title: Edge-Related Activity Is Not Necessary to Explain Orientation Decoding in Human Visual Cortex.
abstract: Multivariate pattern analysis is a powerful technique; however, a significant theoretical limitation in neuroscience is the ambiguity in interpreting the source of decodable information used by classifiers. This is exemplified by the continued controversy over the source of orientation decoding from fMRI responses in human V1. Recently Carlson (2014) identified a potential source of decodable information by modeling voxel responses based on the Hubel and Wiesel (1972) ice-cube model of visual cortex. The model revealed that activity associated with the edges of gratings covaries with orientation and could potentially be used to discriminate orientation. Here we empirically evaluate whether "edge-related activity" underlies orientation decoding from patterns of BOLD response in human V1. First, we systematically mapped classifier performance as a function of stimulus location using population receptive field modeling to isolate each voxel's overlap with a large annular grating stimulus. Orientation was decodable across the stimulus; however, peak decoding performance occurred for voxels with receptive fields closer to the fovea and overlapping with the inner edge. Critically, we did not observe the expected second peak in decoding performance at the outer stimulus edge as predicted by the edge account. Second, we evaluated whether voxels that contribute most to classifier performance have receptive fields that cluster in cortical regions corresponding to the retinotopic location of the stimulus edge. Instead, we find the distribution of highly weighted voxels to be approximately random, with a modest bias toward more foveal voxels. Our results demonstrate that edge-related activity is likely not necessary for orientation decoding.
  
  
  
  A significant theoretical limitation of multivariate pattern analysis in neuroscience is the ambiguity in interpreting the source of decodable information used by classifiers. For example, orientation can be decoded from BOLD activation patterns in human V1, even though orientation columns are at a finer spatial scale than 3T fMRI. Consequently, the source of decodable information remains controversial. Here we test the proposal that information related to the stimulus edges underlies orientation decoding. We map voxel population receptive fields in V1 and evaluate orientation decoding performance as a function of stimulus location in retinotopic cortex. We find orientation is decodable from voxels whose receptive fields do not overlap with the stimulus edges, suggesting edge-related activity does not substantially drive orientation decoding.
authors.affiliation: Department of Cognitive Science, Macquarie University, Sydney, 2109 New South Wales, Australia, susan.wardle@mq.edu.au.|ARC Centre of Excellence in Cognition and Its Disorders, Macquarie University, Sydney, 2109 New South Wales, Australia.|Perception in Action Research Centre, Macquarie University, Sydney, 2109 New South Wales, Australia.
authors.forename: Susan G
authors.identifier: 0000-0003-2216-7461
authors.initials: SG
authors.lastname: Wardle
authors.affiliation: Department of Cognitive Science, Macquarie University, Sydney, 2109 New South Wales, Australia.|ARC Centre of Excellence in Cognition and Its Disorders, Macquarie University, Sydney, 2109 New South Wales, Australia.|Perception in Action Research Centre, Macquarie University, Sydney, 2109 New South Wales, Australia.|Laboratory of Biological Psychology, KU Leuven, 3000 Leuven, Flemish Brabant, Belgium.
authors.forename: J Brendan
authors.identifier: 0000-0002-2402-8724
authors.initials: JB
authors.lastname: Ritchie
authors.affiliation: Department of Cognitive Science, Macquarie University, Sydney, 2109 New South Wales, Australia.|ARC Centre of Excellence in Cognition and Its Disorders, Macquarie University, Sydney, 2109 New South Wales, Australia.|School of Psychology, University of New South Wales, Sydney, 2052 New South Wales, Australia, and.
authors.forename: Kiley
authors.identifier: 0000-0002-2988-3121
authors.initials: K
authors.lastname: Seymour
authors.affiliation: Department of Cognitive Science, Macquarie University, Sydney, 2109 New South Wales, Australia.|ARC Centre of Excellence in Cognition and Its Disorders, Macquarie University, Sydney, 2109 New South Wales, Australia.|Perception in Action Research Centre, Macquarie University, Sydney, 2109 New South Wales, Australia.|School of Psychology, University of Sydney, Sydney, 2006 New South Wales, Australia.
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Carlson
mesh_terms: D001931:Brain Mapping; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D008297:Male; D008959:Models, Neurological; D000072458:Orientation, Spatial / Q000502:physiology*; D010364:Pattern Recognition, Visual / Q000502:physiology*; D010775:Photic Stimulation; D014793:Visual Cortex / Q000502:physiology*
keywords: fMRI decoding; hyperacuity; multivariate pattern analysis; orientation columns; population receptive field mapping; visual cortex