Abstract
The orientation of a visual grating can be decoded from human primary visual cortex (V1) using functional magnetic resonance imaging (fMRI) at conventional resolutions (2–3 mm voxel width, 3T scanner). It is unclear to what extent this information originates from different spatial scales of neuronal selectivity, ranging from orientation columns to global areal maps. According to the global-areal-map account, fMRI orientation decoding relies exclusively on fMRI voxels in V1 exhibiting a radial or vertical preference. Here we show, by contrast, that 2-mm isotropic voxels in a small patch of V1 within a quarterfield representation exhibit reliable opposite selectivities. Sets of voxels with opposite selectivities are locally intermingled and each set can support orientation decoding. This indicates that global areal maps cannot fully account for orientation information in fMRI and demonstrates that fMRI also reflects fine-grained patterns of neuronal selectivity.
Highlights
Visual orientation is known to be represented in columnar preference patterns in the primary visual cortex (V1) at a sub-millimetre scale[1]
Two spirals with orientations 45° and −45°, respectively, relative to the radius are orthogonal to each other everywhere. They are balanced about the radial direction everywhere, and radial preference cannot account for their decodability
Filtering analysis is not able to conclusively determine whether fine-grained activation patterns contribute to orientation decoding because coarse-scale neural effects can give rise to spurious high-spatial frequency functional magnetic resonance imaging (fMRI) pattern information if adjacent voxels have different sensitivity to local neural activity
Summary
Visual orientation is known to be represented in columnar preference patterns in the primary visual cortex (V1) at a sub-millimetre scale[1]. Filtering analysis is not able to conclusively determine whether fine-grained activation patterns contribute to orientation decoding because coarse-scale neural effects can give rise to spurious high-spatial frequency fMRI pattern information if adjacent voxels have different sensitivity to local neural activity. If orientation decoding of gratings and spirals originated solely from coarse-scale radial and vertical preferences, respectively, one would not expect voxels in a local cluster to exhibit reliable opposite preferences.
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