Joseph Cataliotti

Local and global processes in surface lightness perception.

Various demonstrations show that a target of constant luminance can be made to appear darker in perceived lightness merely by introducing an adjacent region of higher luminance. This has often been interpreted as a manifestation of contrast effects produced by lateral inhibition, a relatively local process. An alternative interpretation holds that the highest luminance in such a display serves as an anchor that defines the white level. This interpretation is global in the sense that the anchor need not be located near any particular target in order to serve as its standard. Edge integration processes have been postulated that would enable such remote comparisons, but there is controversy about the strength of these processes. We report a series of experiments in which local and global processes were assessed. Specifically, we tested whether the introduction of a higher luminance has a greater darkening effect on an adjacent target than on a remote target. We found no difference, suggesting that the darkening effect is a matter of anchoring, not contrast, and that edge integration processes required by anchoring are relatively strong.

The effects of figure/ground, perceived area, and target saliency on the luminosity threshold.

Observers adjusted the luminance of a target region until it began to appear self-luminous, or glowing. In Experiment 1, the target was either a face-shaped region (figure) or a non-face-shaped region (ground) of identical area that appeared to be the face’s background. In Experiment 2, the target was a square or a trapezoid of identical area that appeared as a tilted rectangle. In Experiment 3, the target was a square surrounded by square, circular, or diamond-shaped elements. Targets that (1) were perceived as figures, (2) were phenomenally small in area, or (3) did not group well with other elements in the array because of shape appeared self-luminous at significantly lower luminance levels. These results indicate that like lightness perception, the luminosity threshold is influenced by perceptual organization and is not based on low-level retinal processes alone.

Spatial and temporal lightness anchoring

Research in the area of lightness perception has not adequately addressed the influence of previously viewed visual fields on perceived surface reflectance. In the spatial realm, a spot-in-a-void will appear darker when a second surface of higher intensity is placed adjacent to it. The brighter surface takes the role of white, the anchor, and the dimmer is scaled accordingly. We find that when a spot-in-a-void is presented to observers in a light controlled chamber it is influenced by nonretinal temporal relationships mediated by the degree of complexity. We also find that the influence cannot be explained in terms of successive contrast at a high or low level in the visual system, but can be explained by an anchoring model. The present results follow the same rules governing spatial integration and anchoring and thereby support the currently proposed concept of temporal anchoring.

T-junctions, apparent depth, and perceived lightness contrast

Observers performed lightness matches for physically equivalent gray targets of asimultaneous lightness contrast display and displays in which both targets were on the same background. Targets either shared a common line-texture pattern with their respective backgrounds or did not. Results indicate that when targets share a line-texture pattern with their respective backgrounds, a contrast effect is obtained. However, when the target’s pattern is different than the background’s pattern, perceived contrast is significantly reduced and the target appears as a separate 3-D entity. This result applies to both vertically and horizontally oriented displays, to targets that are increments or decrements, and to line-texture patterns that are black or white. Line patterns that are shared by targets and backgrounds result in T-junctions that provide occlusion information. We conclude that targets and backgrounds perceived to be on separate planes because of T-junctions are less likely to be perceptually grouped together and that their luminance values are less likely to be compared with one another.

A vision research apparatus for broad luminance range displays

Lightness, the perceived gray shade of a surface, and the perception of self-luminous surfaces—that is, surfaces that appear to glow—have most often been studied with paper displays and computer-generated stimuli presented on CRT monitors. Although both methods are often effective, experiments that require a wide range of luminance values in the same display are often difficult to conduct with paper and computer displays alone. Also, color mode appearance is often an issue when surface color perception is the topic of research; CRT monitors are essentially light sources themselves and often appear in the luminous mode of color appearance. Here, we describe an apparatus in which the target is an undetected aperture whose luminance is adjustable. Whereas a typical CRT monitor offers a luminance range of about 100:1, much broader luminance ranges are possible with the described apparatus. Unlike a CRT monitor, the stimulus background will always appear in the surface mode of color perception, and the target(s) can appear as either surface colors or luminous colors. Apparatus modifications are possible, including the addition of a stereoscope or an embedded CRT for creating an adjustable region that is computer controlled.