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Categorization and object shape


Graf,  M
Department Human Perception, Cognition and Action, Max Planck Institute for Biological Cybernetics, Max Planck Society;

Bülthoff,  HH
Department Human Perception, Cognition and Action, Max Planck Institute for Biological Cybernetics, Max Planck Society;

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Graf, M., & Bülthoff, H. (2003). Categorization and object shape. Poster presented at 6. Tübinger Wahrnehmungskonferenz (TWK 2003), Tübingen, Germany.

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Shape variability of objects from a basic level category usually can be described well with continuous shape transformations (topological transformations). Previous experiments with line drawings showed that categorization performance depends systematically on the amount of shape transformation, both in speeded categorization tasks and rating tasks (Graf, doctoral dissertation, Wissenschaftlicher Verlag Berlin, 2002). We investigated whether this systematic dependency can be replicated under dierent conditions. Gray-level images of new category members were produced by morphing between members of the same basic level category. Two images were presented sequentially and subjects were required to decide whether they belonged to the same category. The amount of shape transformation between members of the same category was varied systematically. Objects were presented either upright (Experiment 1), in dierent orientations in the picture plane (Experiment 2) or in dierent equidistant positions (Experiment 3). In general, categorization performance deteriorated systematically with increased shape transformation. In Experiment 2, categorization performance depended both on the amount of shape change and orientation change, while there was no signicant interaction between orientation and shape. The eect of shape transformation was found despite position changes (Experiment 3). The systematic dependency of categorization performance on the amount of shape transformation was replicated with gray-level images, for upright objects, image-plane rotated, and for translated objects. This indicates that the systematic dependency on shape changes is not simply due to low-level processes. Processes which compensate for shape and for orientation changes seem independent, which is in agreement with previous ndings for other combinations of transformations. Overall, the ndings strongly support an image-based model of categorization.