Vertical-horizontal illusion Top hat illusion The arc of St. Louis
illusion is also called the vertical-horizontal illusion, which is the
simplest of all the perceptual illusions, and was the first visual
be discovered; and it was discovered by nobody else but the father of
experimental psychology, Wilhelm Wundt. The top hat illusion and the
famous arc in St. Louis, USA, are considered as variations of the vertical-horizontal illusion since the height of the hat or the arc has the exactly same length as their base, as shown above. Wilhelm Wundt described the vertical-horizontal
illusion as the situation where a vertical line looks longer than the
horizontal line of equal length, as shown in the figure above. He attributed
this distortion to asymmetry of the eye movement system. Although this asymmetry
assumption has been invoked many times since to explain distortion illusions,
it must be ruled out because the distortions occur in after-images or in normal
retinal images optically stabilized so as to remain stationary on the eye. In
addition, distortions can occur in several directions at the same time, which
could hardly be due to eye movements. For instance, when the standard
vertical-horizontal illusion is turned sideways (90°) and on its
side as shown in the left figure below, the former vertical line (now the
horizontal line) still looks longer.
The most acceptable and often-cited explanation for the vertical-horizontal illusion so far is that we perceive objects higher in our field of vision (ground) as farther away and perceive the lower part of a figure-ground illustration (figure) as closer (Vecera & others, 2002; Myers, 2003). Next the Ponzo illusion is used to explain that the closer object is perceived as shorter than the farther away object of the same size because our brain may reason that the farther away object must be longer than the closer object if they have the same retinal images. This explanation is proven unsatisfactory when the so-called figure and ground are turned around as shown in the right figure above. The perception is more or less the same no matter how you rotate the vertical-horizontal figure. On the other hand, using one illusion which is not yet fully understood to explain another illusion is itself problematic. Actually, the vertical-horizontal illusion is most effective when the vertical line is sitting in the middle of the horizontal line as shown in the standard figure. When the vertical line and horizontal line are connected at a right angle by the ends of the lines (see the two figures below), the vertical-horizontal illusion effect of these two figures is diminished extensively, and many observers cannot even tell which line is longer. The different illusory effects between standard and all the other figures tell us that the vertical-horizontal illusion is probably not caused primarily by the different perceptions of the two lines, but something else is playing a role.
The above triangle is an isosceles triangle, which has two equal length sides (two c lines) and two equal angles (both at the base). Thus, the c sides are longer than the b side of the triangle. This fact might be the contributing factor for the illusion. If this is the case, all we need to do is to draw an equilateral triangle, which has equal length on all three sides, and draw a vertical line in the middle as the a line in the above figure. If the illusion is caused by the isosceles triangle itself, the vertical line and the horizontal line should look the same length after the two side lines are taken off from the equilateral triangle. However, the figures below show that the vertical line in the equilateral triangle still looks longer than its horizontal line. This means that the illusion is not caused simply by the shape of the triangle. There is definitely something more to it.
