Many of us take our vision for granted. We seem to accurately use our properties of vision with little effort or thinking at all. At times we often see things with our eyes and wonder how can this be possible?
Physicist Richard Feyman once wrote, “It’s quite wonderful that we can see or figure it out so easily. Someone who’s standing at my left can see somebody who’s standing at my right – that is the light can be going this way across or that way across, or this way up, or that way down; it’s a complete network. Some quantity is shaking about, in a combination of motions so elaborate and complicated the net result is to produce an influence which makes me see you, completely undisturbed by the fact that at the same time there are influences that represent the guy on my left side seeing the guy on my right side. The light’s there anyway…it bounces off this, and it bounces off that – all this is going on, and yet we can sort it out with this instrument, our eye” (Introduction to Perception [Online]).
As you can see by this quote, sight is a very complicated process that is easily controlled by our eyes. However, perceptual illusions need a little more explaining.
Take an instant to observe the world around you. If you tilt your head, the rest of the world does not tilt along with you. If you close on of your eyes, you don’t suddenly lose your depth perception. Shades of color seem to vary under different levels and types of illumination. When you move around an object, the shape you see changes, yet the object remains stable (Introduction to Perception [Online]). Just take a peek at the Perceptual Illusions on last page of this report. Even though you may know that you are being fooled, it does not stop the effect from continuing to trick you. This indicates a split between the sensation of seeing it, and the way in which you perceive it. In many cases your cognitive abilities can not influence your perceptions. For any given retinal image, there is an infinite variety of possible three-dimensional structures that can lead to it. Our visual system, however, usually settles for the correct interpretation (Introduction to Perception [Online]). It is when a mistake is made in the interpretation that an illusion occurs.
Perceptual illusions are anything that may seem to cause something to become perceptually confusing. These illusions have a distinct manner of confusing our normal perceptual processes. The Muller-Lyer Illusion and the Ponzo Illusion may be the two most commonly studied perceptual illusions in psychology. Stephen R. Jackson states in his Feb. 2000 article in Journal of Experimental Psychology that appropriate indications such as linear perspective and relative size can exert a powerful effect on the perception of objects. The illusory effects such as those of the Muller-Lyer and Ponzo illusions demonstrate this fact (Jackson 2000).
In the Muller-Lyer Illusion (fig. 1), there are two objects, both having one line and two lined triangles at their ends. When the objects are placed parallel to each other, the line from the top object looks longer than the line from the lower object. Why does it appear to be longer when the two are actually the same length? This has to do with the direction of the two triangles or arrows at the end of the lines. On the lower figure, the open ends of the triangles are facing inward, causing the line to appear shorter than it is. And on the upper figure, the open ends of the triangles are facing outward, causing the line to appear longer than it actually is (Online. Retrieved on October 11, 2000 at http://www.epsychlopedia.net/concepts/sensationAndPerception/index.php?id=sensandperc8).
The Ponzo Illusion (fig 2) has four lines. There are two horizontal lines parallel to each other and two diagonal lines approaching each other on either side of the parallel lines. The top line appears to be longer than the bottom line. This is a result of our perception thinking that the top horizontal line is father away from us, resulting in our brain telling us that the top line is longer than the bottom one. (Online. Retrieved on October 11, 2000 at http://www.epsychlopedia.net/concepts/sensationAndPerception/index.php?id=sensandperc8).
Perceptual illusions also play a role on sports. In a March 1999 article from the Journal of Sport Behavior at the University of South Alabama, sport psychology researchers attempted to determine the best perspective for making accurate judgements with calling pitches a ball or a strike. The standard home plate umpiring position was compared to four other alternative positions. Each of the four alternative umpiring positions were higher and farther from the center of home plate. The four positions were labeled as follows: Inside-Far, in which the umpiring perspective was behind the line of the occupied batters box; Inside-Near, directly behind the outside corner of home plate; Outside-Near, directly behind the outside corner; and Outside-Far, behind the line of the unoccupied batter’s box (Repositioning the home plate umpire to provide enhanced perceptual cues and more accurate ball-strike judgements. 1999). The results from this experiment indicated that the positions behind the outside corner, farther from the batter, generated significantly more accurate calls due to the additional height and distance cues provided when umpires see pitches pass in front of the batter.
Another example of a perceptual illusion is after-images. An after-image is an image that stays with you even after you have stopped looking at a certain object. The back of your eye is lined with light sensitive cells, called cones, which are only sensitive to certain colors of light. For example, stare at the picture of the turquoise, black, and yellow American flag (fig. 3) on the last page of this report. When you stare at the black stars, your black-sensitive cells start to grow tired and lose their sensitivity. So, when you shift your gaze suddenly to a plain white background, you see white minus black where the black-sensitive cells have become fatigued. White light minus black light is white light. That's why the after image you see is white stars. The same thing happens when you stare at the turquoise and yellow because the complimentary color of turquoise is red, and the complimentary color of yellow is blue.
After-imaging is also explained in Eric Haseltine’s article about mental fatigue on page 128 in the December 1999 issue of Discover Magazine. He states that while you were staring at the black, cyan, and yellow flag a battle was raging between the neurons in your brain that sense yellow and blue and the ones that sense turquoise and red. On the blank sheet, you should see an afterimage of the colors of the American flag. The suppressed blue and red activity, which was kept at a minimum as you looked at the flag, now dominates, and those are the colors you see on the blank sheet (Haseltine 1999). The interval that is required for the illusion to vanish corresponds roughly to the rallying time of the neurons used to see yellow and turquoise (Haseltine 1999).
Throughout the years, many researchers have learned about the way in which we perceive the world through countless experiments with the systematic way in which we misperceive the world (Kassin 1998). We are surrounded by perceptual illusions (Wade, 1990; Rodgers, 1998). I hope that the examples that have been illustrated in this report will help you to better understand the world of perceptual illusions. Seeing should not always be believing.
Bridwell, A. M., Ford, G. G., Gallagher, S. H., & Lacy, B. A. (1999, March). Repositioning the home plate umpire to provide enhanced perceptual cues and
more accurate ball-strike judgements. Journal of Sport Behavior, 22, 28-44.
Haseltine, E. (1999, December). Mental Fatigue. Discover, 12, 128.
Introduction to Perception. [Online]. Retrieved October 11, 2000, at
Jackson, S. R. (2000, February). The Ponzo illusion affects grip-force but not grip-aperture scaling during prehension movements. Journal of Experimental Psychology, 26, 418-423
Kassin, S. (1998). Psychology (3rd ed.). Upper Saddle River, NJ: Prentice Hall.
Sensation and Perception-Perceptual Illusions. [Online]. Retrieved November 3, 2000, from http://www.epsychlopedia.net/concepts/sensationAndPerception/index.php ?id=sensandperc8.