Excerpts from
L.Kleine-Horst: "Empiristic theory of visual gestalt perception. Hierarchy and interactions of visual functions" (ETVG). Köln
2001, Part 1, I


The old problem

(addressing laypersons, and going on to scientists)

1. "Optical illusions"

"What is it all about?" This is a question I am often asked when I tell people that I am working on a theory of sight. It is a question that confronts me with a few problems. I can answer this question for a perceptual psychologist in one sentence, but how about for a layperson? A layman can hardly understand how something that seems obvious to him, is "apparent" and "clearly in front of his eyes", should be a problem. To him, I would have to say the following:

Clearly, right in front of your eyes, is a printed page - what do you perceive? A printed page, of course. Fig. 1-1 is a black cross. What do you perceive? A black cross, of course. There does not seem to be a problem, or is there? Now look at the cross a little bit more closely. You do not necessarily perceive a cross, but maybe two straight lines crossing each other. Now there is a problem, because you do not know whether there is a cross or two straight lines. You perceive two crossing lines especially if you focus on the intersection, one line even lies in front of the other - and all of a sudden everything is the other way round: the line that used to be in front is now behind the other line, and then they switch again. The lines keep on switching back and forth for as long as you look at the intersection. Now there is a significant problem because it is absolutely certain that the two lines do not move objectively. Therefore you perceive something that does not exist, what is called an "optical illusion".


Figure 1-1. One cross or two crossing lines?


Figure 1-2. Müller-Lyer illusion

So you now see that what looked obvious in the beginning is now anything but obvious, it is even completely incomprehensible. There are  many such  illusions.   Many  striking  examples  are  given  in scientific literature. One of the most famous is the Müller-Lyer illusion, which has found its way into the literature of popular science. Please look at Fig.1-2A. You will find nothing unusual about it. Fig. 1- 2B is - seen by itself - in no way a remarkable figure. B is different from A in that the two diagonal lines point inward, and in that the middle line is apparently shorter than in A. Other than that there is nothing unusual about it - until you compare the lengths of the two middle-lines.  It comes as a big surprise: they are the same length! You  would expect, however, that two lines that "are" the same length also "look" the same length. Again you have to realize that you were "tricked" by your senses. Another example: you see the moon "move" behind the clouds. As a child you perceived this movement as an actual movement of the moon. As an adult you know that the moon "is fixed" in the sky (or at least that it does not move so quickly). It has to be the clouds that move in front of the moon. Despite this knowledge you have the overpowering impression that it is the moon that moves. You tell yourself again: "it's an illusion". You now realize that one of the major problems that confront the perceptual psychologist is that the actual perception deviates from what the observer expects. But why does the observer expect a certain perception? In the case of the cross it is clear that the lines do not "really" switch back and forth, the middle-lines of the Müller-Lyer pattern are not "really" of a different length, and the moon does not "really" move. Of course, one may ask the question: how does one define "real"?.

Another example: you are in a gorgeous winter landscape: white snow, deep blue skies. You take a photograph of the scenery. But after developing the film you are disappointed: the snowy landscape has a "tinge of blue", and Barbara's bright yellow anorak looks a dirty gray color. How did it happen? Well, because the snow of the landscape put blue light into the lens of the camera; the snow reflected the blue from the sky. The reflected blue also reached your eye, but the human visual system abstracts the blue from all the other colors in the landscape, and "overlooks" it. This is why one does not notice the blue in the snow with the naked eyes (unless one happens to be either an experienced photographer or painter), and that the colors of the objects appear as they "are". The camera, on the other hand, cannot perform these abstractions. It mixes the colors, which is why the white snow is tinged with blue and the yellow anorak looks gray when mixed with blue.

You might say that this is an "illusion" just as in the preceding cases, but you are more likely  not to. You do not feel tricked by your visual system. You "know" that the anorak is "really" yellow, and that the snow is "really" white. For this reason you expect to  see these things in these colors. In this case the illusion makes biological sense, because it presents the things in our surroundings in the colors that we are used to, which enable us to identify them as these particular things.

We do not judge such an illusion to be an illusion; we term it a "veridical perception".  Just as we are unable to say what defines an illusion we are unable to say what defines a "veridical perception". How is the stimulus given in Fig.1-1 perceived "veridically"? As a "cross" or "two crossing lines"? You cannot decide. And even if you were able to give important reasons for one of the alternatives, the question of why we can claim that the "stimulus" consists of "lines" (since we have to experience "lines" in either case) remains. What really gets stimulated are single visual receptors. Therefore the experience of a "line" has to be based on the stimulation of many receptors, which are organized in a certain way on the retina. Does this certain order belong to the stimulus? In any case, the stimulus is the light energy and not the excitation of the receptor stimulated by the light.

