Masking exposes multiple global form mechanisms.

Previous work suggests there are mechanisms at intermediate levels of visual processing specialized for the detection of radial and circular form. The evidence in favor of specialized global form mechanisms is derived from structure detection experiments that have told us very little about their bandwidth or number. To address these related questions, we examined the effects of configural backward masking on human observers' ability to detect global structure in arrays with different spiral forms. Each array consisted of 100 Gabors randomly positioned within a circular annular window. Observers judged which of two sequentially presented Gabor arrays contained global structure. One array contained Gabors with random orientations; the other contained Gabors with a variable proportion of orientations coherent with a randomly chosen spiral pitch. At its offset, each array was immediately followed by a backward masking Gabor array with a fixed spiral pitch angle. When mask and test had the same spiral pitch, we found an approximately three-fold elevation of structure detection thresholds that was not explained by local orientation masking. The magnitude and breadth of tuning around each masking angle was predicted by a simple model consisting of at least eight detectors broadly tuned for different spiral forms.