result, we know that c line is 11.8% longer than b line after the
We can look at the triangle on the left as a mountain. Mountains are made up of earth and rock materials. The outermost layer of the Earth or the Earth's crust is composed of six plates. When two plates move or collide each other, vast land areas are uplifted, resulting in the formation of mountains. There are five major types of mountains; but fold mountains are the most common type of mountains. Examples of fold mountains are the Himalayas (Asia), the Alps (Europe). They are formed due to collision of two plates, causing folding of the Earth's crust. Now, we can imagine that the two side lines are the plates, which meet with each other at the peak of the triangle, or the peak of the mountain if you like to think this way, and collide each other there. Each side has a force 11.8% more than the vertical line. The collision of these two forces results in the peak being uplifted 23.6% higher. Thus, when we look at the vertical-horizontal illusion, we are looking at a longer vertical line which has been pushed up by two side lines invisible on our retina. Whether this perceptual process has happened in our brain is unknown. However, we know that it is possible due to its predictive power. I have predicted that the perceived longer portion of the vertical line is due to addition of the extra lengths from two side lines ( c lines in the triangle), and that the vertical line and horizontal line will look the same if the addition of the extra lengths from two side lines is removed from the vertical line. The result, as you can see, is consistent with this prediction. Therefore, it is not over my head to claim that my explanation is superior to the other existing explanations simply because of its predictive power, if not its convincing sensibility. The folding of the Earth's crust is only an imaginary force to explain the height illusions. However, I believe that there exists a force, which is beyond our subjective understanding and manipulation, that contributes to the longer looking of the vertical line in the height illusions. There are many forces in the universe that we cannot see with our naked eyes, such as gravitational force, electrical force. We know their existence and function based on their effects. Thus, I postulate that the height illusions are caused by an invisible force rather than perceptual errors or perceptual manipulation since our understanding of the perceptual error cannot correct the illusions at all. The force or power that makes the vertical line look longer is objective and beyond our perceptual control, just like the gravitational for or other natural forces which are not subject to our volition.
I don't blame you if you want to leave the website at this point because you think what I am suggesting is far-fetched and senseless. However, I urge you to have some patience and stay a little bit longer. My explanation of the vertical-horizontal illusion may be unsatisfactory, but it is a stepping stone for a more complicated and more important illusion, the Müller-Lyer illusion. The similar explanation for the Müller-Lyer illusion has much more predictive power, which can actually predict all those line illusions caused by crossing lines or curves. The approach I have taken to explain the size illusions is to assume that the force or whatever the influence exists independently of our brain; hence it is out of the control of our brain, which is pretty much like the gravitational force. When we fall out of an airplane in the sky, we will fall to the center of the Earth, following the law of gravitation. There is nothing we can do about it; it is beyond our control. Similarly, I believe that the visual illusions are not something that is created or constructed by our brain based on our experience and erroneous interpretations as believed by the conventional wisdom. I believe that the illusions are caused by a force, just like the gravitational force or the electromagnetic force in the nature, which is real and tangible, and independent of our brain and its processing mechanism (if there is any). Although the vertical-horizontal illusion is the simplest of them all, sometimes the simpler things are harder to understand and explain. I find that the more complicated an illusion is, the more satisfactory my explanation turns out to be. For instance, I am most pleased with my explanation of the moon illusion; and the moon illusion is the most complicated illusion among all the size illusions. On the other hand, the explanation for the vertical-horizontal illusion can be backed up by the predictive power of the similar explanations for all the other illusions.
Some things happen not because of sure, unbreakable laws, but for reasons that we cannot explain. So we just have to go along with what we observe. When we see something happen again and again and again, it is probable that it will continue to happen. It is important to know the true causes of why things happen. However, most of time we simply cannot determine what the causes are, and what we can actually strive for is to find out the regularity of happening. It is up to you to decide whether I have found the true causes of the size illusions because I am not sure myself. Nevertheless, nobody should deny that I have discovered the regularities of these major size illusions.
Coren, S. & Girgus, J.S. (1978). Seeing is Deceiving: The Psychology of Visual Illusions. New Jersey: Lawrence Erlbaum Associates, Publishers.
Gregory R. L. (1997). Eye and brain: The psychology of seeing (5th ed.). Princeton University Press.
Michaels, C. F., & Carello, C. (1981). Direct perception. Englewood Cliffs, New Jersey: Prentice-Hall, Inc.
Turvey, M. T. (1996). Dynamic touch. American Psychologist, 51, 1134-1152.
Vecera, S. P., Vogel, E. K. & Woodman, G. F. (2002). Lower region: A new cue for figure-ground assignment. Journal of Experimental Psychology: General, 131(2), 194-205.
Related Information on the Web:http://www.opticalillusion.net/optical-illusions/the-verticalhorizontal-illusion/