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Graded Size Sensitivity of Object-Exemplar–Evoked Activity Patterns Within Human LOC Subregions

¹Institut National de la Santé et de la Recherche Médicale, U562, Gif/Yvette, France; ²Commissariat à l'Énergie Atomique, Direction des Sciences du Vivant, Institute d'Imagerie Biomedicale, NeuroSpin, Gif/Yvette, France; ³University Paris-Sud, Orsay, France; and ⁴Brain Imaging Center, Department of Neurology, University of Frankfurt, Frankfurt, Germany

Submitted 26 February 2008; accepted in final form 12 July 2008

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 thus far mostly been studied using adaptation paradigms because the neuronal layout of object selectivities was thought to be beyond the resolution of conventional functional MRI (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) and 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, whereas the same comparison was at chance performance in early visual (calcarine) cortex. Analyzing subregions, 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 showing object identity and 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.