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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
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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.
