inhibitory motifs in retinal direction selective circuit are only recruited in the noisy visual condition in On pathway. Based upon findings in chapter 3, chapter 4 further investigated the mechanistic implementation of noise resilience by lateral inhibition motif. We found that one particular form of lateral inhibition, the mutual inhibition of lateral inhibition, prevents use-dependent synaptic suppression triggered by competing signals, thus maintains the strength and fidelity of synaptic transmission in the circuitry. Since feature selectivity, including direction selectivity, relies on the veto of spiking activities to "null" stimuli. This silencing of neuronal spiking requires timely cancellation of excitation by inhibition. In chapter 5, we investigated how retinal direction selective circuit maintains the ~ms time-scale covariation of inhibition and excitation for robust direction selectivity.

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