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Shape Processing in Basic Level Categorization—: An fMRI Investigation

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http://pubman.mpdl.mpg.de/cone/persons/resource/persons83873

Dahl,  C
Department Physiology of Cognitive Processes, Max Planck Institute for Biological Cybernetics, Max Planck Society;
Department Human Perception, Cognition and Action, Max Planck Institute for Biological Cybernetics, Max Planck Society;

http://pubman.mpdl.mpg.de/cone/persons/resource/persons83944

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

http://pubman.mpdl.mpg.de/cone/persons/resource/persons84023

Erb M, Kourtzi,  Z
Department Human Perception, Cognition and Action, Max Planck Institute for Biological Cybernetics, Max Planck Society;
Department Physiology of Cognitive Processes, Max Planck Institute for Biological Cybernetics, Max Planck Society;

http://pubman.mpdl.mpg.de/cone/persons/resource/persons83839

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

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Citation

Dahl, C., Graf, M., Erb M, Kourtzi, Z., Grodd, W., & Bülthoff, H. (2004). Shape Processing in Basic Level Categorization—: An fMRI Investigation. Poster presented at 7th Tübingen Perception Conference (TWK 2004), Tübingen, Germany.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0013-DA0F-5
Abstract
How is basic level categorization achieved in the human brain? Deforming shape (morphing) transformations are well suited to describe the shape variability of members of common basic level categories. Behavioral experiments showed that categorization performance deteriorates systematically with increasing amount of morph transformation [1, 2]. A possible explanation for these ndings is that categorization relies on time-consuming compensation processes (deformable template matching). If spatial compensation processes are involved, then categorization might not only comprise the ventral visual pathway, as generally assumed, but also the dorsal stream. We investigated this question with functional MRI. Objects from 25 common basic level categories were generated by morphing between two members of the same category (using 3ds max). Eleven subjects participated in two tasks, starting with the categorization task. Subjects had to decide as fast as possible whether two sequentially presented objects belonged to the same basic level category. The transformational distance between category members was varied (event-related design). In a second task, the same observers perceived intact morphing sequences, scrambled morphing sequences, and static presentations of different morph exemplars (block design). fMRI data were acquired on a 3T scanner (Siemens Trio), measuring 24 slices of 64x64 voxels every two seconds (resolution of 3x3x5 mm A ). In the categorization task, the response latencies for same trials increased with increasing morph distance between two category members. Correspondingly, the contrast long vs. short morph distance revealed an increasing BOLD signal in LOC (lateral occipital complex). Moreover, activation increased also in the superior parietal cortex (BA 7) and in the frontal cortex (BA 44). Control analyses showed that this pattern of activation cannot be reduced to task difculty, or increasing dissimilarity between the objects. In the second task we found dorsal activation for the comparison between intact vs. scrambled morphing sequences. This activation spot was close to the dorsal activation in the categorization task, but was not identical. The results suggest that basic level categorization is not limited to the ventral pathway, but rather relies on a network of ventral, dorsal and frontal activation. The activation within this network is systematically dependent on the amount of shape transformation. The dorsal activation seems related to compensational processes taking place in parietal cortex, i.e. spatial (deforming) transformation processes. These ndings are in accordance with an alignment approach of object recognition and categorization.