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Poster

Categorization and Object Shape: Behavioral Data and Global fMRI Analysis

MPG-Autoren
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/persons83839

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

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Zitation

Dahl, C., Graf, M., Erb M, Grodd, W., & Bülthoff, H. (2004). Categorization and Object Shape: Behavioral Data and Global fMRI Analysis. Poster presented at 10th Annual Meeting of the Organization for Human Brain Mapping (HBM 2004), Budapest, Hungary.


Zitierlink: http://hdl.handle.net/11858/00-001M-0000-0013-D907-E
Zusammenfassung
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 (Figure 1). Behavioral experiments showed that categorization performance deteriorates systematically with increasing amount of morph transformation (Dahl, Graf, and Bülthoff, 2003; Graf, 2002). A possible explanation for these findings is that categorization relies on time-consuming compensation processes (deformable template matching). We tested, using fMRI, whether spatial (deforming) compensation processes are involved in categorization, and hypothesized that such processes would additionally activate the dorsal stream. Objects from 25 common basic level categories were generated by morphing between two members of the same category (using 3ds max). Subjects were familiarized with all objects before the experiment. 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 (updating task, block design). fMRI data were acquired on a 3T scanner (Siemens Trio), with 24 slices of 64x64 voxels every two seconds (resolution of 3x3x5 mm3), and whole brain coverage. Behaviorally, we found that in the categorization task, the response latencies for same trials increased with increasing morph distance between two category members. Correspondingly, the fMRI analysis, contrasting long vs. short morph distance, revealed an increasing BOLD signal in LOC (lateral occipital complex) in both hemispheres. Moreover, we found activation in the superior parietal cortices (BA 7) and in the right frontal cortex (BA 44) (Figure 2). Control analyses showed that this pattern of activation cannot be reduced to task difficulty, or increasing dissimilarity between the objects. In the second task we found dorsal activation for the comparison between intact vs. scrambled morphing sequences (and additional bilateral activation in LOC when intact morphing sequences were compared to scrambled and static sequences). The parietal activation, which was located close to the dorsal activation in the categorization task (Figure 3), seems related to the perception of continuously morphing objects. These 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 systematically depends 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 findings are in accordance with an alignment approach of object recognition and categorization.