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J Neurophysiol (July 16, 2008). doi:10.1152/jn.90305.2008
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Submitted on February 26, 2008
Revised on June 23, 2008
Accepted on July 12, 2008

Graded size-sensitivity of object-exemplar-evoked activity patterns within human LOC subregions

Evelyn Eger1*, Christian A Kell2, and Andreas Kleinschmidt1

1 INSERM U.562
2 Brain Imaging Center Frankfurt

* To whom correspondence should be addressed. E-mail: evelyn.eger{at}gmail.com.

A central issue for understanding visual object recognition is how the cortical hierarchy represents incoming sensory information and transforms it across successive processing stages. The format of object representation in the human brain has so far mostly been investigated using adaptation paradigms because the neuronal layout of object selectivities was thought to be beyond the resolution of conventional fMRI. Recently, however, multivariate pattern recognition succeeded in discriminating fMRI responses of object-selective cortex to different object exemplars within a given category. Here, we use increased spatial fMRI resolution to explore size sensitivity and tolerance to size change of response patterns evoked by object exemplars across a range of three sizes. Results from Support Vector Classification on responses of the human lateral occipital complex (LOC) show that discrimination of size (for a given object) as well as discrimination of objects across changes in size depended on the amount of size difference. Even across the largest amount of size change accuracy for generalization was still significant in LOC, while the same comparison was at chance performance in early visual (calcarine) cortex. Analyzing sub-regions, we further found an anterior-posterior gradient in the degree of size sensitivity and size generalization within the posterior-dorsal and anterior-ventral parts of LOC. These results speak against fully size-invariant representation of object information in human LOC and are hence congruent with findings in monkeys demonstrating object identity as well as size information in population activity of inferotemporal cortex. Moreover, these results provide evidence for a fine-grained functional heterogeneity within human LOC beyond the commonly used LO/fusiform subdivision.







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