Building a Common Framework for Understanding the Dimensions of Achromatic Perception: Brightness, Lightness and Perceived Illumination

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Determining the basic mechanisms of human achromatic visual perception is important to understanding the operation of human vision as a whole. This proposal is concerned with lightness, brightness and transparency perception, and is aimed primarily at understanding how the visual system parses incoming patterns of light into separate representations of surface reflectance and illumination intensity. The research could have implications for the design of visual displays, virtual reality systems, artificial vision systems, and effective lighting systems. The project integrates the basic research goals with the training of undergraduate students as well as public outreach in the state of North Dakota. Though the initial visual information entering the visual system is a distribution of light intensity (an image) on the back surface of the eye (the retina), the retinal image itself is inherently ambiguous because the intensity at each point in the retinal image is given by the product of the amount of incident light (illumination) and the surface reflectance. Without additional information (or prior assumptions), the relative contributions of these causes cannot be uniquely determined. This is known as the "inverse problem" in vision. The goal of the proposed research is to identify the prior assumptions and processing strategies that the visual system uses to solve this problem. In the experiments, observers make apparent intensity (brightness) and apparent reflectance (lightness) matches of surfaces (both rendered and real) under homogeneous illumination and in the presence of visible shadows and transparencies. Both behavioral responses and pupillary responses will be measured. Measuring both brightness and lightness perception under the carefully controlled conditions of the experiments is expected to increase current understanding of how the visual system parses incoming patterns of light into the separable components of surface reflectance and illumination intensity in the computations that enable perceived lighting to occur. The investigators ultimately aim to explain why people often vary in the accuracy of their judgments about lighting.