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Cerebral Cortex Advance Access originally published online on August 9, 2007
Cerebral Cortex 2008 18(5):1029-1041; doi:10.1093/cercor/bhm139
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© The Author 2007. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Critical Spatial Frequencies for Illusory Contour Processing in Early Visual Cortex

Chang'an A. Zhan1 and Curtis L. Baker, Jr2

1 Department of Physiology, McGill University, 2 McGill Vision Research Unit, Department of Ophthalmology, McGill University, Montreal, QC, Canada

Address correspondence to McGill Vision Research, 687 Pine Ave W, H4-14, Montreal, QC, Canada H3A 1A1. Email: curtis.baker{at}mcgill.ca.

Single neurons in primate V2 and cat A18 exhibit identical orientation tuning for sinewave grating and illusory contour stimuli. This cue invariance is also manifested in similar orientation maps to these stimuli, but in V1/A17 the illusory contour maps appear reversed. We hypothesized that this map reversal depends upon the spatial frequencies of the inducers in the illusory contours, relative to the spatial selectivities of these brain areas. We employed intrinsic signal optical imaging to measure orientation maps in cat A17/18 to illusory contours with inducers at spatial frequencies from 0.15 to 1.6 cpd. A17 illusory contour maps were indeed reversed compared with grating-driven maps for inducer spatial frequencies <1.3 cpd, whereas A18 maps were invariant. Simulations based on known neurophysiology demonstrated that map reversal can arise from linear filtering, and map invariance can be explained by a nonlinear (filter-rectify-filter) mechanism. The simulation also correctly predicted that A17 could show invariant maps when the inducer spatial frequency is sufficiently high (1.6 cpd), and that A18 maps could reverse at lower inducer frequencies (0.18 cpd). Thus, the map reversal or invariance to illusory contours depends critically on the relationship of the inducer spatial frequencies to the spatial filtering properties of neurons in each brain area.

Key Words: form-cue invariance • non-Fourier • optical imaging • orientation maps • second-order


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