Local and global contributions to shape discrimination

Abstract

Humans are remarkably sensitive in detecting small deviations from circularity. In tasks involving discrimination between closed contours, either circular in shape or defined by sinusoidal modulations of the circle radius, human performance has been shown to be limited by global processing. We assessed the amount of global pooling for different pattern shapes (different radial modulation frequencies, RF) when circular deformation was restricted to a fraction of the contour. The results show that the improvement in performance depends on the modulation frequency (the pattern shape) when increasing the number of cycles of an RF pattern. Global processing only extends up to modulation frequencies between 5 and 10. For higher frequencies, performance can be predicted by probability summation. Position uncertainty cannot explain these effects. In a circumstance where global pooling exceeds probability summation (RF=5), we split the pattern up into five identical segments conserving the total amount of information presented. Thresholds are significantly affected by different global arrangements of these segments: (a) Occluding small parts of the pattern shows a significant effect on the position of occluders with performance lowest when gaps are placed at the points of maximum curvature. (b) