In Fig. 1, which line is longer?
Our eyes will tell us that the first line seems longer than the second, but in fact two lines are the same. This famous image consisting of stylized arrows is called Müller-Lyer illusion. When we see the picture, though we are asked to place a mark on the figure at the midpoint, our perception will automatically place our vision more towards the “tail end”. Illusions are situations when human’s perception differs from generating stimulus in a meaningful, even misleading way.
Optical illusions are fascinating, and sometimes even challenging because they demonstrate that what we see is just what we perceive through eyes: we receive the information, interpreted it and yielded the internal representation of the world.
Psychologists and evolutionary biologists have researched the proposition of illusory perceptual discrepancies. The direction of these researches has been changed throughout history, from purely concentration on the phenomena to the predicted analysis. With enough knowledge of the nervous system, detailed researches of neural structure and function in perspectives of the behavioral research proceed to geometrical and physical directions.
We can classify illusions into six categories: Luminance and contrast, Motion, Geometric or angle illusions, 3D interpretation (impossible figures), Cognitive effects and Color. Here are some examples related with some of these categories.
Luminance and contrast
The “Hermann Grid” (Fig. 2) is that if you examine the picture, you will see gray dots at the intersections of the white “streets”. This was explained on the concept of lateral inhibition. We see the worlds as our retinal ganglion cells encode and thereby compress it. Moreover, the appearance of gray patches highlights the additional role of cortical processing, i.e. orientation selective neurons.
In Fig. 3 the disks seem to expand slowly. One explanation is that when the asymmetric luminance (black to gray to white) appears, we will perceive the asymmetric luminance steps drive motion detectors. Meanwhile, human’s eye movements affect the modulation with our automatic adaptation or possible suppression.
Geometric or angle illusions
As J Zöllner discovered in 1860 (Fig. 4), the parallel lines intersected by short lines with right angles appear to be unparalleled. One illustration was that we feel like these parallel lines appear to be closer because we overestimated the small angles affected by the crossings of short lines.
As Gerald mentioned (2, Gerald), compared with common cube, figures of the Penrose (Fig. 5) and Escher type require specific parsing of the figure’s components and deductive reasoning to reveal the impossibility of their physical realization. These so-called “impossible figures” require human beings to imagine a physical world, take time to simulate and get a perception, but in fact neglect some other sensory. Basically, they can be created through illusory contours, time discrepancies and perspective drawings.
Bach, Michael, and Ch M. Poloschek. “Optical illusions.” Adv Clin Neurosci Rehabil 6.2 (2006): 20-21.
Westheimer, Gerald. “Illusions in the spatial sense of the eye: Geometrical–optical illusions and the neural representation of space.” Vision research 48.20 (2008): 2128-2142.
Coren, Stanley, and Joan S. Girgus. Seeing is deceiving: The psychology of visual illusions. Lawrence Erlbaum, 1978.