Optimal sampling of visual information for lightness judgments

Abstract

The variable resolution and limited processing capacity of the human visual system requires us to sample the world with eye movements and attentive processes. Here we show that where observers look can strongly modulate their reports of simple surface attributes, such as lightness. When observers matched the color of natural objects they based their judgments on the brightest parts of the objects; at the same time, they tended to fixate points with above-average luminance. When we forced participants to fixate a specific point on the object using a gaze-contingent display setup, the matched lightness was higher when observers fixated bright regions. This finding indicates a causal link between the luminance of the fixated region and the lightness match for the whole object. Simulations with rendered physical lighting show that higher values in an object's luminance distribution are particularly informative about reflectance. This sampling strategy is an efficient and simple heuristic for the visual system to achieve accurate and invariant judgments of lightness.lightness constancy | lightness perception | visual perception | attention J udging the lightness of visual stimuli has been studied for centuries, since the original investigations by Weber (1) and Fechner (2). The light reaching the eye is the product of the illumination and the reflectance of the object, and also depends on the scene geometry (3). However, only the proportion of reflected light is an invariant property of the object and thus of great importance for vision. There are several well-established factors that support lightness constancy in the face of these challenges. On the one hand, lateral inhibition between retinal neurons filters out shallow intensity gradients, which are mostly caused by illumination effects (4, 5). On the other hand, more complex factors also have an effect on lightness perception, such as object shape (6-9) or the interpretation of transparent surfaces (10, 11). However, eye movements have been almost completely neglected so far, even though a general influence of viewing behavior has been shown for some color constancy tasks (12)(13)(14)(15). This finding is surprising because the visual system needs to sample the local properties of objects and this is accomplished by moving the eyes and the focus of spatial attention around. Because visual acuity, luminance sensitivity, contrast sensitivity, and color sensitivity change with retinal eccentricity (16-18), our visual system has to stitch together its representation of the world from many small samples to analyze the visual scene in detail. Peripheral vision is not only characterized by poor resolution, but also the appearance of basic visual features-like spatial frequency, luminance, or chromatic saturation-is distorted in the periphery of the visual field (19-22). Eye movements may then be used to select relevant information, even for stimuli that are above threshold in peripheral vision. We investigated whether the distribution of fixations on an object has an e...

The Interaction Between Vision and Eye Movements

“…There was an illumination gradient present in the scene with the papers, leading to a dissociation between luminance and reflectance for the individual paper chips. We found a significant effect of the fixation position only for the reflectance matches, and the chosen paper chips for fixation of light regions were much closer to the reflectance values rendered on the screen (Toscani et al, 2013a).…”

“…We investigated the effect of saccadic sampling on lightness perception (Toscani, Valsecchi, & Gegenfurtner, 2013a, 2013b. We asked our observers to match the hue, saturation, and lightness of a uniformly colored object placed on a pedestal in front of a Figure 2.…”

The many colours of ‘the dress’

“…so that it would match for them the colour of the dress [3]. We also had them match the colour of the lace parts of the dress.…”

Information extraction from shadowed regions in images: An eye movement study

“…In fact, it has been shown that influences of specific instructions in color constancy experiments (Arend & Reeves, 1986) can be understood, at least in part, by the pattern of eye movements that observers make during the task (Cornelissen & Brenner, 1995). Moreover, recently it has been shown that where observers fixate in an image directly influences lightness judgment (Golz, 2010;Toscani, Valsecchi, & Gegenfurtner, 2013a, 2013b. These studies also show that observers, trying to match the hue of an object in a cast shadow to a reference patch, tend to spend more time gazing at the part in the higher illumination.…”