Developing an early vision model of lightness perception: experimentand theory

点击翻译按钮获取中文摘要

The goal of this project is to characterize the computations that allow the human visual system to extract meaningful perceptual categories from the retinal input. In particular, we study how the perceptual attribute of surface lightness, i.e. perceived surface reflectance, is computed from the luminance pattern that stimulates the photoreceptor array in the retina. At present there is no computational model of lightness perception. The problem of lightness perception lies in the ambiguity of the luminance signal, because the same luminance value can result from different surface reflectances that are seen in different illuminations. Traditionally lightness perception has been studied in simplified stimuli in which targets of varying luminance have been presented on backgrounds of varying luminance. However, such stimuli have been void of information about the surface properties of a target and about the depth and illumination structure of the scene in which the target surface was seen. Recent evidence suggests that this unnatural viewing situation has seriously misled our understanding of visual perception and has prevented us from observing visual mechanisms as they would operate under more realistic conditions. Such under-constrained stimuli might have introduced perceptual ambiguities that would not occur under more natural conditions. Here we pursue a different approach using realistic stimuli of moderate complexity. The stimuli that we use are variegated checkerboard patterns that are computer-rendered images of three-dimensional scenes in which we can specify the reflectances of each check in the checkerboard as well as their illumination. They have proved to be a well-controlled experimental test bed to study the computations involved in lightness perception, because they allow us to capture perceptual performance in relation to the external world properties of visual stimuli. They are simple enough to rebuild them as real-world three-dimensional models, and at the same time they allow the successive increase in complexity to study the influence of mechanisms of scene segmentation on lightness perception. We will elaborate an early vision model of lightness perception that is based on the contrast at luminance edges and that has emerged from a number of recent findings from our own group and from other researchers. We will use the model as the starting point and theoretical framework for the research questions described in this proposal.