What do visual illusions tell us about information processing?

Visual illusions help us to understand how the visual pathways organise images into objects. In the following analysis I have given examples of three different types of illusions and detailed how they arise. I have chosen to focus on what visual illusions tell us about perception, as this is the most relevant section, although information processing also includes the areas of memory and attention. I will outline how visual illusions provide supporting evidence as to the way cognitive processes allow us to, in general, make accurate perceptions of objects from visual stimuli.

A visual illusion arises when the visual stimuli cannot be perceived in a way that accords with what we can measure. Illusions are cases where we find significant differences between perceived and measured reality. The term perception “refers to the means by which information acquired via the sense organs is transformed into experiences of objects, events, sounds, tastes etc” (Roth, 1986, p81). Vision is arguably the most important sense for humans and in order to understand more about visual perception and hence what visual illusions can tell us about information processing, it is first worth noting the way in which the visual system is constructed. Below is a very brief and over-simplified outline of the processes involved.

The cornea bends incoming light rays from the visual stimulus so they fall directly onto the retina at the back of the eyeball. The retina is a layer of light receptors (photoreceptors) and nerve cells at the rear of the eye and it is here that the transduction of light into neural energy takes place. The retina contains two types of photoreceptors: rods and cones, and a variety of nerve cells. Cones provide our perception in daylight conditions and allow us to see colour. Rods enable us to see under dim conditions (although they only allow black and white perception). The photoreceptor cells convert light into neural information, which is then transmitted via the bi-polar cells to the ganglion cells and out of the eye into the brain via the optic nerve. The optic tracts continue on each side of the brain and travel through the lateral geniculate nucleus (LGN) to finally converge on the area of the brain called the visual cortex. Colour, motion and form are processed in anatomically separate parts of the visual cortex. A visual perception of an object is made when processing from each of these areas is integrated. (Source:Rookes, P. & Wilson, J., Perception).

Veridicle perception is perception which matches the actual physical situation. This is most likely to occur in well lit, natural environments in which lots of information is available. However, if the conditions are such that we receive inaccurate information, a breakdown of this process can occur and we experience a visual illusion.

There are three types of illusions analysed in greater detail below:

Distortion - systematic differences between measured and perceived size, shape etc.
Ambiguity - when there is more than one way of perceiving the stimulus.
Fiction - seeing things that are not actually there.

Distortions – The Muller-Lyer Illusion

This is an illusion in which the line with the outward fins (a) is perceived as being longer than the line with inward fins (b), despite the fact they are exactly the same length.

There are several theories as to the cause of this illusion. It can be explained in terms of misapplied size-constancy theory. According to Gregory (1970), the figures above can be thought of as simple perspective drawings of three-dimensional objects.

Size constancy is defined as “objects that are perceived to have a given size regardless ...

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Distortions – The Muller-Lyer Illusion

This is an illusion in which the line with the outward fins (a) is perceived as being longer than the line with inward fins (b), despite the fact they are exactly the same length.

Size constancy is defined as “objects that are perceived to have a given size regardless of the size of the retinal image”, (Eysenck, M., Cognitive Psychology 4the, p536). Effectively, because we know that objects far away are not really smaller, we perceive them to be bigger than their retinal size. In the case of the Muller-Lyer illusion, if the viewer makes length judgements on the basis of size constancy, line (a) which appears to be further away will be judged as longer. Gregory suggests that this mechanism which helps us to maintain stable size perception in the real, three-dimensional world could lead us to misinterpret two dimensional pictures. We assume that the further away object must be bigger than its retinal image hence perceived as being bigger, i.e. misapplied size constancy.

If the Muller-Lyer illusion is cognitive, in that it is learned because of experience with corners and perspective, then people with less familiarity with corners should show less distortion. Empirical evidence to support this theory has been produced by Pedersen and Wheeler (1983). They investigated the illusion with two groups of American Navajo Indians. One group had lived all their lives in typically Western architecture (i.e. rectangular buildings with outside edges and inside corners). The other group had been brought up in typical Navajo houses and were less familiar with the environment of Western housing. It was found that the first group were significantly more susceptible to the illusion.

However, DeLucia and Hochberg (1991) have shown that the Muller-Lyer illusion still occurs where there are no depth cues. The pictures below show stimuli similar to those used in their experiments. The distance A to B is the same as between B and C, and yet the distance appears larger, just as it does in the two dimensional version. It is obvious that the spaces between the two sets of fins are not at different depths therefore misapplied size constancy cannot be a factor. However, the illusion still exists.

Coren (1981) suggested that the illusion could be explained in terms of eye movement patterns. He showed that the eyes track a longer path in line (a) with the fins pointing outwards than in line (b), with the fins pointing inwards, thus line (a) is judged to be longer. This is associated with incorrect comparison theory, according to which our perception of visual illusions is influenced by parts of the figure not being judged. Evidence for this theory is shown by the fact the size of the illusion was greatly reduced when the fins were in a different colour from the vertical lines. Presumably this makes it easier to ignore the fins.

In reality, it is likely that the Muller-Lyer illusion arises as a combination of both misapplied size constancy and incorrect comparison theory.

Ambiguity – Necker Cube.

Constructivist theory is founded on the belief that perception is based on a process of inference. Helmholtz argued that, on the basis of the sensations we receive, we draw conclusions about the nature of the object or event that the sensations are most likely to represent. Modern constructivists suggest that the information supplied to the sensory organs from the environment is frequently ambiguous and has no clear-cut analysis. This means the observer has to solve the problem (or construct a best guess) as to the identity of the stimulus. In other words, the observer has to use indirect, top-down processing to perceive an accurate interpretation.

