Linear and non-linear mechanisms in the perception of stereoscopic slant and transparency.

This thesis explored the role of contrast disparities in stereopsis, and the nature of the encoding of surface slant from stereoscopic cues. Contrast disparities may be defined as interocular differences in the position of image regions corresponding in terms of image contrast, rather than luminance. It was found that, for simple plaid stimuli, stereoscopic slant thresholds could be predicted from disparities in the plaid's components. Further, the perceived slant of grating and plaid stimuli was found to be underestimated, with the degree of underestimation for plaids depending on the orientation of their component gratings. These results may be explained in terms of the orientation and spatial frequency disparities in the Fourier components of the stimuli, and are consistent with the notion that orientation disparities provide the primary cue to stereoscopic slant (Rogers and Graham, 1983). For plaids with orthogonal components, differing in contrast and spatial frequency, stereoscopic transparency was observed. Transparency was also observed in stimuli for which depth was defined by contrast modulation disparities. Transparency was only perceived for crossed disparities of the contrast modulation, such that the modulation appeared to lie in front of its carrier. This asymmetry was not evident if additional luminance disparities were introduced to the image. These results support the view that stereopsis has access to independent, linear and nonlinear channels (Hess and Wilcox, 1994). However, it was found that adaptation to the carrier of a contrast modulated stimulus increased the minimum contrast at which contrast disparities could be detected, suggesting that significant nonlinearities in stereopsis are preceded by a stage of linear filtering. These results were explained using a model in which luminance and contrast disparities are processed by independent linear and nonlinear mechanisms,s haring a common linear filtering stage early in processing. A ck n ow led gem en ts I would like to thank my supervisor, Keith Langley, for his help, advice and support throughout the thesis. Oliver Braddick and David Fleet also provided many helpful comments and ideas. I am grateful to many others at UCL for their encouragement and support. I would particularly like to thank Colin Clifford, Tom Hartley, Stevie Sackin, Peter Howell, Alan Johnston, Mark Gardner, Nico Preston, John Draper, Kate Bradford, and the Psychology Allstars . I would also like to thank David Green and George Houghton, for interesting me in Psychology as an undergraduate. Finally, I would like to thank Isabel, Jo, Paul(s), Carol, Ben, Mikie, Arshad, Garreth, Helen and the Loughborough posse for their friendship throughout. This thesis is for my parents.