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

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 and previous experiences are used, deceiving people into seeing something that is incorrect. There are however several criticisms of using constructivism to prove top down theory in illusions. One way in which is that one of the main principle of constructivism is that perception reached by hypothesis is prone to error, however in everyday life it can be seen that this is not the case. In addition, many of the experiments that prove the constructivist theory are artificial and inconsistent with normal life.  

Visual illusions can also be explained from a bottom up perspective. This primary form of cognition is low level and uses serial processing as ‘one process is completed before the next begins’ (Eysenck, 2004). This form of perceptual information processing is frequently ‘data driven’ and is predominantly involuntary, not being influenced by context or hypothesis. Visual illusions can be used to explain this type of perceptual processing in a number of ways, one of which is in illusions which involve fiction. One example of which is the illusion of the Mach Bands (see figure 2). This illusion was created by Italian psychologist Ernst Mach and can be seen practical terms in medicine when examining x-rays. Mach Bands is a fictional illusion as the eye tends to interpret either a bright or dark vertical band near the area where there is a strong gradient change in the color, when in fact none exists. This is supposedly due to ‘lateral inhibition of the receptors in the eye’ however it can also be attributed to the ‘statistical strategy of visual perception’ (Lotto, Williams, Purves, 1999). This illusion can be explained through bottom up perception processes and constasts sharply with top down theory as it does not use previous knowledge or context to delude the viewer, it is purely an automatic, low-level mechanism without hypothesis or expectation.

An argument which supports this approach is the theory of Direct perception, maintained by the American psychologist James Gibson. He claimed there is ‘information available in sensory stimulation’ (Eysenck, 2004) and his main principles include ‘structured light contains visual information from the environment’, and that this provides ‘information about the layout of objects’ also that perception involves picking up this information ‘with little or no information processing being involved’ (Eysenck, 2004). These principles were proven by Gibson in 1950 using his pilot experiment in which he observed the pilots processing information from the environment with little or no expectation or hypothesis involved in relation to factors such as direction speed and altitude. When this approach is applied to visual illusions, it supports the bottom up theory for the explanation of them in that it shows visual information being absorbed without considering context. However there are several criticisms of Gibson’s direct theory of perception, one of which being that processes are more complex than stated in his experiments, and another that his principles apply much more to some aspects of perception than to others. He also was incorrect in disregarding the use of ‘internal representations to understand perception’ (Eysenck, 2004) for example memory which was later proved erroneous by the work of Menzel.

Although as shown, visual illusions can be explained using a bottom up or top down approach, many visual illusions cannot be clearly or fully categorized into either group. Often, visual illusions can be seen to have elements of both as perceptual processes and an example of this is the Müller-Lyer Lines illusion (see figure 3).

Eysenck, M.W. (2004). Principles of Cognitive Psychology. Hove: Psychology Press

http://plato.stanford.edu/entries/relativism/supplement1.html

Einhäuser, Wolfgang, Martin, Kevan A. C. & König, Peter (2004) Are switches in perception of the Necker cube related to eye position?. European Journal of Neuroscience 20 (10), 2811-2818.

Lotto RB, Williams SM, Purves D (1999). "Mach bands as empirically derived associations". Proceedings of the National Academy of Sciences 96 (9): 5245-50

Gibson, J. J. (1979). The Ecological Approach to Visual Perception. Westport: Greenwood Press