24 Lightness Perception and Lightness Illusions EDWARD

The amount of light coming to the eye from an object depends on the amount of light striking the surface, and on the proportion of light that is reflected. If a visual system only made a single measurement of luminance, acting as a pho-tometer, then there would be no way to distinguish a white surface in dim light from a black surface in bright light. Yet humans can usually do so, and this skill is known as lightness constancy. The constancies are central to perception. An organism needs to know about meaningful world-properties, such as color, size, shape, etc. These properties are not explicitly available in the retinal image, and must be extracted by visual processing. The gray shade of a surface is one such property. To extract it, luminance information must be combined across space. Figure 24.1 shows the well-known simultaneous contrast effect, which demonstrates a spatial interaction in lightness perception. The two smaller squares are the same shade of gray. However, the square in the dark surround appears lighter than the square in the light surround. Illusions like these are sometimes viewed as quirky failures of perception, but they help reveal the inner workings of a system that functions remarkably well. Here we will consider how lightness illusions can inform us about lightness perception. The visual system processes information at many levels of sophistication. At the retina, there is low-level vision, including light adaptation and the center-surround receptive fields of ganglion cells. At the other extreme is high-level vision, which includes cognitive processes that incorporate knowledge about objects, materials, and scenes. In between there is mid-level vision. Mid-level vision is simply an ill-defined region between low and high. The representations and the processing in the middle stages are commonly thought to involve surfaces, contours, grouping, and so on. Lightness perception seems to involve all three levels of processing. The low-level approach to lightness is associated with Ewald Hering. He considered adaptation and local interactions , at a physiological level, as the crucial mechanisms. This approach has long enjoyed popularity because it offers an attractive connection between physiology and psy-chophysics. Figure 24.2(a) shows the receptive field of an idealized center-surround cell. The cell exhibits lateral inhibition: light in the center is excitatory while light in the surround is inhibitory. A cross-section of the receptive field is shown in figure 24.2(b). This cell performs a local comparison between a given …