Cerebral Cortex Advance Access originally published online on September 8, 2005
Cerebral Cortex 2006 16(6):896-906; doi:10.1093/cercor/bhj033
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Boundary Cue Invariance in Cortical Orientation Maps
McGill Vision Research Unit, Department of Ophthalmology, McGill University, Montreal, QC, Canada H3A 1A1
Address correspondence to Chang'an A. Zhan, McGill Vision Research Unit, Department of Ophthalmology, McGill University, 687 Pine Avenue West, H4-14, Montreal, QC, Canada H3A 1A1. Email: changan.zhan{at}mcgill.ca.
We effortlessly perceive oriented boundaries defined by either luminance changes (‘first-order’ cues) or texture variations (‘second-order’ cues). Many neurons in mammalian visual cortex show orientation preference to both types of boundaries, but it is uncertain how they contribute to perceptual orientation cue-invariance at the neuronal population level. Using optical imaging in cat A18, we observed highly similar orientation preference maps to first-order and a variety of second-order visual stimuli. Thus the neuronal representation of coarse-scale boundary orientation appears to be invariant to the characteristics (including local orientation) of the fine-scale textures by which those boundaries are defined. A common feature of second-order visual stimuli is that modulation shifts their Fourier energy for boundary orientation to the higher spatial frequencies of their constituent textures — our results suggest a common neural mechanism (demodulation) mediating visual processing of many kinds of texture boundary. The similarity between orientation maps to different stimuli implies that second-order responsive neurons are homogeneously distributed across the cortical surface. Such homogeneously cue-invariant orientation representation could provide a neural substrate for perceptual form-cue invariance, and reflect an optimal organization for encoding orientation information in natural scenes.
Key Words: form-cue invariance • illusory contours • non-Fourier • optical imaging • second-order
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