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This thesis presents a study of how edges are detected and encoded by the human visual system. The study begins with theoretical work on the development of a model of edge processing, and includes psychophysical experiments on humans, and computer simulations of these experiments, using the model. The first chapter reviews the literature on edge processing in biological and machine vision, and introduces the mathematical foundations of this area of research. The second chapter gives a formal presentation of a model of edge perception that detects edges and characterizes their blur, contrast and orientation, using Gaussian derivative templates. This model has previously been shown to accurately predict human performance in blur matching tasks with several different types of edge profile. The model provides veridical estimates of the blur and contrast of edges that have a Gaussian integral profile. Since blur and contrast are independent parameters of Gaussian edges, the model predicts that varying one parameter should not affect perception of the other. Psychophysical experiments showed that this prediction is incorrect: reducing the contrast makes an edge look sharper; increasing the blur reduces the perceived contrast. Both of these effects can be explained by introducing a smoothed threshold to one of the processing stages of the model. It is shown that, with this modification,the model can predict the perceived contrast and blur of a number of edge profiles that differ markedly from the ideal Gaussian edge profiles on which the templates are based. With only a few exceptions, the results from all the experiments on blur and contrast perception can be explained reasonably well using one set of parameters for each subject. In the few cases where the model fails, possible extensions to the model are discussed.

This thesis investigated with regard to the perception of abstract and social stimuli: (1) What constitutes a visual pattern? (2) Whether people possess a proclivity towards one particular pattern type. (3) When is patterning imposed or detected by the visual system? The abstract stimuli consisted of checkerboard patterns and the social stimuli consisted of faces or social groups. Initially the term "pattern" was defined as an image that contains redundant information. This was illustrated by a bias when defining patterns by members of the public towards images that contain both repeated and reflective symmetry, or a low number of possible variants and therefore reduced information content, i. e. more redundancy. Similarly reflective symmetry was identified as a key property in defining faces. The effect of symmetry type on early visual processing was investigated further in a series of backward masking experiments on both abstract and facial stimuli (Chapters 6& 7). The results of the masking experiments suggest a bias during early visual processing for patterns that contain symmetry (i. e. repetition or reflection), or share common fate compared with randomly generated patterns or distorted faces. A top-hemifield and LVF bias was observed in the early detection of patterns. Patterns that take advantage of these properties such as the eyes within the face were suggested as having a perceptual advantage. Patterning appears to be imposed at all stages of visual processing. At early stages of visual processing, repetition (and in the face the eyes) was observed as having an early perceptual advantage over reflection (and in the face the mouth). However at later stages of processing repetition appeared to be processed serially and no longer had a perceptual advantage over reflection (ISIs >42ms). Reflection was suggested as having a perceptual advantage post V1 (ISIs >96ms). Patterning continues throughout a visual scene from the local level to the global level, as such both the human face and human social groups can be perceived as patterns. This was illustrated by a series of experiments investigating the effect of patterning on the perception of images presented in the periphery of a scene (Chapter 8).

Previous research has indicated that information carried by the Magnocellular (M) pathway may be used in written word recognition and reading, although these findings are far from conclusive. The precise nature of this potential role of the M pathway in word recognition is also unclear, with some researchers suggesting that it is to convey word-level shape information whilst other researchers have indicated that the M pathway’s role may relate to attentional selection. Eight experiments are reported that used isoluminant stimuli to investigate the validity of these claims. Experiment 1 examined the use of Heterochromatic Flicker Photometry to create isoluminant stimuli and in particular, the effect of stimulus type on the luminance ratios obtained. Experiments 2-4 investigated the recognition of words, pseudowords and illegal nonwords under isoluminant and non-isoluminant conditions in a Reicher-Wheeler task. Experiment 5 was a further Reicher-Wheeler task experiment in which case type (lowercase, UPPERCASE, and MiXeDcAsE), stimulus type and target luminance were varied. The recognition of isoluminant and non-isoluminant letter and nonletter strings were compared in Experiments 6 and 7. Experiment 8 used a similar design to previous experiments to investigate whether the recognition of isolated letters might also use M pathway information. These experiments revealed that with lowercase words, accuracy in the Reicher-Wheeler task is reduced at isoluminance in comparison to perception under non-isoluminant conditions, indicating that M pathway information is used in the recognition of these stimuli. Furthermore, this reduction in accuracy at isoluminance was shown to extend to pseudowords and illegal nonwords, to words and nonwords presented in upper and mixed case, and to isolated letters. However, with nonletter strings, no reduction in accuracy at isoluminance was obtained in the Reicher-Wheeler task. The implications of these findings for the various theoretical explanations of the M pathway’s role in written word recognition are discussed.