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Illusions

Amazing Perceptions

The visual system performs many remarkable tasks. The brain receives fragments of information from approximately 1 million axons in each of the optic nerves. It combines and organises these fragments into the perception of a scene/objects with different forms, colours, and textures, residing at different locations in three-dimensional space. Even when our eyes move exposing the photoreceptors to new patterns of visual information, our perception of the scene before us does not change. We see a stable world, not a moving one, because the brain keeps track of our own movements and those of our eyes and compensates for the constantly changing patterns of neural firing that these movements cause.

‘Perception is a rapid, automatic, unconscious process; it is not a deliberate one, in which we puzzle out the meaning of what we see. We do not first see an object and the perceive it; we simply perceive the object. Yes, occasionally we see something ambiguous and must reflect about what it might be or gather further evidence to determine what it is, but this situation is more problem solving than perception.’

(Carlson, 1993, p133)

Background Information on perception

Sight is such a familiar phenomenon it is difficult to conceptualise the myriad of complex, computational processings involved when simply recognising a face. Experiments on computer generated pattern recognition illustrate the amazing sophistication of the brain's visual system. The most powerful computers do not begin to match the strategies employed by the brain in order to simultaneously recognise movement, colour and form.

A great deal of progress has been made in understanding how such processing occurs. A simplistic idea stated that visual perception was attained via ‘serial processing’ – a linear, hierarchical system of information processing by cells from the retina through to the visual cortex. It was also thought that cells operating in this hierarchical system had differing receptive properties ranging from simple, to complex and to super complex.

In more recent times it has been suggested that in addition to serial processing, cells in different regions of the visual cortex are sensitive to particular perceptual features of objects; movement, colour and form. Furthermore, information is carried along to each of these three regions by independent neural pathways.

The current view, that the ability of the visual system to ‘perceive’ the environment is a holistic, active and ultimately a creative process (involving a great deal more that just the information supplied by the retina), was first conceived early in the 20^th century.

This view was developed by a group of German psychologists led by, Max Wertheimer, Kurt Koffka and Wolfgang Kohle, who founded the school of Gestalt psychology. The German term Gestalt means ‘configuration‘, ‘form’ or ‘image’. The central theory held by the Gestalt psychologists was that the act of perception creates a Gestalt–an image which represents the ‘organisation of sensations by the brain.’ They argued that the creation of three–dimensional sight was the result of the brain’s ability to organise sensations into ‘stable patterns’ or ‘perceptual constancies.’ The visual system was designed to follow ‘rational principles’ of movement, colour, form and distance and thereby organise incoming sensations. In other words, the brain makes particular assumptions about what is being seen, based in part upon experience, as well as, from the pattern of ‘neural wiring’ involved with vision.

Visual illusions and perceptual constancies were used by the Gestalt psychologists to illustrate their theory and the brain's strategies for perception. The ‘visual illusion’ exhibits all illustrate strategies employed by the brain in order to perceive the visual environment.

(The notes above were derived from Kandel et al., 1991, Principles of Neural Science, Chapter 30: Perception of Motion, Depth and Form, p 441-443)

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