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Abstract:
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.
