Computational Modeling of Mental and External Reasoning with Sketches Sven Bertel, Thomas Barkowsky, Peter König, Holger Schultheis, Christian Freksa {bertel, barkowsky}@sfbtr8.uni-bremen.de, mail@peterkoenig.info, {schulth, freksa}@sfbtr8.uni-bremen.de Many spatial reasoning tasks involve external diagrams and mental images. Successful problem solving is often furthered by the close functional coupling of spatial reasoning with diagrams and images. For human problem solving of geographic tasks, we present the computational model NEVILLE which proposes a visual model of how mental images and external sketches interact. Behavioral adequacy for a selected set of phenomena such as limited working memory capacity and mental rotation of objects are among the goals realized. Spatial Problem Solving with Sketches and Mental Images Mental images are constructed from knowledge fragments retrieved from long-term memory and the interpretation associated with these fragments becomes part of the image (cf. Logie, 2001). As a consequence, structurally reinterpreting an image in mind is often hard. However, when the image gets externalized (i.e., when its content gets re-represented in terms of a sketch or diagram) its content can be easily reinterpreted (Verstijnen et al., 1998). Internal and external representations comple- ment one another as representational and procedural limitations of one form are oftentimes compensated for by the other. For example, external representations are durable and stable, internal representations are volatile and flexible. In the case of spatial problem solving (e.g., in reasoning about geographic knowledge), sketches serve as external representations. Mental images form their internal counter-parts and they are constructed in working memory from spatial knowledge fragments (Barkowsky, 2002). When people solve a geographic problem, much interaction takes place between the drawing and their mental image. Some modifications—like the rotation of a single element—can easily be applied to a mental image but are impossible on a sketch without erasing the object or cutting out paper. On the other hand and unlike in the case of mental images, there hardly exist practical restrictions on capacity for drawings.