The constructivist approach has led to the discovery of a wide range of interesting perceptual phenomena. Processes resembling those postulated by constructivist theorists probably underlie most of these phenomena. However, many theorists disagree strongly with the constructivist viewpoint. They are unconvinced of the central assumption that perceivers resemble the great detective Sherlock Holmes as they struggle to make sense of the limited "fragmentary scraps of data" available to them. Some of the major problems for the constructivist approach will now be discussed.

First, this approach appears to predict that perception will often be in error, whereas in fact perception is typically accurate. If we are constantly using hypotheses and expectations to interpret sensory data, why is it that these hypotheses and expectations are correct nearly all the time? Presumably the environment provides much more information than the "fragmentary scraps" assumed by constructivist theorists.

Second, many of the experiments and demonstrations carried out by constructivist theorists involve artificial or unnatural stimuli. Of particular importance, many studies supporting the constructivist approach (e.g., Bruner et al., 1951; Palmer, 1975) involved presenting visual stimuli very briefly. Brief presentation reduces the impact of bottom-up processes, allowing more scope for top-down processes (e.g., hypotheses) to operate.

Third, it is not always clear what hypotheses would be formed by observers. Let us return to the study (Ittelson, 1951) in which someone walks backwards and forwards along the rear wall of the Ames room. Observers could interpret what they are seeing by hypothesising that the room is distorted and the person remains the same size, or by assuming that the room is normal but the person grows and shrinks. The former hypothesis strikes the authors as more plausible, but observers favour the latter.

Fourth, constructivist theorists such as Gregory have not succeeded in providing satisfactory explanations of most visual illusions. The classic visual illusions seem to depend on a range of factors, and so the search for a general theory (e.g., misapplied size constancy) is likely to prove fruitless.

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