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Roger B. H. Tootell, Kathryn J. Devaney, Gheorghe M. Postelnicu, Leslie G. Ungerleider; Cortical fMRI maps in response to 3D morphs between head and house. Journal of Vision 2007;7(9):104. doi: 10.1167/7.9.104.
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© ARVO (1962-2015); The Authors (2016-present)
As in early visual cortex, accurate functional maps should clarify the fundamental information processing steps in inferotemporal (IT) cortex. Tanaka's group suggested that macaque IT is organized based on continuously-varying cortical columns, optimized for specific geometrical features (the ‘feature map’ hypothesis). More recently, human fMRI described larger IT areas responding selectively to specific objects (FFA and PPA for faces and places, respectively), general object categories (animate, tool), or expertise.
Different hypotheses predict correspondingly distinct fMRI maps in IT, in response to virtual 3D objects that were morphed systematically between two shape extremes. These extremes (head and house) were easily recognizable; the intermediate shapes were less (or un-) recognizable. All objects had identical lighting and surface reflectance; other properties (retinotopic extent, position and viewpoint) were equated wherever possible. FMRI data were acquired in response to selected shapes along the morph continuum, using extensive signal averaging. The response to each single shape was resolved independently (‘single stimulus’ imaging).
The feature map hypothesis predicts a continuous shift in the topography of cortical activity, as stimulus shapes changed along the morph continuum. In contrast, the FFA-PPA hypothesis predicts changes in response amplitude in each presumptive area, without any topographic shift. Preliminary results (n = 5) suggest that: 1) recognizable and unrecognizable shapes produce equivalent levels of fMRI activity throughout IT; 2) these virtual shapes (lacking surface reflectance variation) nevertheless activate FFA and PPA selectively, like real-life objects; 3) intermediate shapes may produce higher activity than either head or house, in specific cortical regions; 4) intermediate shapes produce systematic variations in fMRI amplitude in FFA and PPA - but no topographic shifts. Collectively, this evidence suggests that FFA and PPA are classical cortical areas, with differing functional selectivity. However the ultimate functional selectivity may only be approximated by ‘faces’ or ‘places’
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