Studies of Brightness, Brightness Contrast and Lightness: Methods and Modeling

North Dakota State University Fargo, Fargo ND

Investigators

Abstract

Brightness is a fundamental quality of vision. A central problem in brightness perception is understanding how and when the visual system is able to separate the physically invariant reflectances of surfaces from their potentially changing illumination. The luminance distribution falling on the photoreceptor array, and the neural activity this engenders, is the algebraic product of the reflectance of objects in the environment and their illuminance. The recovery of surface reflectance and illuminance information from the raw luminance signal is thus an ill-posed problem, in that there are myriad combinations of illumination and reflectance that can potentially give rise to a given intensity distribution, and in the absence of additional information there is no algorithmic solution. We know from everyday experience, however, that the visual system solves the inverse problem in a satisfactory manner much of the time. The heuristic used by the visual system must, therefore, involve the use of additional sources of information and/or assumptions. Indeed, much of the current debate among researchers in brightness and lightness perception, and the focus of this research project, centers on discovering the mechanisms the visual system uses to solve this problem. A careful examination of the work of Arend and Spehar [(1993) Perception & Psychophysics, 54, 446-468] makes it clear that lightness defined only as perceived reflectance is under-specified, since it refers to three very different types of achromatic judgments that are not comparable. This is a direct result of the fact that, in its heuristic solution to the inverse problem, the visual system uses different strategies to estimate surface reflectance in different situations. The fact that lightness judgments are based on different information under different conditions, and that the various definitions of the term lightness refer to either all or only a subset of these different types of lightness judgments, has created confusion in the literature. Thus, "lightness" data generated in one study may or may not be comparable or relevant to what other investigators study using the same name. The proposed experiments are designed to resolve large discrepancies in the literature over the effects of perceived illumination and the effects of perceived depth on lightness perception. It is hypothesized that these discrepancies are the result of different studies tapping different types of lightness judgments. To definitively resolve this issue we will obtain brightness, brightness contrast and lightness judgments for a set of critical stimuli. A clear distinction between inferred-lightness judgments (cognitive judgments of target lightness) and lightness judgments based on brightness or brightness contrast (sensory-level judgments of surface color or contrast) is an essential step in critically evaluating models of brightness and lightness perception. These data will be compared to predictions of both a multiscale filtering model of brightness perception [Blakeslee & McCourt (1999) Vision Research, 39, 4361-4377] as well as to an anchoring model of lightness perception [Gilchrist et al. (1999) Psychological Review, 106, 795-834]. The project will integrate research and education by involving undergraduate students as much as possible in every phase of the research process. Effort will be made to involve students from underrepresented groups, such as females and Native Americans. In addition to the benefits conferred on society by expanding knowledge of basic sensory processing, the present research will be of broad applicability to any area impacted by the interface between sighted humans and machines of all descriptions.