Full primal sketch

Various processes need to be applied to the raw primal sketch to identify its underlying structure or organisation. This is needed, because the information contained in the raw primal sketch is typically ambiguous and compatible with several underlying structures. Marr (1976) found that it was valuable to make use of two rather general principles when designing a program to achive perceptual organisation:

1. The principle of explicit naming.

2. The principle of least commitment.

According to the former principle, it is useful to give a name or symbol to a set of grouped elements. The reason is that the name or symbol can be used over and over again to describe other sets of grouped elements, all of which can then form a much larger grouping. According to the principle of least commitment, ambiguities are resolved only when there is convincing evidence as to the appropriate solution. This principle is useful, because mistakes at an early stage of processing can lead on to several other mistakes.

With respect to the principle of explicit naming, Marr's program assigned place tokens to small regions of the raw primal sketch, such as the position of a blob or edge, or the termination of a longer blob or edge. Various edge points in the raw primal sketch are incorporated into a single place token on the basis of Gestalt-like notions such as proximity, figural continuity, and closure (see Chapter 2). Place tokens are then grouped together in various ways, in part on the basis of the grouping principles advocated by the Gestaltists. Some examples of the ways in which place tokens are combined are:

• Clustering: place tokens that are close together can be combined to form higher-order place tokens.

• Curvilinear aggregation: place tokens that are aligned in the same direction will be joined to produce a contour.

Section summary

Marr provided one of the first detailed accounts of the initial processes in visual perception. As such, it has been very influential. Marr's (1976, 1982) visual processing program for the full primal sketch was reasonably successful. One reason why the grouping principles applied to place tokens work is because they reflect what is generally the case in the real world. For example, visual elements that are close together are likely to belong to the same object, as are elements that are similar. The program works well although it typically does not rely on object knowledge or expectations when deciding what goes with what. However, there were cases of ambiguity when the program could not specify the contour or perceptual organisation until supplied with additional information.

Marr (1982) assumed that grouping is based on two-dimensional representations. However, grouping can also be based on three-dimensional representations (e.g., Rock & Palmer, 1990, see Chapter 2). Enns and Rensick (1990) found that their participants immediately perceived which in a display of block figures was the "odd-man-out". They were able to do this even though the figures differed only in their three-dimensional orientation. This suggests that three-dimensional or depth information can be used to group stimuli.

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