Encoding the shape of objects within the visual environment is one of the important roles of the visual system. This thesis investigates the proposition that human sensitivity to a broad range of closed-contour shapes is underpinned by multiple shape channels (Loffler, Wilson, & Wilkinson, 2003). Radial frequency (RF) contours are a novel type of stimulus that can be used to represent simple and complex shapes; they are created by sinusoidally modulating the radius of a circle, where the number of cycles of modulation defines the RF number (Wilkinson, Wilson, & Habak, 1998). This thesis uses RF contours to enhance our understanding of the visual processes which support shape perception. The first part of the thesis combines low and high RF components, which Loffler et al. have suggested are detected by separate global and local processes respectively, onto a single contour and shows that, even when combined, the components are detected independently at threshold. The second part of the thesis combines low RF components from across the range where global processing has been demonstrated (up to approximately RF10) onto a single contour in order to test for interactions between them. The resulting data reveal that multiple narrow-band contour shape channels are required to account for performance, and also indicate that these shape channels have inhibitory connections between them. The third part of the thesis examines the local characteristics which are used to represent shape information within these channels. The results show that both the breadth (polar angle subtended) of individual curvature features, and their relative angular positions (in relation to object centre) are important for representing RF shapes; however, processing is IV not tuned for object size, or for modulation amplitude. In addition, we show that luminance and contrast cues are effectively combined at the level where these patterns are detected, indicating a single later processing stage is adequate to explain performance for these pattern characteristics. Overall the findings show that narrow-band shape channels are a useful way to explain sensitivity to a broad range of closed-contour shapes. Modifications to the current RF detection model (Poirier & Wilson, 2006) are required to incorporate inhibitory connections between shape channels and also, to accommodate the effective integration of luminance and contrast cues.
| Identifer | oai:union.ndltd.org:ADTP/185653 |
| Date | January 2008 |
| Creators | Bell, Jason |
| Publisher | University of Western Australia. School of Psychology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Jason Bell, http://www.itpo.uwa.edu.au/UWA-Computer-And-Software-Use-Regulations.html |
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