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Abstract

The shape variability of objects from the same basic-level category can be conceptualised by transformations which continuously change object shape. Morphing between members of a basic-level category allows to systematically vary the shape of common objects. We demonstrated that categorisation performance depends on the amount of shape transformation, using a sequential matching task (Graf and Bülthoff, 2003, EuroCogSci, Osnabrück, Germany). We further investigated shape dependence of basic-level categorisation with name matching and rating tasks. We created five category members (1, 25, 50, 75, and 100 morphs) for twenty-five basic-level categories, based on morphable 3-D object models. Objects were rendered as grey-level images and presented in a canonical orientation. In the speeded name-matching experiment, subjects had to decide whether an object corresponded to a category name. In a second experiment, subjects were required to rate the typicality of the objects, ie they had to rate how well the objects corresponded to their mental image of the category. Both name-matching latencies and typicality ratings depended systematically on the amount of shape transformation. Name-matching latencies were significantly shorter for exemplars that were more in the middle of the morphing sequence. These exemplars were also rated as more typical in experiment 2. The findings confirm the systematic dependence of categorisation performance on the amount of shape transformation. Shapes that are rated as more prototypical are categorised faster. This shape-dependence in basic-level categorisation is reminiscent of the orientation-dependence in recognition, where the canonical perspective is rated as more typical and recognised faster [Palmer et al, 1981, in Attention and Performance IX Eds J Long, A Baddeley (Hillsdale, NJ: Lawrence Erlbaum Associates) pp 135 - 151]. Overall, the results support an extension of the image-based model of recognition (Bülthoff et al, 1995 Cerebral Cortex 3 297 - 260) to basic-level categorisation.