Constructivist Theories

Helmholtz (1821-1894) argued that the inadequate information provided by the senses is augmented by unconscious inferences, which add meaning to sensory information. He assumed these inferences were unconscious, because we typically have no awareness that we are making inferences while perceiving. A good example of unconscious inference is the "hollow face" illusion (Gregory, 1973, see Chapter 2). The face is hollow, but the shading and other cues are consistent with a solid face. As a result of our expectations, we see a solid face. We continue to do so even when we "know" the face is hollow, indicating that conscious knowledge is not influencing perception.

The approach advocated by Helmholtz, which we will call the constructivist approach, remains influential. Theorists such as Bruner (1957), Neisser (1967), and Gregory (1972, 1980) all subscribe to assumptions resembling those originally proposed by Helmholtz:

• Perception is an active and constructive process; it is "something more than the direct registration of sensations.. .other events intervene between stimulation and experience" (Gordon, 1989, p. 124).

• Perception is not directly given by the stimulus input, but occurs as the end-product of the interactive influences of the presented stimulus and internal hypotheses, expectations, and knowledge, as well as motivational and emotional factors.

• Perception is influenced by hypotheses and expectations that are sometimes incorrect, and so it is prone to error.

The flavour of this theoretical approach was captured by Gregory (1972). He claimed that perceptions are constructions, "from floating fragmentary scraps of data signalled by the senses and drawn from the brain memory banks, themselves constructions from the snippets of the past." Thus, the frequently inadequate information supplied to the sense organs is used as the basis for making inferences or forming hypotheses about the external environment.

Contextual information can be used in making inferences about a visual stimulus. Palmer (1975) presented a scene (e.g., a kitchen) in pictorial form, followed by the very brief presentation of the picture of an object. This object was appropriate to the context (e.g., loaf) or inappropriate (e.g., mailbox). There was also a further condition in which no contextual scene was presented. The probability of identifying the object correctly was greatest when it was appropriate to the context, intermediate when there was no context, and lowest when it was inappropriate.

According to constructivist theorists, the formation of incorrect hypotheses or expectations leads to errors of perception. Ittelson (1952) argued that the perceptual hypotheses formed may be very inaccurate if a visual display appears familiar but is actually novel. An example of this is the well known Ames distorted room (see Chapter 2). The room is actually of a peculiar shape, but when viewed from a particular point it gives rise to the same retinal image as a conventional rectangular room.

It is perhaps not surprising that observers decide that the room is like a normal one. However, what is puzzling is that they maintain this belief even when someone inside the room walks backwards and forwards along the rear wall, apparently growing and shrinking as he or she proceeds! The reason for the apparent size changes is that the rear wall is not at right angles to the viewing point: one corner is actually much further away from the observer than the other corner. As might be expected by constructivist theorists, there is a greater likelihood of the room being perceived as having an odd shape and the person walking inside it remaining the same size when that person is the spouse or close relative of the observer.

Another illustration of the possible pitfalls involved in relying too heavily on expectations or hypotheses comes in a classic study by Bruner, Postman, and Rodrigues (1951). Their participants expected to see

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