What does Navon's (1977) reveal about the perception of shape?

University Degree Biological Sciences

Kathryn Smith

20089003

PSY3018

Visual Perception with Neuropsychology

Graham Smith

What does Navon's (1977) reveal about the perception of shape?

Roth (1986) defines perception as 'the means by which information is acquired through the sense organs and transformed into experiences of objects, events, sounds and taste.'

The constructivist psychologist, Richard Gregory (1998) describes such perceptions to be 'like the predictive hypothesis of science.' His theory of 'hypothesis testing' suggests that data from the retinal image interacts with previous appropriate knowledge, creating 'psychological data', whereby the hypotheses try and make sense of it all. Gregory's hypothesis theory can be explained by an experiment carried out by Palmer (1975). He presented a pictorial scene to participants (e.g., a kitchen scene), followed by a brief presentation of a picture of an object. It was found that the probability of identifying an object correctly was highest when the object was relevant to the context (e.g., a loaf), intermediate when no context was given (i.e. no picture was shown before the object picture) and lowest when the object was irrelevant to the picture (e.g., a drum). (Cited in Eysenck & Keane, 2000). Such findings cannot be explained by any direct processing theories, which are usually driven by bottom-up processing but only by indirect constructivist theories like that of Gregory's hypothesis testing (1973).

The constructivist approach to perception is concerned with how perceptions are constructed by the mind. Any approach to perception that is concerned with perceptual processing is included within the constructivist approach (Goldstein, 1999). Constructivist processing theories are mainly dependent on internal processes and driven by both top-down and bottom-up processes. It was Hermann Von Helmholtz (1821-1894) who formed the early roots of the constructivist approach. He thought of visual perceptions as 'unconscious inferences' which add meaning to sensory information and believed these inferences to be conscious as one is not commonly aware that inferences are being made while perceiving (cited in Eysenck & Keane, 2000). Helmholtz (1894) proposed the 'likelihood principle', which assumes that a person perceives an object that is likely to be the cause of sensory stimulation. Goldstein (1999) explains this principle. ‘If a number of objects could have caused a particular pattern of light and dark on the retina, the object that is most likely to occur in that particular situation will be perceived.’ It is Gregory's (1973) hypothesis testing that follows on from the likelihood principle.

Gregory (1973) and other constructivist theories like Bruner (1957) and Neisser (1967) all believe in the assumptions, which Helmholtz (1894) had proposed previously: that perceptions are influenced by hypotheses and expectations which are at times incorrect. This therefore implies that perception is 'prone to error' (cited in Eysenck & Keane, 2000). Ittelson (1952) argued if a visual display appears familiar but is actually unfamiliar, the perceptual hypotheses formed could be inaccurate. The Ames distorted room is a good example of this. The room is an unusual shape, however when it is viewed from a certain angle, it actually looks like ...

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Gregory (1973) and other constructivist theories like Bruner (1957) and Neisser (1967) all believe in the assumptions, which Helmholtz (1894) had proposed previously: that perceptions are influenced by hypotheses and expectations which are at times incorrect. This therefore implies that perception is 'prone to error' (cited in Eysenck & Keane, 2000). Ittelson (1952) argued if a visual display appears familiar but is actually unfamiliar, the perceptual hypotheses formed could be inaccurate. The Ames distorted room is a good example of this. The room is an unusual shape, however when it is viewed from a certain angle, it actually looks like a normal rectangular room. In reality, one corner of the room is actually further away from the observer than the other corner and so when a person walks backwards and forwards along the back wall, they appear to have grown and shrunk. (Goldstein, 1984 as cited in Hilgard's Introduction to Psychology, 2002). However, according to constructivists, if the person in the distorted room happens to be a close relative of the observer, then there is a greater chance that the room would be seen as having an odd shape.

We often view objects from angles at which their 'true' shapes are not reflected in the retinal image they project. For example, rectangular doors often project trapezoid shapes and round cups often project elliptical-shaped images. Just as with size constancy, the perceptual system maintains constancy in terms of shape. However, shape and size constancy do not always work. When we look down at people from the top of a very tall building, they do not look more like ants to us, even though we know they are people.

Motivational and emotional states may also influence perceptual shape, and in turn, visual perception. One of the main assumptions of the constructivist approach is that it is dependent on internal processes. A study by Bruner and Goodman (1947) illustrated this. They studied motivational factors by asking rich and poor children to estimate the sizes of coins. They found that poor children overestimated the sizes of coins as apposed to rich children. The major bias here, however is that poor children may have overestimated the size of the coins because of the importance of money to them as apposed to rich children. However, rich children may have been more familiar with coins and so estimated their sizes more accurately. A modified version of this experiment was conducted by Ashley et al. (1951). They hypothesised participants into believing they were either rich or poor and found that the 'poor' condition overestimated the size of the coins as apposed to the rich condition. This is a strength of the constructivist approach as it can account for the role of motivation and emotion in perception since it strongly acknowledges the importance of internal states in determining perception (cited in Eysenck & Keane, 2000).

Similar to Gregory's hypothesis theory is a schema theory. This assumes that a cue calls upon a mental schema, rather than a hypothesis, which Gregory suggests. Anderson (1980) defines schemas as 'large complex units of knowledge that encode properties that are typical of instances of general categories and omit properties which are not typical of the categories'. An important function of schemas is to allow a person to gather relevant information from where it is needed but absent. The idea is similar to Gregory's hypothesis theory; however, this provides a richer account of the information that is called up by the cue. As already mentioned, Gregory's (1973) constructivism based hypothesis theory accounts for a number of visual illusions where the 'unnatural' use of cues leads for the misapplications of hypotheses (e.g. Ames 'distorted room' described above).

Also most of the evidence used in support of the constructivist approach is based on artificial stimuli that do not occur naturally in the real world. Studies such as that of Bruner et al. (1951) and Palmer (1975) presented visual stimuli only briefly, reducing the likelihood of using bottom-up processing and giving chance for hypotheses (top-down processes) to be used (cited in Eysenck & Keane, 2000).

Navon (1977) tested the idea that the whole is perceived before the parts that make it up by presenting participants with various stimuli. Navon distinguished between the global (or whole like features of a stimuli) and the local (or more specific and 'part-like' features). Each stimulus consisted of a large (global) letter made up of many small (local) letters. In some cases, the global and local letters matched and in some cases they did not. Participants had to identify either the large or the small letter as quickly as possible. Navon found that the time taken to identify the large letter was unaffected by whether the large letters matched or not. However, the time taken to identify the small letters was affected by whether the large letters matched or not, such that when the large letter was different, response times were longer. This suggests that it is difficult to avoid processing the whole and that global processing necessarily occurs before any more detailed perceptual analysis.

According to Kimichi (1992) assumption regarding global advantage cannot be rightly inferred, until more is known about perceptual units of a stimulus structure, such discrepancies may advocate improper characterisation of perceptual processing. In line with Kimichi (1992), it is therefore considered that the true nature of perceptual processing must be treated carefully in respect to the mapping of one processing style onto another, as no specific processing strategy is necessarily inherent of the same characteristics as another.

Reference

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Navon, D., & Norman, J. (1983). Does Global Precedence really depend on Visual Angle? Journal of Experimental Psychology: Human Perception & Performance, 9 (6), 955-965.

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