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 becomes further away, this causes the image on our retina to reduce in size, leading us to be able to realise that the actual size of the object has not altered. Therefore, in the Ponzo Illusion, bar A is perceived to be longer than bar B even though they are the same size – viewing it as farther away leads us to assume that it is longer. This is known as the ‘misapplied size-constancy theory’, where the misapplication of our existing knowledge of the world leads to a fault in perception. In addition, it supports the top-down theory in showing that we have not only used our basic cognitive processes to perceive the information but have also used more complex systems (to show that we recognise both bars, A and B, were the same size).
Gregory (1970, 1980) demonstrates another visual illusion arising out the ‘misapplied size-constancy theory’: the Muller-Lyer Illusion (fig.2). In this diagram, although both lines are of equal length, one line appears to be longer than the other. Yet again, the ‘depth theory’ demonstrates how two-dimensional images are attempted to be perceived the same way in which we would perceive a three-dimensional object. In the Muller-Lyer Illusion, the arrow pointing inwards (A) indicates that the surfaces are slanting towards you whilst arrow B implies that the surfaces are moving away from you (fig.3), leading viewers to believe that the corner of A seems to be much closer than the corner of B. Based on the ‘depth explanation’, if both lines are equal in size, but one looks as if it is further away than the other, it should in fact be longer, thus creating the illusion. ‘Interposition’ is another cue to depth and occurs when objects overlap, leading the overlapped object to be considered further away. An example of this can be seen in the Kanizsa’s Illusory Square (fig.4), where the absent segments of the four black circles lead us to assume that there is a white square in front cover it. Once again, knowledge of depth is added to the image enabling us to form something which is not actually there, providing evidence that a more complex cognitive process is taking place.
The bottom-up (direct) approach, on the other hand, emphasizes that information supplied by the visual environment is sufficient enough for moving around as well as interacting with the environment (without the role of any internal processes). Gibson’s ecological approach, derived from the bottom-up approach, argues that perception supplies the information necessary for organization of action; in addition, action along with movement facilitates accurate perception. He assumed that the pattern of light reaching the retina consists of all the visual information from the environment, and that this optical array provides explicit information about the layout of objects. Therefore Gibson’s ecological approach is a bottom-up process as it involves “picking up” information, without applying any other additional information. Relating this approach to the Muller-Lyer Illusion may lead to the explanation that the illusion is created simply due to the eye movement of the viewer. Boyce and West (1967) conducted experiments where eye fixations and movements were recorded and found that the length of time taken to look from one end of a line to the other was more time-consuming when lines with the arrow head pointing inwards were observed. These findings are further supported by other findings which showed that as the angles of the arrow heads decreased, the larger the difference in the length of movement. In addition, it was found that the illusion was still present even when the arrow heads were replaced with circles, although the strength of the effect is not as strong. This leads to the assumption that although depth cues are imperative in explaining the Muller-Lyer Illusion, the integration of the line into the object at the ends should also be considered.
Another visual illusion which can be explained using the bottom-up approach is the Hermann Grid (fig.5). As the viewer focuses on the grid, dark spots appear in the intersections of the white crosses formed by the black squares. However, if your eyes are focusing directly on these spots, the illusion disappears. The occurrence of this illusion can be explained through a neural explanation with reference to receptive fields and lateral inhibition. Receptive fields are the result of a group of receptors which is formed through the intensity at a point in the visual system. The stimulation at the centre of the receptive field may have the opposing effect to the stimulation at its edge – receptors in the centre act excitatory during the resulting signal, whilst receptors surrounding the edge behave inhibitory.
Bruner, Postman and Rodriguez’s (1951) experiment demonstrates how an illusion can involve both bottom-up and top-down processing. The experiment consisted of briefly presenting participants with a selection of playing cards and found that when participants were shown black hearts, some had claimed that they had seen brown or purple hearts. From this, it has indicated that both colours (red and black) were taken into account when responding – the colour red initiated the top-down process as our existing knowledge and expectations know that hearts on playing cars should be red, however this contrasts with the bottom-up process which tells us that the response should be black as that was the colour of the information presented.
Evidence that top-down processing exists is provided through the Ponzo and Muller-Lyer illusion. Other factors may also influence the affect of the illusion, i.e. eye movement; however the illusion still occurs even when eye movement is not present, although the illusion appears to be weaker. This is because two-dimensional illusions are still attempted to be perceived through our existing knowledge of interpreting the world surrounding us – illusions emphasise this through leading us to falsely apply this knowledge, thus making it apparent. Simple examples, such as an ambiguous drawing (fig.6), show that in order to recognise either or both images, the viewer must have an existing knowledge of what both a rabbit and a duck look like. On the contrary, illusions such as the Hermann Grid provide evidence that bottom-up processing is also used in some cases, at times even in conjunction with top-down processing. In conclusion, a combination of both approaches, top-down and bottom-up, appears to provide a better theory. This being because the bottom-up approach fits well in good viewing conditions, whilst the top-down theory comes into use under bad viewing conditions or lack of stimulus clarity.
Bibliography
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Michael Eysenck: Principles of Cognitive Psychology. Psychology Press, 2nd edition, 2001
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Randolph Blake and Robert Sekuler: Perception. 5th edition
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Lothar Spillman and John S. Werner: Visual Perception. The Neurophysiological Foundations
Michael Eysenck, Principles of Cognitive Psychology, Psychology Press, 2nd edition, 2001
