Several illustrations of our disposition to see the environment as structured instead of chaotic were given in the previous sections, when I discussed the phenomena of object constancy and motion perception. The imposition of this kind of high-level structure on the visual world is an apotheosis of the processes of perceptual organization, linking vision to general cognition and language. Experimental evidence suggests that this connection is bilateral, and that the cognitive level can influence visual perception. The most directly relevant experiments are those in which subjects exhibit object superiority effects, for example, the facilitation of the perception of a low-level feature, such as a line segment, by virtue of its appearance as a part of the projection of a three-dimensional object (Weisstein and Harris, 1974).
In many top-down effects (including object superiority), the perceptual phenomenon is better characterized as categorization than recognition. The manner of cognitive involvement in perception is thus more flexible and more general than mere recollection of previously encountered stimulus exemplars. Experiments carried out by Rosch and her collaborators as part of a wider study of the structure of categories (see, e.g., Rosch et al., 1976) showed that people tend to perceive and describe objects at a certain level of detail. Importantly, this basic category level can be independently defined in terms of visual perception, language, and general cognitive development.
Figure: Past experience may influence the emergence of a percept where bottom-up organization and grouping alone fails (photography by R. C. James).
A less direct, but not less compelling, demonstration of the top-down principle is shown in Figure 3. Rock (1984, p.130) remarks that we would be unlikely to see a Dalmatian in this figure, unless past experience affected our perception of it. Some of the features of the processes involved in perceptual learning and memory are mentioned in the next section.