A SEQUENCE OF OBJECT-PROCESSING STAGES REVEALED BY FMRI IN THE HUMAN OCCIPITAL LOBE

Functional magnetic resonance imaging was used in combined functional selectivity and retinotopic mapping tests to reveal. object-related vi sual areas in the human occpital lobe. Subjects were tested with right , left, up, or down hemivisual field stimuli which were composed of im ages of natural objects (faces, animals, man-made objects) or highly s crambled (1,024 elements) versions of the same images. In a similar fa shion, the horizontal and vertical meridians were mapped to define the borders of these areas. Concurrently, the same cortical sites were te sted for their sensitivity to image-scrambling by varying the number o f scrambled picture fragments (from 16-1,024) while controlling for th e Fourier power spectrum of the pictures and their order of presentati on. Our results reveal a stagewise decrease in retinotopy and an incre ase in sensitivity to image-scrambling. Three main distinct foci were found in the human visual object recognition pathway (Ungerleider and Haxby [1994]: Curr Opin Neurobiol 4:157-165): 1) Retinotopic primary a reas V1-3 did not exhibit significant reduction in activation to scram bled images. 2) Areas V4v (Sereno et al., [1995]: Science 268:889-893) and V3A (DeYoe et al., [1996]: Proc Natl Acad Sci USA 93:2382-2386; T ootell et al., [1997]: J Neurosci 71:7060-7078) manifested both retino topy and decreased activation to highly scrambled images. 3) The essen tially nonretinotopic lateral occipital complex (LO) (Malach et al., [ 1995]: Proc Natl Acad Sci USA 92:8135-8139; Tootell et al., [1996]: Tr ends Neurosci 19:481-489) exhibited the highest sensitivity to image s crambling, and appears to be homologous to macaque the infero-temporal (IT) cortex (Tanaka [1996]: Curr Opin Neurobiol 523-529). Breaking th e images into 64, 256, or 1,024 randomly scrambled blocks reduced acti vation in LO voxels. However, many LO voxels remained significantly ac tivated by mildly scrambled images (16 blocks). These results suggest the existence of object-fragment representation in LO. Hum. Brain Mapp ing 6:316-328, 1998. (C) 1998 Wiley-Liss, Inc.