Visual illusion

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‘Sensation involves registering and forming a code for light, sound, etc. Through natural selection, our sensory systems have been designed to develop these for information about objects and events in our environment. The ability to interpret this information, to extract from it meaningful and useful representations of our world is called perception.1’

Perception involves a set of processes by which sensory information is selected, organized and interpreted in a way that can be understood.

The automization of perception involves two processes. Most perceptual illusions derive from the context in which the illusory elements appear and can be described in terms of the bottom-up and top down processes. The bottom up processes consist of integrating sensory information and forming a hypothesis of an object, and top down processes involve using previous knowledge in order to relate it to the new stimulus, hence interpret the information. This process occurs automatically in which a new stimulus requires us to form a hypothesis and then use our preexisting knowledge to interpret this information. There is no doubts to this happening automatically but people need to keep in mind that some objects can deceive us into believing that the object we see is either not there or we interpret it in a completely different manner to what it really is.

The automization of perception is involved in visual illusion where perception plays a major role in perceiving things in different forms.

Visual perception involves many complex physiological and psychological processes that intereact during the selection, organization and interpretation of visual stimuli. These processes usually occur automatically. Most of the time, our perception of the world closely matches the physical environment around us, however making mistakes in visual perception can occur.

There are many examples that occur throughout the day that result from visual illusions. A visual illusion is a misinterpretation of real sensory stimuli. It is an experience in which there is a mismatch between our perception and what we know as physical reality. Psychologists have identified over hundreds of visual illusions, the most famous being the muller-lyer illusion, the ponzo illusion, the ames room illusion and lastly the moon illusion, which I will be discussing shortly.

We need to keep in mind that illusory effects are unavoidable, that is even if know that we are looking at an illusion and understand we will still experience the same affects. Visual illusions demonstrate the important role our brain plays in constructing our view of the world. They also demonstrate the effect of context on the formation of our perception.

Before discussing the various illusions, I would like to touch on the pictorial cues that play a major role in perception. Many monocular (those which require the use of one eye) depth cues are referred to as pictorial cues. Linear perspective is the apparent convergence of parallel lines as they recede into the distance. Interposition is when one object partially obscures another, the partially obscured object will be perceived as further away than the object which obscures it.

Texture gradient, refers to the gradual diminishing of detail that occurs in the surfaces as they recede into the distance compared with objects in the visual field which are seen with fine detail. Relative size involves judging the size of an object in relation to other objects. It refers to the tendency to visually perceive the object that produces the largest image as being closer. Height in the visual field refers to the location of objects in our visual field whereby objects which are located closer to the horizon are perceived as being further away than objects located away from the horizon. In the following paragraphs we can see how these various pictorial cues demonstrate how visual illusions are able to deceive people’s interpretations.

The Muller-Lyer illusion is a illusion in which one of the two lines of equal length, each of which has opposite shaped ends is incorrectly perceived as being longer than the other. Psychologists have had various explanations for this illusion. One approach is explained according to biological factors. We might have the tendency to misperceive simple, geometric patters which appear in 2D form. Psychologists can also explain this illusion through a cognitive approach and this suggests that this misinterpretation can occur due to the person not using appropriate mental stratergies when making a perceptual interpretation. The reason to why we misinterpret the length of the two lines can explained through how we incorrectly apply the principle of size constancy. This leads us to make an interpretation that when the two lines appear to be at different distances, and one that appears further away must be longer.

The Ponzo illusion is known as the perceptual illusion of unequal length of two identical horizontal lines shown within two converging lines. This illusion is formed by the presence of the converging lines at the right and left which represent depth cues of linear perspective. The Ponzo illusion is explained through the apparent depth or distance theory. The converging lines resemble a scene such as railway tracks. Depth cues such as linaer perspective lead us to believe that the upper line is further away from the lower line. The principle of size constancy leads us to make an interpretation that when two lines appear to be at different distances from the observer, and cast retinal images of equal size, then the line which appears further away must be longer.

Both the Muller-Lyer and Ponzo illusions demonstrates that processes involved in maintaining size constancy can lead us to perceive size of tow objects as different from what should be perceived on the basis of our knowledge of the retinal image’s size.

Unlike the Muller -Lyer illusion and the Ponzo illusion which were artificially created, the Moon illusion is one that occurs naturally. The moon illusion occurs when the moon appears to larger on the horizon than when it ishigh up in the sky, even tough the retinal image is equal in both situations. We need to keep in mind that obviously the moon doesn’t change in its size as it moves across the sky. Here the pictorial cue height in visual field is used where objects which are located closer to the horizon are perceived as being further away from the objects located further away from the horizon. An object on the ground , hence being below the horizon gets higher in visual field and is therefore perceived as being smaller as it moves away towards the horizon. The line that appears to form between the ground and horizon result in the misinterpretation of this visual illusion.

The Ames room is another famous illusion. The illusion allows people to perceive that one person is bigger than the other. A rectangular room is cut on a angle, thus one side of the room is bigger then the other. There is a point of focus at the centre of the room. A person is placed in the corner on the smaller side of the room, and another person of the same height is placed in the corner of the bigger wall. The object beside the people will depend on where they are standing. The person standing at the small wall will have objects that are smaller object then themselves, next to them and vice versa. Once this is all put together using size constancy, the illusion is complete. Therefore, the person at the smaller corner is perceived as larger then the person at the long wall.

The room is distorted. But viewed with one eye through a peephole, its trapezoidal walls produce the same images as those of a normal rectangular room viewed with both eyes. Presented with the camera’s one-eyed view, the brain makes the reasonable guess that the room is normal and perceives the people as changing size.

As we can see, pictorial cues play a major role in the automization of perception. When these cues are applied to objects they seem to allow people to believe what they are seeing must obviously be accurate. By applying these pictorial cues people do not focus on performing a hypothesis, and following a procedure in order to reach a conclusion, however with the aid of pictorial cues believe that there interpretations seem to be rational.

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