A large amount of digital visual data is being distributed and communicated globally and the question of video quality control becomes a central concern. Unlike many signal processing applications, the intended receiver of video signals is nearly always the human eye. Video quality assessment algorithms must attempt to assess perceptual degradations in videos. My dissertation focuses on full reference methods of image and video quality assessment, where the availability of a perfect or pristine reference image/video is assumed. A large body of research on image quality assessment has focused on models of the human visual system. The premise behind such metrics is to process visual data by simulating the visual pathway of the eye-brain system. Recent approaches to image quality assessment, the structural similarity index and information theoretic models, avoid explicit modeling of visual mechanisms and use statistical properties derived from the images to formulate measurements of image quality. I show that the structure measurement in structural similarity is equivalent to contrast masking models that form a critical component of many vision based methods. I also show the equivalence of the structural and the information theoretic metrics under certain assumptions on the statistical distribution of the reference and distorted images. Videos contain many artifacts that are specific to motion and are largely temporal. Motion information plays a key role in visual perception of video signals. I develop a general, spatio-spectrally localized multi-scale framework for evaluating dynamic video fidelity that integrates both spatial and temporal aspects of distortion assessment. Video quality is evaluated in space and time by evaluating motion quality along computed motion trajectories. Using this framework, I develop a full-reference video quality assessment algorithm known as the MOtion-based Video Integrity Evaluation index, or MOVIE index. Lastly, and significantly, I conducted a large-scale subjective study on a database of videos distorted by present generation video processing and communication technology. The database contains 150 distorted videos obtained from 10 naturalistic reference videos and each video was evaluated by 38 human subjects in the study. I study the performance of leading, publicly available objective video quality assessment algorithms on this database. / text

Chinese characters are used by about one-fifth of the world population. Each character can generally be analyzed and represented at three orthographic levels including single stroke, component and whole character. Regardless of the number of strokes a Chinese character has, it always occupies a square area. Such special spatial layout leads to a great variation in stroke density, i.e., visual complexity, among Chinese characters. Here in this thesis, the effects of visual complexity on Chinese character recognition were examined to investigate the visual processing of Chinese characters in both central and peripheral vision. Empirical findings from four psychophysical studies will be reported. First, the efficiency of detecting and recognizing Chinese characters of different complexity levels was studied. Human recognition of a more complex Chinese character was found to be generally less efficient than that of a simpler one. The relationship between efficiency and complexity had a log-log slope of approximately -1, suggesting that the basic features for recognizing a single Chinese character might be less complex than the simplest object used, which is a single stroke. Analysis results of the detection and recognition efficiency were consistent with a two-stage processing model of Chinese character recognition—feature detection followed by feature integration. Results in peripheral vision suggested that the feature integration process was adversely influenced by internal crowding. Second, the spatial frequency characteristics of recognizing Chinese characters of different complexity levels were examined. The results showed that the peak tuning frequency for recognizing a more complex Chinese character was higher and the bandwidth of the spatial tuning functions was narrower than for recognizing a simpler character. In addition, the effects of size on such spatial frequency characteristics depended on the complexity level. The comparison of human performance against the corresponding performance of a CSF-limited ideal observer model implied that the processing mechanism of recognizing simple and complex characters was different. Moreover, similar results were observed among native Chinese and non-Chinese readers, suggesting that expertise might not influence front end processing properties such as the spatial frequency characteristics of Chinese character recognition. Third, the importance of utilizing spatial frequency information of different orientations (i.e., horizontal versus vertical) in Chinese character recognition was investigated. Results showed higher efficiency in utilizing vertical than horizontal stroke information for Chinese character recognition among native Chinese readers whereas non-Chinese readers used both vertical and horizontal stroke information similarly. Fourth, the effects of the target-flanker similarity in complexity on crowding in Chinese character recognition were examined. The results suggested that the behavior of feature integration process in peripheral vision might be consistent with summation theory, which stresses on the relationship between the processing capacity of neurons and the processing demand of stimuli. The “excessive feature integration explanation” of crowding was also discussed in the context of the current findings. Building on previous research in the literature, results from the four studies in this thesis were synthesized in a proposal of a visual information processing model for Chinese character recognition. / published_or_final_version / Psychology / Doctoral / Doctor of Philosophy

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Figure assignment entails competition between object properties on opposite sides of borders. The figure is perceived on the side of the border that wins the competition. Ample evidence indicates that configural familiarity is among the competing object properties. We investigated whether priming the semantics of a familiar object suggested along one side of a border can increase its likelihood of winning the competition. To prime the semantics, we presented brief masked exposures of object names before brief masked exposures of displays where a portion of a familiar object was suggested on one side of a central border separating two equal-area, black-and-white regions. Participants reported whether the figure lay on the left or right side of the central border and were unaware of the presence of the word prime. These experimental primes named either the Same Object (SO) or a Different Object (DO) as the familiar object suggested in the display. In the DO condition, the word named an object either in the Same Category (DO-SC) or a Different Category (DO-DC) as the familiar object suggested in the display, where superordinate category was defined as natural versus artificial objects. We also used non-words as control primes. We hypothesized that, if semantic activation influences figure assignment, participants in the SO and DO-SC conditions should be more likely than participants in the DO-DC condition to perceive the figure on the side where the familiar object lies following experimental primes than control primes. We did not observe differences between experimental and control prime in any condition. However, we did obtain a Prime Context Effect, in that participants were more likely to perceive the figure on the familiar side of the border in the SO and DO-SC conditions than in the DO-DC condition. The Prime Context Effect shows that participants discerned the relationship between the masked word prime and the semantics of the familiar object suggested in the display, and this led them to change their strategy on both experimental and control trials. We also found that behavior changed over the course of the experiment: Participants in the DO-DC condition perceived the figure on the familiar side of the border more often in the second half of the experiment, on both experimental and control trials. This pattern suggests that over the course of the experiment, they learned to rely more on information from the display than from the prime, perhaps by restricting their attention to the time when the figure-ground display appeared. Participants in the DO-SC condition perceived the figure on the familiar side of the border more often on experimental trials in the second half of the experiment, whereas their performance on control trials did not differ in the first and second half. We hypothesize that participants in the DO-SC condition learned to match the superordinate semantics of the experimental prime and the display, leading to semantic priming. Taken together, these results show that (1) participants can quickly learn the relationship between experimental primes and target displays and can change their strategy accordingly, and (2) semantic activation can affect figure assignment.

The present work is an examination of the mechanisms underlying temporal processing in vision. Recent studies have shown that when observers are asked to identify two objects presented in rapid succession, identification of the first object is quite accurate, while identification of the second object is poor when it folows the first at very brief inter-target intervals (i.e. 200-500 ms). This second-target deficit is known as the attentional blink. According to bottleneck models, the attentional blink occurs because processing of the first target prevents the second target from gaining access to high-level processing. A strong prediction of this account is that if processing time for the first target is increased, the magnitude of the attentional blink should also increase. This prediction is confirmed in experiments. It is argued that these results strongly support bottleneck models as an account of the attentional blink in particular and of temporal processing more generally.

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