spring or turning a gear wheel, make it easier to imagine the resulting spring forces and gear movements.

People are versatile in their ability to generate and use images. They can call to mind the physical appearance of static objects or dynamic events they have directly experienced through vision, audition, and touch (Kosslyn and Koenig, 1992). They can generate spatial images from nonspatial forms of input such as reading text (Franklin and Tversky, 1990), listening to conversation, or ideas they have imagined on their own (Finke, 1989). Although imagery often results from visual input and is often described in visual terms, spatial images are not necessarily visual; they are accessible to persons who lack life experience seeing (de-Beni and Cornoldi, 1988; Cornoldi and Vecchi, 2003; Farah, 1989). Children younger than about 7 years of age do not tend to use imagery strategically to help them learn new information, but even pre-school age children can generate and process images based on perceptual input. Kosslyn et al. (1990) showed that children in kindergarten can look at a visual stimulus and then generate an image of it later in order to make decisions about what they saw. Ray and Rieser (2003) showed that children 3–4 years of age can listen to short stories and generate spatial representations of the story in order to judge the relative locations of objects described in the story.

Psychologists distinguish memory in terms of its structures and the types of knowledge that are represented. A classic model of memory (Baddeley, 1986) includes three major structural types: sensory storage, short-term or working memory, and long-term memory (although a unitary view of memory is a viable alternative; see Cowan, 1997). Memory is highly selective, and much of what is perceived is forgotten or reconstructed as a result of organizing schema (for a classic treatment of this issue, see Bartlett, 1932).

Three features of the memory system can help us to understand how and why spatial thinking does and does not work: the automaticity of overlearning, the use of strategies to reduce memory demands, and the capacity of multiple working memory subsystems to operate in parallel.

The first feature is that calling to mind overlearned materials is more automatic than recalling less familiar materials, thus placing a smaller demand on working memory. The idea of automaticity might explain why attempts to use spatial aids such as maps or graphics to understand historical events sometimes interferes with learning instead of facilitating it. If the content and form of the map or graph are relatively unfamiliar, then too much working memory capacity is required to process both the unfamiliar form and the intended content of the representation.

A second feature is the skillful use of strategies to reduce demands on long-term memory. For example, chunking numbers by combining them into larger units, each of which is meaningful, is an effective memory strategy. It is easier to remember a list of numbers in this form [526 924 018 682] rather than this form [526924018682]. Even more effective is a way to chunk information so that it relates to things (or meaningful chunks) one already knows well. It is more difficult, therefore, to remember this list of arbitrarily defined chunks [219 171 918 194 119 45] than this list of meaningful chunks [the start and end years of the United States participation in the two World Wars are 1917–1918 and 1941–1945].

Most of the research on the use of memory strategies is aimed at understanding memory for verbal materials. We need to find out about analogous memory strategies for spatial information—what are efficient ways to “chunk” the information in graphics to make it easier to remember? What are efficient ways of chunking new graphics so they can be related to graphics that have already been learned?

Perceptual learning processes can lead to the rapid and accurate identification of the patterns that are central to expertise (Fahle and Poggio, 2000; Gibson, 1969; Gibson and Pick, 2000). Examples include pilots learning to judge air and ground speed (Haber, 1987); machinists and architects learning to “see” the three-dimensional shape of a solid object or house from top, side, and front views (Garling and Evans, 1991); radiologists learning to spot tumors on X-rays (Lesgold,

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