How can visual illusions illustrate top down processes in perception? Contrast this with a visual illusion that can be explained through bottom up processes.

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Caroline Hewitt

How can visual illusions illustrate top down processes in perception? Contrast this with a visual illusion that can be explained through bottom up processes.

        There are many suggestions to explain how visual illusions can be perceived.  These suggestions include physical illusions, bottom up illusions and top down illusions.   An example of a physical illusion is how a straight stick when placed in water appears bent.  Here the illusion has occurred before the light has entered the eye and so is a physical illusion.  Bottom up and top down illusions however involve the processing after the light has entered the eye.  Bottom up processes are processes which take information into the eye and then make judgements about the nature of the visual world based solely on this information.  Hering who suggested that it was the innate ability of the visual system that led to how things were perceived illustrates this. Top down processing however involves using prior knowledge and experience about the structure of the world to influence how something is perceived.  Helmholtz who felt that the perception of a stimulus was based on visual experience illustrated this.  The following illusions show examples of how both processes can be used to explain perceptions.

        An example of a visual illusion that can be explained by top down processing is the Muller Lyer illusion (figure 1).  In this illusion the lines in both A and B are the same length however the arrows pointing inwards in A make the line appear longer than when the arrows point outwards as in B.  Gregory explained this illusion in 1970 by suggesting that the lines are perceived as being three dimensional rather than two-dimensional.  This is shown in figure 2 where A is shown as the inside of the room and B shown as the outside.  By perceiving the objects in this way A becomes further away than B.  However given that the lines are of the same size by applying the principle of size constancy it can be concluded that A is perceived as being longer than B.  For this to be perceived knowledge about what the outside and inside of a building looks like must be taken into account.  This therefore shows how prior knowledge is used when the image is perceived and so shows how the Muller Lyer illusion is perceived through top down processing.

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        Similarly the ‘Necker Cube’ illusion (figure 3) can also be explained through top down processing.  In this illusion the cube flips between two different interpretations of the picture whereby one of the faces of the cube can appear to be at the front of the cube but also can appear to be at the back of the cube.  Work carried out by Wheatstone illustrates how this illusion can be explained through top down processing.  He conducted experiments with geometrical solids and three-dimensional models and made the following observations.  He found that by following the lines of the cube with the ...

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