In a natural environment, humans are not able to process all information available to the visual system simultaneously. As a consequence, we attend to different subparts of the input one after the other. Under natural viewing conditions, these shifts of attention are associated with changes in fixation. This so-called overt attention therefore provides an objective measure of attention shifts. In my thesis, I investigate the influence of color on overt visual attention. I present human subjects with different categories of color-calibrated images and record their eye-movements. In the first series of experiments, images of 7 different categories (Face, Flower, Forest, Fractal, Landscape, Man-Made object, and Rainforest) are presented either in natural color or grayscale. With regard to the influence of color on overt attention, I find two extreme categories: while in Rainforest images all color features examined are salient, none is salient in Fractal. In all other categories, color features are selectively salient. This shows that the influence of color on overt attention depends on the type of image. In the second series of experiments, I use only Rainforest images. These are presented to color-normal or deuteranope subjects with several modifications in the color domain. I find a causal influence of color-contrast on overt attention, processed in a way that it is not specific to either the red-green or blue-yellow color channel. In the case of color blindness, a slower high-level compensatory mechanism affects the selection of fixation points. These experiments show that there is no single color feature, which influences overt attention in all possible environments. I provide evidence that different levels of the visual hierarchy are involved in the computation of saliency.

To see is to act. Most obviously, we are continuously scanning different regions of the world with the movement of our eyes, head, and body. These overt movements are intrinsically linked to two other, more subtle, actions: (1) the prior process of deciding where to look; and (2) the prediction of the sensory consequences of overt movements. In this thesis I describe a series of experiments that investigate different mechanisms underlying the first process and that evaluate the existence of the second one. The aiming of eye movements, or spatial visual selection, has traditionally been explained with either goal-oriented or stimulus-driven mechanisms. Our experiments deal with the tension of this dichotomy and present further evidence in favor of two other type of mechanisms, not usually considered: global orienting based on non-visual cues and viewing biases that are independent of stimulus and task. Firstly, we investigate whether stimulus-driven selection based on low-level features can operate independently of top-down constraints. If this is the case, the inhibition of areas higher in the hierarchy of visual processing and motor control should result in an increased influence of low-level features saliency. The results presented in Chapter 2 show that inhibition of the posterior parietal cortex in humans, by a permanent lesion or by transient inhibition, result in similar effects: an increased selection of locations that are characterized by higher contrast of low-level features. These results thus support a selection system in which stimulus-driven decisions are usually masked by top-down processes but can nevertheless operate independently of them. Secondly, we investigate how free-viewing selection can be guided by non-visual content. The work in Chapter 3 indicates that touch is not only an effective local spatial cue, but that, during free viewing, it can also be a powerful global orienting signal. This effect occurs always in an external frame of reference, that is, to the side where the stimulation occurred in the external world instead of being anchored to the side of the body that was stimulated. Thirdly, we investigate whether selection can operate even without reference to any sensory stimulus or goal. Results from our experiments presented in Chapters 2 to 5, demonstrate normal and pathological biases during free-viewing. First, patients with neglect syndrome show a strong bias to explore only the right side of images (ch. 2). In contrast, healthy subjects present a strong leftward bias, but only during the early phase of exploration (ch. 3 & 4). Finally, patients with Parkinson’s disease show a subtle overall bias to the right and no initial leftward bias (ch. 5). The results described so far indicate that visual selection operates based on diverse mechanisms which are not restricted to the evaluation of visual inputs according to top-down constraints. Instead, selection can be solely guided by the stimulus, which can be of a multimodal nature and result in global rather than local orienting, and by strong biases that are also independent of both stimuli and goals. The second part of this thesis studies the possibility that eye movements result in predictions of the inputs they are about to bring into sight. To investigate this with electroencephalography (EEG) we had to first learn how to deal with the strong electrical artifacts produced by eye movements. In Chapter 6, a taxonomy of such artifacts and the best way of removing them is described. On this basis, we studied trans-saccadic predictions of visual content, presented in Chapter 7. The results were compatible with the production of error signals after a saccade-contingent change of a target stimulus. These errors signals, coding the mismatch between trans-saccadic predictions and sensory inputs, depend on the reliability of pre-saccadic input. The violation of predictions about veridical input (presented outside the blind spot) results in stronger error signals than when the pre-saccadic stimulus is only inferred (presented inside the blind spot). Thus, these results support the idea of active predictive coding, in which perception consists in the integration of predictions of future input with incoming sensory information. In conclusion, to see is to act: We actively explore the visual environment. We actively select which area to explore based on various competing factors. And we make predictions about the sensory consequences of our actions.

The human eyes are always moving. Even during periods of fixation when visual information is acquired, a persistent jittering of the eyes (ocular drift) is occasionally interrupted by small rapid gaze shifts (microsaccades). Though much has been learned in the last 20 years about the perceptual roles of fixational eye movements, little is known about the consequences of their active control for fine pattern vision and depth perception. Using custom techniques for high-resolution eye-tracking and precise control of retinal stimulation, this dissertation describes three studies that investigated the consequences of controlled fixational eye movements for visual perception of fine patterns in two and three dimensions. The first study addresses whether fixational eye movements are controlled to meet the needs of a demanding visual task and their contributions to visual acuity. We show that in a standard acuity test, humans actively tune their drifts to enhance relevant spatial information and control their microsaccades to precisely place stimuli within the foveola. Together these eye movements contribute 0.15 logMAR to visual acuity, approximately two lines of an eye chart. The second study addresses the perceptual and computational impact of tuning ocular drift. We show that humans are sensitive to changes in visual flow generated by drifts of different sizes. Changes in sensitivity are fully predicted by changes in effective power of luminance modulations delivered by drift, suggesting that drift acts as a mechanism for controlling the effective contrast of the retinal stimulus. The third study addresses the impact of binocular fixational eye movements on fine depth perception. We show that these movements, specifically the opposing movements of the eyes (vergence), are beneficial for stereovision. In the absence of disparity modulations from fixational vergence, fine depth perception is significantly impaired. The research described in this dissertation advances the field in several fundamental ways by showing that (a) contrary to traditional assumptions, ocular drift is tuned to the demands of the visual task; (b) the precise spatiotemporal structure of the luminance changes from ocular drift predictably impacts visual sensitivity; and (c) stereoscopic vision is a dynamic process that uses temporal disparity modulations generated by fixational vergence. / 2024-02-02T00:00:00Z

京都大学 / 0048 / 新制・課程博士 / 博士(文学) / 甲第21491号 / 文博第796号 / 新制||文||672(附属図書館) / 京都大学大学院文学研究科行動文化学専攻 / (主査)教授 蘆田 宏, 教授 Anderson James Russell, 教授 Emmanuel MANALO / 学位規則第4条第1項該当 / Doctor of Letters / Kyoto University / DGAM

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