Processing boundary and region features for perception
Creators: name: Coningham, Alexander
Publisher: name: The Australian National University
Year: 2021

description: A fundamental task for any visual system is the accurate detection of objects from background information, for example, defining fruit from foliage or a predator in a forest. This is commonly referred to as figure-ground segregation, which occurs when the visual system locates differences in visual features across an image, such as colour or texture. Combinations of feature contrast define an object from its surrounds, though the exact nature of that combination is still debated. Two processes are likely to contribute to object conspicuity, the pooling of features within an object's bounds relative to those in the background ('region' contrast) and detecting feature contrast at the boundary itself ('boundary' contrast). Investigations of the relative contributions of these two processes to perception have produced sometimes contradictory findings, some of which can be explained by the methodology adopted in those studies. For example, results from several studies adopting search-based methodologies have advocated nonlinear interaction of the boundary and region processes, whereas results from more subjective methods have indicated a linear combination. This thesis aims to compare search and subjective methodologies to determine how visual features (region and boundary) interact, highlight limitations of these metrics, and then unpack the contributions of boundary and region processes in greater detail. The first and second experiments investigated the relative contributions of boundary strength, regional orientation, and regional spatial frequency to object conspicuity. This was achieved via a comparison of search and subjective methodologies, which, as mentioned, have previously produced conflicting results in this domain. The results advocated a relatively strong contribution of boundary features compared to region-based features, and replicated the apparent incongruence between findings from search-based and subjective metrics. Results from the search task suggest nonlinear interaction and those from the subjective task suggest linear interaction. A unifying model that reconciles these seemingly contradicting findings (and those in the literature) is then presented, which considers the effect of metric sensitivity and performance ceilings in the paradigms employed. In light of the findings from the first and second experiments that suggest a stronger contribution of boundary information to object conspicuity, the third and fourth experiments investigated boundary features in more detail. Anecdotal reports from observers in the earlier experiments suggest that the conspicuity of boundaries is modulated by information in the background, regardless of boundary structure. As such, the relative contributions of boundary-background contrast and boundary composition were investigated using a novel stimulus generation technique that enables their effective isolation. A novel metric for boundary composition that correlates well with perception is also outlined. Results for those experiments suggested a significant contribution of both sources of boundary information, though advocate a critical role for boundary-background contrast. The final experiment explored the contribution of region-based information to object conspicuity in more detail, specifically how higher-order image structure, such as the components of complex texture, contribute to conspicuity. A state-of-the-art texture synthesis model, which reproduces textures via mechanisms that mimic processes in the human visual system, is evaluated respect to its perceptual applicability. Previous evaluations of this synthesis model are extended via a novel approach that enables the isolation of the model's parameters (which simulate physiological mechanisms) for independent examination. An alternative metric for the efficacy of the model is also presented.