What I want to demonstrate by posing these questions is that it makes no sense to talk about "veridical perception", as equally as it does not make sense to talk about "illusions". Illusions and veridical perception do not exist. The terms "illusion" and "veridical perception" are terms from a theory which seems to have been mistaken. They are terms from traditional psychology, which seems to have constructed a concept  for itself which fails to accurately depict the organization of visual perception. If, despite this, I use these expressions (however, mostly in quotation marks), it is because everyone knows roughly what is meant by them; they should not be taken as belonging to a "final" scientific explanation.

2. Sensory perception

The aforementioned examples hint at the scientific fields concerned with visual perception, especially with gestalt perception. Now you will be introduced to the deeper problem the science of perception faces, a problem that becomes particularly acute when dealing with "illusions". If you look at Fig.1-1, the stimulus pattern will be depicted on the back of your eye. The back of the eye (i.e. the concave inner wall of the eye) consists of millions of light sensitive cells lying close together which are called "photoreceptors" or "visual receptors". Each photoreceptor becomes activated when it is "stimulated", i.e. when it is impinged on by light energy. The stronger the energy, the intenser the "sensation" of brightness. The relationship between strength of stimulus and strength of sensation was discovered by science, and expressed in a mathematical formula. It was also discovered that the perceived color is dependent on the wavelength of the light. If, for example, the light stimulus has a wavelength of 480 nanometers, the observer has a perception of blue; a perception of yellow corresponds to a wavelength of 570 nanometers.

The visual system is constructed in such a way that each locus of the retina is assigned a certain "local value". This means that a light source we look at is depicted (=optically projected) on the middle of the retina, the "fovea". This also implies that light impinging on the fovea "appears" to us as coming from the direction in which we are looking, at that moment. A stimulus impinging on a small area of the retina 1° vertically above the fovea will be perceived as coming from a place exactly 1° vertically below the direction in which we are looking. ("Below" because objects are depicted "upside down" on the retina). Thus each locus of the retina corresponds to a certain direction in space,  relative to the direction in which we are looking, at any particular moment.

Taking all this into consideration, we can understand that when we look at the middle of the cross in Fig.1-1 we perceive in a certain direction relative to the line of sight, "here is something bright (white)" and "there is something dark (black)". We cannot develop, however, this explanation any further by means of the receptors' function. We cannot explain why we experience a "line", why this line appears "straight", or why we perceive "two" lines "crossing". We are even less capable of explaining why these lines constantly switch their position in the dimension of depth, in relation to each other; we cannot even explain why one line seems to lie "in front" of the other. All these perceptions are of a different kind to the perception of brightnesses and colors at certain locations. In order to be able to perceive a "line", the organism has to be capable of somehow grouping together the brightness perception of many, single visual receptors, a single receptor alone cannot achieve this.

When I speak of "brightness" in the context of my theory, I am also speaking of colors. I view the perception of colors as a further development of the perception of brightness. Not all brightnesses are colored, but all colors are bright. A hierarchical relationship seems to

be involved, in which the sensory function mediating the perception of color is one step higher than the sensory function mediating the perception of brightness.

3. Gestalt perception

The real achievement of the early "Gestalt psychologists" is to have pointed out time and time again that a percept, in contrast to what the "Elementarists" believed, should not be understood as resulting from the senses alone. In the view of the Gestaltists, there are further factors of perception of an unknown kind, which they called "gestalt-tendencies" or "Prägnanz-tendencies". If the Elementarists are to be believed, the stimulation of each photoreceptor would result in the appearance of a certain brightness, a certain color, at a certain location in the frontal plane, at a certain depth and at a certain time; the total percept would therefore be a mosaic of single sensations. In their experimental work, the Gestalt psychologists did not find such single sensations; instead, they found that perceptual experience is always holistic. In their view details are not individual, but parts of a "whole" which they called, for their field of research, a "Gestalt", thus the origin of the term "Gestalt perception". They referred to the Elementarists' hypothesis of constant stimulus/experience relation- ships as the "constancy hypothesis" (Metzger, 1954).  

I shall attempt to expand and make more precise the term "gestalt perception", a term defined somewhat narrowly and imprecisely by the early "Gestalt psychologists". By "gestalt perception" I mean perception insofar as it is not "purely" sensory perception, as long as it is not able to be explained only by the luminance, the wavelength of the light, the location of the stimulated photoreceptor within the retina, the binocular disparity, and the time of the stimulation. To see a "line" is such a gestalt perception. To perceive "two" objects is such a gestalt perception. To see blue when the light has a wavelength of 500 nanometers is such a gestalt perception, because at this wavelength one would have to perceive "green" if the perception were based on the sensory function of the receptors alone.

The theoretical problem of gestalt perception can be thus defined:

How may the deviations of our perception from the theoretically, i.e. according to the "constancy hypothesis", expected "pure" sensory perception be understood?

"The new way to solve the problem"

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