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Ben S. Webb, Neil W. Roach, Jon W. Peirce; Masking exposes multiple global form mechanisms at intermediate levels of visual processing. Journal of Vision 2007;7(9):208. doi: 10.1167/7.9.208.
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© ARVO (1962-2015); The Authors (2016-present)
Human perceptual studies have demonstrated the existence of mechanisms at intermediate levels of visual processing that are specialised for the detection of radial and circular form. However, these configurations are only two members of a family of forms defined by their pitch angle, where radial and circular form have pitch angles of 0 and 90 degrees, respectively. It is not clear whether the visual system encodes the entire family of spiral forms with two cardinal mechanisms or multiple mechanisms each tuned to a different pitch angle. To determine which encoding strategy the visual system favours, we examined the effects of configural backward masking on human observers' ability to detect global structure in Gabor arrays with different pitch angles. Each array (generated anew prior to each trial) consisted of 100 Gabors (carrier sf 6 c/deg, envelope sd 0.166 deg) randomly positioned within a circular annular window (inner diameter 1 deg, outer diameter 10 deg). Four observers judged which of two stimulus intervals contained global structure. One interval (the ‘foil’) contained Gabor elements with random orientations; the other (the ‘test’) contained Gabor elements with a variable proportion of orientations coherent with a pitch angle chosen from the range 0–90 degrees. Both test and foil were followed by a masking image composed of Gabors with a given pitch angle. When mask and test had the same pitch angle we found a 2–3 three-fold elevation in thresholds for detecting global structure that could not be explained by local orientation masking. The magnitude and bandwidth of this effect was similar for all pitch angles tested, suggesting that two cardinal mechanisms are insufficient to encode the entire family of spiral forms. We conclude that there are multiple mechanisms at intermediate levels of visual processing tuned for spiral form.
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