Why does color always cheat us? The multiple colors of black and white

Our visual system sometimes makes colors appear on a black and white image. How is this possible since the image objectively contains no spectral characteristic that can be interpreted as a color by our brain? We then speak of subjective colors because they differ from one observer to another, and some do not perceive them. It turns out that the time parameter plays a key role in the perception of these illusory colors. By what mechanism?

Rotating black and white disks provide colorful sensations

In 1838, the German philosopher and psychologist Gustav Fechner (1801–1887) wanted to make graded gray rings by rotating a disc with black areas, increasing in size from the center, on a white background (Fig. 1). To his surprise, he observed not gray rings but colored rings. The color also depended on the direction of rotation!

Fig. 1. The spinning Fechner disc in rotation makes perceive colored rings and not gray.

Fechner’s article recounting his observations, however, went unnoticed. Much later, in 1894, the Englishman Charles Benham (1860–1929), a journalist and inventor in his spare time, took up the idea and made a spinning top, the Toton de Benham, which was sold in toy stores a great commercial success. When spinning this top containing black circle arcs on a white background, colored rings appear (click on the image in Figure 2). The color order with respect to the center is reversed when the direction of rotation changes. These illusory colors are called “subjective colors of Fechner-Benham”.

Fig. 2. Benham’s spinning disc produces the sensation of light-colored rings in many observers. To see the effect of this rotation, click here.

The origin of these colors is not completely elucidated. The conditions necessary but not sufficient to observe them are temporal modulation and spatial modulation of the stimulus. This is what Benham’s record achieves with the alternation of dark and light parts produced by the short arcs and the two black and white half-discs. Opinions converge to invoke differences in the speed of transmission to the brain of retinal photoreceptor signals (cones) and pretreatment by retinal neurons. In addition, since the perception of colors varies significantly from one individual to another, it is conceivable that the subjective colors can be perceived differently.

The spokes of MacKay

Look at Figure 4 designed by the British Donald MacKay in 1957. Many observers perceive in this figure in black and white colored moirés and movements in the central part. These subjective colors are again explained by temporal and spatial modulations, but they do not come this time from a movement of the drawing, but from the movements of the eye itself. Indeed, the latter never remains fixed, even when staring: it is animated by rapid movements, random, and low amplitude, called micro jolt.

Here is a small experiment which proves the existence of micro jolt of the eye. Attach the black dot to the center of the image below for about 30 seconds, then move your gaze to the white disk. You will then see that the remanent image of the grid is animated by micromovements.

fig 3 experience Micro jolt

Fig. 4. MacKay rays cause colorful moirés and movements in the center of the image. By scrolling the document on your screen, you will also notice the appearance of transient rings. The micros jolts of the eye are responsible for these observations. Source: Project Lite.

Producing the impression of movements or vibrations by taking advantage of the properties of the eye and the illusions of optics, such as the objective of works of op art (or optical art). Figure 5 gives an example: it is a reproduction of the painting that the artist Isia Leviant painted in 1981 and christened The Enigma. By staring at the center of this painting, you probably perceive clouds turning in the colored rings.

Fig. 5. The Enigma, painting by Isia Leviant

Fig. 6When you remove the rays of MacKay, the feeling of movement disappears.

This illusion has intrigued scientists. It has not escaped you that these rings are superimposed on the spokes of MacKay that provide a feeling of movement. These are somehow “transferred” to the rings. Without the rays, the illusion disappears (Fig. 6). After much debate and controversy, S. Martinez-Conde and his colleagues showed that the speed of rotation of the clouds in the rings was directly related to the frequency of micro-jolts of the eye, which refutes the other interpretations putting into play the brain and not the eye.

The importance of the micros jolts of the eye

The micro jolts of the eye are essential for most of the visual perception. When you fix your gaze on a letter of this note, its image on the retina moves permanently because of the micromovements of the eye whose amplitude is of the order of the size of the letter. Thus, groups of different photoreceptors are solicited, which “refreshes” the neuronal activity several times a second. Retinal neurons thus remain active, thanks to micro jolts; without them, the image would disappear. The information transmitted to the visual cortex is analyzed, reorganized, and “in fine”, the brain provides a coherent and stable image.