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Visual adaptation in central and peripheral vision
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Peripheral vision occupies as much as 99.9% of the visual field beyond the fovea and serves important functions such as getting the gist of a scene and capturing important objects for central vision to be further processed. Phenomenon such as crowding, Flashed Face Distortion effect (FFDE) indicates that visual processing in peripheral vision may be fundamentally different from that in central vision. Visual adaptation is widely used as a tool to study the underlying mechanism of the visual system and may reflect how visual coding strategies are different in central versus peripheral vision. In this dissertation, we reported a series of experiments focusing on visual processing in central versus peripheral vision, including visual adaptation and FFDE. In Chapter 2, we tracked the time course of contrast detection threshold and tilt aftereffect in central versus peripheral vision. We found that not only the adaptation magnitude was much stronger, but the temporal dynamics of buildup and decay were much slower in peripheral vision, pointing to qualitatively difference of visual processing in central vs. peripheral vision. Visual adaptation has been widely studied with a constant stimulus. However, in real life, we are constantly adapting to different stimuli over time. In Chapter 3, we investigated the effects of temporal frequency on face adaptation in central vision. We found that when adapting to a constant face interleaved with a blank interval, the magnitude of aftereffects was independent of the temporal frequency of the face presentation, given the total adaptation duration was the same. When adapting to a pair of faces alternating between two different genders, the overall aftereffect was determined by the temporal frequency of the face presentation. Specifically, it was dependent on the last adapting face when each face was presented for 8s (i.e., at 0.0625 Hz) or longer and was dependent on the average of adapting faces when each face was presented 4s (i.e., at 0.125 Hz) or shorter. These results illustrate the timescale of face gender adaptation when alternating faces were used as adaptors. In chapter 4, we investigated FFDE, where faces were perceived as distorted when they were continuously presented in peripheral vision. We showed that FFDE was preserved when continuous faces were presented to different eyes and when faces of the same identity but with different facial expressions were presented. The results of Chapter 4 indicate that FFDE did not depend on low-level mechanisms such as retinal adaptation or presenting different face identities. In chapter 5, we investigated how FFDE and facial expression averaging depended on the temporal frequency of face presentation in peripheral vision. We found that the magnitude of FFDE decreased while the expression averaging performance increased as temporal frequency increased, indicating a transition from contrastive distortion to averaging. Overall, in this dissertation, we showed how visual adaptation might be qualitatively different in central versus peripheral vision and how the visual system interpreted multiple stimuli over time. We also investigated the periphery-specific visual phenomenon FFDE and its relation to interocular presentation, face identity, and the temporal frequency of presentation. The results of this dissertation shed light on the distinctive mechanisms of visual processing in the central versus the peripheral visual fields.