Top-down processing is used to describe the higher, more cognitive influences on perception. It is based on the idea that sensory information from the retina is insufficient to explain how we interpret visual information. In addition, stored knowledge about the world is required in order to make sense of the visual input. Gregory wrote in his book Eye and Brain (1990, p219): “The sense organs receive patterns of energy, but we seldom see merely patterns: we see objects. A pattern is a relatively meaningless arrangement of marks, but objects have a host of characteristics beyond their sensory features.” Although the top-down processing involved in perception is believed to be largely unconscious and instantaneous, perception is seen to be indirect because information has to be processed at a level beyond the sensory level in order to be recognised accurately.

Three assumptions of modern constructivist theory were put forward by Eysenck & Keane (1995):

Perception is an active and constructive process involving more than the direct registration of sensations.
Perception occurs indirectly as the end-product of the interaction between the stimulus input and the internal hypotheses, expectations and knowledge of the observer. Motivational and emotional factors can also play a part in this perceptual processing.
Perception is influenced by individual factors and this means that errors will sometimes be made, leading to inaccurate perceptions.

Ambiguous figures such as the Necker Cube support the idea that top-down processes can influence perception. If the cube below is concentrated on hard, it is found that the cube suddenly seems to jump and presents itself in a new orientation. A change in depth perception here alters the perceived shape between a cube and a truncated pyramid. Once it has happened, the cube continually jumps back and forth between the two orientations. Gregory explained this by saying that the drawing is ambiguous. At first sight, most people test the hypothesis that the drawing represents a cube resting on a flat surface, such as a table. However, there is no surrounding context in the picture (i.e. there is no table drawn), and so it is suddenly possible to see an alternative interpretation of the cube, such as mounted on a wall, coming out towards the viewer. In the absence of further information such as a table or a wall, the pictures offer no clue as to which of the interpretations is most plausible. Hence the viewer switches between the two.

In everyday life however, the viewing conditions are such that there will normally be enough contextual information to remove any ambiguity and lead to the correct identification of the stimuli.

The Necker Cube

Fiction – The Mach Band Effect

The Mach Band effect is a physiological illusion as a result of bottom-up processing. “This approach emphasises how the sensory receptors register the stimuli, with information flowing from this level upwards to the higher, more cognitive levels” (Matlin, M. & Foley, H., Sensation & Perception 3rd Edition, p460). The process begins with an analysis of sensory inputs and is based on the properties of the stimulus such as the distribution of light and dark areas, or the arrangement of lines and edges in the visual scene. The information that is acquired from these sensory inputs is then transformed and combined until we have formed a perception. This information is then transmitted upwards from the bottom level (i.e. the sensory input) to the higher, cognitive levels.

The Mach bands illusion is such that observers perceive bright and dark bands within a single stripe. The interior edges of the diagram all seem to be distinct and appear to have a faint lighter border to the right and a faint darker border to the left of each edge. In reality, each stripe has uniform intensity.

Mach Bands

If the Mach bands do not exist in the picture, where do they come from? This can be answered by covering all but one of the stripes, which eliminates the bands. From this it can be derived that without the influence of the adjacent stripes, the single stripe is accurately perceived as having a single intensity. Based on this observation, Mach (1914) proposed that the illumination of one area of the retina causes receptors in that area to affect the response of the receptors in a nearby area of the retina. This is known as lateral inhibition, defined as “inhibition of neural activity for points near the part of the retina that is stimulated by light”, (Sensation & Perception, 3rde, p466). It is a mechanism that allows a single ganglion cell to be selectively sensitive to contrast within its receptive field, rather than simply responding to the total amount of light directed onto the field.

It can be construed that Mach bands provide evidence of lateral inhibition because the physical contrast that already exists is exaggerated further. This demonstrates how perception does not always match the physical stimulus and is not, therefore, determined by our direct contact with objects but by indirect contact. It is based on the properties of the receptors that transduce stimuli into electricity and the way this is processed in the nervous system. This indirect property of perception sometimes causes a mismatch between the physical characteristics of the stimuli and our perception of them; the light and dark bands are not present in the physical stimulus, they exist in our perception of the stimulus because of the way visual systems responds to the stimulus.

Conclusion

The analysis above examines the various ways in which we organise our perceptions. Each type of illusion described provides further evidence for the cognitive processes that psychologists believe enable us to interpret the information we receive from our environment, and transform it into accurate perceptions of objects and events. A breakdown of veridicle perception results from a visual illusion and arises when the visual stimuli cannot be perceived in a way that accords with what we can measure.

One of the main theories for the Muller-Lyer illusion is that it occurs as a result of misapplied size constancy. We assume the line with outward fins, which appears to be further away, as longer. This supports evidence as to why we are able to perceive the size of an object, regardless of the fact objects further in the distance have a smaller retinal image. Through the higher cognitive process of size constancy, we are able to maintain stable size perception in the three-dimensional world.

The Necker Cube can be seen in two different orientations. The observer continually fluctuates between each orientation because there is a lack contextual information to support which angle is the correct one. This supports the constructivist theory that perception involves more than the direct registration of sensations. Instead, the observer is required to draw on stored knowledge about the world in order to make accurate identification of stimuli. This is known as top-down processing.

The analysis is by no means extensive. Gregorys’ theory of top-down processing with respect to the Necker cube is strongly contested by psychologists such as Gibson who feel perception is direct and all the information is contained in the visual display (i.e. bottom down processing). Analysis of the Mach band effect gives evidence that bottom-down processing does occur. With Mach bands, observers perceive bright and dark bands within a single stripe whereas in reality, each stripe has uniform intensity. Our visual system is equipped to highlight edges in a process called lateral inhibition. Mach bands provide evidence of lateral inhibition because the physical contrast that already exists is exaggerated further.