BRIGHTNESS ILLUSIONS: WHY WE SEE CONTRAST

ACHROMATIC FOREGROUND-BACKGROUND CONTRASTS

DESCRIPTION

In FIGURE 1 below, each of the four small, gray squares within a larger square emits exactly the same physical brightness; that is, physical intensity of light energy. Because our eyes receive exactly the same amount of light from each of them, they should all therefore appear to be exactly the same shade of gray. Nevertheless, these four central gray squares appear unequal in perceived brightness; the darker their background the lighter the smaller gray square appears.

Since a computer monitor is a variable light source, brightness contrast can be observed dynamically. Turn down the brightness control of your monitor to a low, but not dim level of illumination. As you slowly increase the level of illumination from its lowest value, the centre gray square in FIGURE 1B should grow relatively brighter compared to the centre square in FIGURE 1A which becomes relatively dark. This phenomenon is called simultaneous brightness contrast or lateral brightness adaptation and is explained by the theory of lateral inhibition. See the commentary below.

FIGURE 1

A.	B.
C.	D.

COMMENTARY

In our everyday visual experience we unconsciously follow an absolute theory of vision, namely, we assume that the perception of a scene in a small area of the visual field is determined entirely by the amount of light which emanates from that part of the scene and then becomes incident on the corresponding part of the retina. FIGURE 1 shows this simple stimulus-response view to be greatly overstated if rendered as a theory of vision to explain our subtly graded perception of brightness contrasts.

The change in the sensitivity of one part of the retina by the level of brightness excitation in an adjacent retinal area is called lateral inhibition and accounts for most of the contrast effects in FIGURE 1 above. In FIGURE 1A for example, the white background strongly stimulates the photoreceptors on the part of the retina where the white part of the image is formed; these photoreceptors in turn inhibit the photoreceptors stimulated by the part of the retina where the gray image is formed. The inhibitory connection between the "white-stimulated photoreceptors" and the "gray-stimulated photoreceptors" is accomplished with aptly named horizontal cells in the retina. The absolute physical intensity of light emitted by a target is generally NOT what is informative for the perceptual system; the visual system responds mostly to variations in light intensity across the visual field which interact with each other within the visual pathways -- brightness contrast is in the eye of the beholder.

CHROMATIC FOREGROUND-BACKGROUND CONTRASTS

DESCRIPTION

Brightness contrast is also evident in colour. In FIGURE 2 below each of the four central gray squares is surrounded by different coloured backgrounds, and the gray squares appear to vary in brightness, even though they send the same physical level of energy to the eye. For example, the gray squares with the blue backgrounds appear brighter than the gray square on the yellow background.

COMMENTARY

This variation in apparent brightness among the gray squares occurs even if all background colours also emit equal levels of brightness or physical energy. The reason is that different colours have different levels of apparent brightness even if their physical light intensity is held constant. At the same time, the colours in the surrounding square inhibit photoreceptors for the same colour in the part of the visual field controlled by the centre gray square; therefore a yellow background will cause a gray square to appear relatively blue (subjectively darker) compared to the same gray square made to appear yellow (subjectively brighter) by a blue background. The observer may note that the gray squares reveal a slightly coloured tint which is complementary to the colour of their backgrounds. These special relations of colours to each other are part of a more complete discussion of simultaneous colour contrast and colour illusions. Stare at each image in turn and the shift your gaze to the dot in the white space below. In FIGURE 2A, for example, one has the sensation of a cyan background surrounding a red square in place of a red background surrounding a cyan tinted gray square.

A

B

C

D

MACH BANDS

DESCRIPTION

Another important expression of brightness contrast occurs with the interesting and more complex phenomenon known as Mach bands, named after the physicist, Ernst Mach, who studied them in the 1860s. In FIGURE 3, best viewed in a dim to moderately lit room, there is a uniformly dark-gray area followed by a gradually increasing level of luminance to the right, like a graded shadow, until a uniform white area becomes apparent. However, the observer does NOT report a uniform change in brightness but rather he notes a fuzzy edged, vertical black line toward left of centre and another blurry vertical white line to the right of centre. Neither of these black or white bands are actually present in the physical stimulus itself; for example, there is exactly the same amount of white or physical luminance within the vertical white band as in the uniform white area to its right. In fact it is technically impossible for us to render the uniform white area to the right any whiter, though it appears slightly gray beside the white band.

COMMENTARY

The bands are strictly a function of the activity of the visual system, namely, lateral inhibition. Retinal cells in the uniformly bright area on the right of FIGURE 3 inhibit each other through other horizontally arranged cells which make connections among them. However, cells receiving the same amount of light from the region of the apparent white band are less completely inhibited because cells to their left receive less stimulation from the gray area of the stimulus. Therefore, being relatively more active than those cells to their right, these less inhibited cells produce the experience of the white band and the region to the right appears slightly more gray.

The opposite is the case for the retinal cells receiving stimulation from the area of the black band. Retinal cells in the uniformly dark area on the left of FIGURE 3 are not inhibiting each other very much because they receive little light from the stimulus. However, cells receiving the same small amount of light from the region of the apparent dark band are even less excited because cells to their right receive more stimulation from the lighter gray area of the stimulus and thus inhibit them. Cells in the region of the dark band, being poorly stimulated to begin with, are now inhibited further by the more active gray cells to their right, thus producing the illusion of a dark band.

CHROMATIC MACH BANDS

DESCRIPTION

Mach bands can also be rendered in colour. In FIGURE 4 each differently coloured vertical panel has a physically uniform luminance. However, the perceived brightness of the panels is NOT uniform. Most panels appear to have a bright glow along their left side and a dark shading along their right side; the pointers in FIGURE 4 make one example of each explicit. The panels were made by sampling at equal intervals from an original stimulus which was made up of a completely uninterrupted sliding gradient of colour from blue through red to yellow. Reducing the original, smooth gradient to a limited number of uniformly coloured panels shows that Mach bands can appear anywhere within the brightness continuum. Can you explain why the bands are reversed in the case of the blue panel on the extreme left?

COMMENTARY

Mach bands rendered in colour are also explained by the same principles of lateral inhibition as achromatic Mach bands above. After all, whether we are looking at chromatic or achromatic stimuli, we use the same type of photoreceptor for day time vision, namely, cones.