Academic journal article Perception and Psychophysics

The Phantom Illumination Illusion

Academic journal article Perception and Psychophysics

The Phantom Illumination Illusion

Article excerpt

A novel brightness illusion in planar patterns is reported. The illusion occurs, for example, when surfaces with a luminance ramp shaded from black to white are positioned on a black homogeneous background, so that each white end of the surfaces faces a single point of the plane of the pattern. The illusion consists of the enhancement of the brightness of the background in a relatively wide area around the white ends of the surfaces. A parametric study was conducted in which participants were asked to rate the difference in brightness between the parts of the background inside and outside a virtual circle formed by disks with different luminance ramps. The results show that mean ratings of brightness depended on the luminance of the background, the luminance range of ramps, and the kind of ramp. Discussion of these results with reference to other brightness illusions (assimilation, neon color spreading, anomalous surfaces, visual phantoms, grating induction, and the glare effect) shows that the phantom illumination illusion derives from processes producing the perception of ambient illumination.

In this article, I present the phantom illumination (PI) illusion, a new brightness effect occurring in planar patterns. Figure 1 shows examples of the illusion. In Figure 1A, five disks shaded from black to white form a virtual pentagon on a homogeneous black background, with the white end of each luminance ramp facing the center of the pentagon. During an informal interview, about 25 naive observers reported that the area of the background within the virtual pentagon looked brighter than that in the rest of the background. Some observers reported the impression that the illumination of this area differed from that of the rest of background. No observer reported that the background looked darker than the target area. Since all observers described the illusion as an enhancement of the brightness of the part of the background enclosed by the luminance ramps rather than as a depression of the brightness of the rest of the background, such an enhancement may be considered the main perceptual feature of the illusion.

In general, let us call surfaces with a luminance ramp potentially capable of producing the PI illusion inducers, and let us call any area of the background whose brightness is affected by inducers the target area (T). Figures 1B and 1C show that inducers of different shapes forming different geometric configurations produce the PI illusion when the bright ends of the inducers face a single point of the ideal plane of the pattern. This point is infinitely distant from the inducers in Figure 1B and is relatively close to the inducers in Figure 1C. Figure 1D, in which the man and the child are on a dark background with uniform luminance, shows that the PI illusion may also occur in rather complex visual scenes, thus suggesting that pictorial 3-D information from inducers may be important for the occurrence of the illusion.

In one of its simplest forms, as for example in Figure 1A, the PI illusion recalls other brightness effects. With reference only to stationary achromatic patterns, the following is a list of brightness illusions having some aspect in common with the PI illusion.

Assimilation (Bezold, 1876). Figure 2A provides an example of assimilation (Musatti, 1953). All six backgrounds have the same luminance. When observers look at the patterns in Figure 2A with a global viewing attitude, they may describe each of the three backgrounds on the left as looking lighter than each of the three backgrounds on the right. That is, when the small achromatic disks shown in Figure 2A are placed on the respective backgrounds and the observer's viewing attitude is global, the brightness of the backgrounds may be reported to change to a value closer to the brightness of the disks. However, when observers inspect the backgrounds analytically, they often report the reverse, that is, that the three right backgrounds are lighter than the three left backgrounds (Kanizsa, 1979, 1980). …

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