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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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 different types of visual illusions, many of which can be shown to illustrate different mental processes in perception. Types of illusion including those involving ambiguity, distortion and fiction can be seen to be processed using varied mental methods and can be categorised into physical, physiological, and cognitive illusions. Physiological, or bottom up processing is ‘directly affected by the stimulus input’ (Eysenck, 2004) as supported by Gibson, where as cognitive or top down theory is a constructivist approach upheld by those such as Bruner and Neisser, stating that processes are ‘influenced by the individuals expectation and knowledge rather than simply the stimulus itself’ (Eysenck, 2004). Both methods of processing however can be illustrated through the explanation of certain visual illusions.

Top down processing is a high level and secondary form of perception and is affected by our expectations and beliefs as well as being hypothesis or expectation driven. Visual illusions can be used to illustrate this type of perceptual processing in a number of ways, one of which is in illusions which involve ambiguity. One example in which this can be seen is the Necker cube (see figure 1). This optical illusion was first published in 1982 in Switzerland by Louis Albert Necker. It has an ambiguous nature as it can be interpreted in more than one way. It the intersection of the two lines, it is unclear which is in the front therefore meaning that it can be understood in two different formats consequently using multi-stable perception. Upon seeing two different images when looking at clearly only one visual stimulus it can be concluded that the difference in perception has to be attributed to another source. This can be seen to be a result of top down processing as it is the context that the illusion is in and the previous experiences in the same area that mean that the viewer sees more than is really there. The illusion can also be proved top down by the fact that they are somewhat under conscious control in that the perception can be altered by choice.  

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Supporting this approach is Constructivist theory, in which advocates Bruner, Neisser and Gregory all insist the emphasis of the role of top down processes as the one of prominent importance in perception. This theory of indirect perception follows the principle that perception is the ‘end-product’ of ‘hypotheses expectations and knowledge’ (Eysenck, 2004). These ideas have been proven for example Palmer’s kitchen scene experiment in 1975 where items were identified correctly more often if put into their context. When this approach is applied to visual illusions, it supports the top down theory for the explanation of them in that context ...

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