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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Cognitive Psychology                Student Number:  4054513

How can Visual Illusions Illustrate Top-down Processes in Perception?

Contrast this with a Visual Illusion that can be explained through Bottom-up Processes.

        Illusions can be described as a ‘distortion of sensory perception’ – the human visual system is deceived into perceiving something which is either not present or misled into incorrectly perceiving what is present.  The brain processes information gathered by the eye, in which, on the face of it, a percept that does not measure up with the physical measurement of the stimulus source is given.  Illusions can be used to highlight the processes behind our normal perception.  The cognitive system is arranged to allow for two types of processing in perception – bottom-up processing and top-down processing.  Information entering the cognitive system can either run from the bottom to the top (bottom-up processing) or in the opposite direction (top-down processing).  Bottom-up processing is a more basic perceptual system, compared to top-down processing and refers to aspects of recognition which rely mainly on the brain’s reception of stimulus information from sensory receptors.  Whereas top-down processing implies that information entering the cognitive system can be influenced by our existing knowledge, expectations and thoughts and is therefore guided by higher-level cognitive processes.  Information flowing upwards enables the more basic cognitive systems to absorb the information, arrange it and then pass it onto the more complex system to allow for further interpretation.

        The top-down (or concept driven) theory signifies the importance of conceptually driven processes.  In terms of perception, the constructivist approach assumes that our reality is formed through putting together raw fragments of sensory information and that our expectations of reality influence our perceptions; several other proceedings intervene between stimulation and experience, such as existing knowledge and expectation of the stimulus – perception is therefore the finished product of the interactive influences.  Illusions affect our visual system into perceiving something different to reality, meaning that we are unable to perceive based solely from the information we are given.  Therefore, many visual illusions support the constructivist approach – retinal information is insufficient for fully specifying how objects in the world are perceived.  Constructivist theorist, Gregory (1972) proposed that perceptions get constructed from “floating fragmentary scraps of data signalled by the senses and drawn from the brain memory bank.”  Furthermore, he argued that many of the classical visual illusions are prone to be error arising out of incorrect expectations and hypotheses, for example, viewers may have inappropriately utilized the perceptual concept of three-dimensional objects, in which they have obtained through their real-life experiences, to two-dimensional figures.

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        An example of a visual illusion which involves top-down processing is the Ponzo Illusion (fig.1), where we interpret the top of the drawing to be moving back into the distance, such as train tracks.  In this illusion, the top horizontal line is perceived as being further away, thus it appearing longer, than the bottom horizontal line.  The concept of ‘size- constancy’ can be used to explain this, which refers to the fact that ‘an object is being perceived as having the same size whether it is looked at from a short distance or a long distance’.  When an object ...

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