Visual Illusions

 

It is generally believed that visual illusions are simply curiosities but by studying illusions it is possible to come to an understanding of how normal vision works - and how it can be fooled by unexpected cues, such as those provided in many ads containing semi-subliminal material. And how knowledge of such rules can be put to use in constructing semi-subliminal and manipulative adverts.

Normally we do not expect illusions in dealing with the everyday world. We have learned that the world is 3 dimensional. We thus interpret all visual signals as if they came from a 3 dimensional object. It is this 'over learning' which allows us to look at pictures, photographs and paintings and 'see' them as if they were 3 dimensional objects, even though we know they are clearly only 2 dimensional representations.

In certain circumstances, the fact that we are presented with incomplete information, as is always the case with two dimensional pictures, leads to inappropriate conclusions. For example, in the figure on the right we can either perceive faces or a vase. Viewing the image normally means we can never 'see' both at the same time.

In the Kanizsa triangle illusion, presented on the Imagination page, it seems as though there is a brighter triangle superimposed on another triangle in the background. Despite appearances, there is, in fact, no difference in brightness levels between the central area of this image and the surrounding area. Similarly with the length of the lines in the illusion shown below. They seem to be different lengths but, as measurement will demonstrate, they are in fact identical in length. You can prove this to yourself by drawing the same shapes on any piece of paper.

This illusion occurs because we know that element in the real world that are nearer to us are larger than those that are further away. Thus, on the assumption that one line is closer than the other, the visual system 'decrees' that it must be smaller than what lies further away. We thus perceive what we would expect to occur naturally in the real world rather than the inappropriate 2 dimensional representation that does not reflect real world reality. Rules regarding one object obscuring another that is further away also play a part in determining how we view two dimensional images - and judge illusions such as the Kanizsa triangle.

Two dimensional representations, whether presented on this site or in advertising, can only be interpreted on the basis of cues concerning colouring and shading. The rest of our understanding draws upon our knowledge of the real world, lit from above, with objects in front obscuring those behind, those closer appearing larger, etc.

Here we have an image of Labrador dog. Once its position is identified (the rollover indicates the position of its head) subsequent viewing leads to easy 'recognition'. On initial viewing, however, it may take some time to identify the dog because all the visual system 'picks up' is a montage of black and white. Our understanding of the real world leads to the organization of the visual input into meaningful images: a dog, a pathway, the base of a tree, etc. This also applies even if the image is further degraded as in the illustration on the right. Most individuals identify this as a man riding a horse.

In these illustrations there was no intention to mislead viewers. However, if this type of imagery were included in advertising the intentions of the advertisers ought to be considered suspect. Advertising agencies know that 'over learning' the rules that apply to reality leads viewers to accept as normal that which is clearly abnormal when considered retrospectively.

The following illusions indicate some additional processes which occur normally but which on occasion can lead us to make false judgements. Some of the illusions also illustrate how we can make sense of incomplete and ambiguous images, the sort that appear regularly in manipulative and semi-subliminal advertising. As you will discover, application of standard visual rules can lead one to identify with some degree of precision what many would consider simply variations in texture without any meaning.

First, close your right eye and stare at the red cross. Then slowly move closer to the screen. The red dot will eventually disappear from your view when it is focused on the 'blind spot' on your retina. This is the area where the optic nerve passes through the retina. As a result there are no photoreceptors to respond to sensory input and whatever is in front of that spot cannot be seen. However, we do not have a blind spot in our visual field because the brain improvises 'information' from the surrounding area and 'fills in' the 'blind spot'. We see a complete image. In this case the lines surrounding the red spot fill the space. In real life information from the scene we are viewing 'fills in the gap' in our vision.

Next, stare at the cross in the middle of the coloured patches for a while and then shift your gaze to the cross in the grey area you will see patches of colour appear there. This is generally because the neurones that are sensitive to one colour become fatigued after use and become 'overridden' by their complementary colours. You thus see what is not there: clockwise from the upper left (red) you perceive green, red, blue and yellow instead of the original red, green, yellow and blue.

Look now at the two dark lines in the illustration on the right. The one on the right seems distinctly longer than that on the left. Our knowledge of perspective 'tricks' the brain into 'seeing' the line on the left as short, because it is 'closer'. Even when we know we are looking at a two dimensional image and not a three dimensional scene our visual systems are still bamboozled.

The small triangles in the centres of the two larger triangles are identical in colour. However, because our perceptual system takes all of a scene or context into account when determining colour, we see one as being slightly darker than the other because of the surrounding colours. Consider this when you look at the Kanizsa triangle on the Imagination page.

 It was noted above that we learn that reality has certain characteristics and these are taken into account when we judge what we are seeing. Here is an excellent example. The two illustrations alongside are identical, except that that one has been rotated 90 degrees relative to the first. This simple rotation leads us to judge them differently. The differential judgements arise because we take into account the angle at which light is presumed to 'hit' an object i.e. from the sun up above. So we do not just see a 2 dimensional object, we see it as if it were in the real 3 dimensional world.

The first illustration seems nicely balanced but when turned 90 degrees one can perceive an X of concave and convex circles respectively.

The illustration on the left gives the impression that the long lines are converging or diverging. This is an illusion created by our judgement of the cross hatched lines. The long lines are in fact parallel to each other and it is the action of brain cells that gauge orientation as they respond to a mixture of alignment signals that that mislead us.

Where the white lines cross you should be able to see small, dark, spots. These do not actually exist. Our perception of them is actually created by brain cells that respond selectively to black and white. To produce a crisp, clear, image the retinal cells that surround the area responding to input are either inhibited or excited. Where there is a pattern such as in this illusion, instead of clarifying our perception of the world this process 'produces' the impression of spots where none in fact exist.

When one is looking at ambiguous aspects of semi-subliminal advertising the same processes are at work trying to detect 'edges' around incomplete figures. Instead of perceiving incomplete figures we 'see' meaningful figures. For example, the image of the Devil on the left has no outline or edge, yet nevertheless we see it as a face, rather than as ambiguous grey blotches. You may also note that certain areas of the image seem brighter than others, despite the background being a uniform colour. In ads such imagery would be embedded in ads and what we perceived would be influenced by the surrounding context and our previous knowledge.

If successfully embedded then we would not consciously notice embedded figures such as those believed to be embedded in the following extracts from Marlboro adverts. See below for additional commentary related to these extracts, in particular the image on the left.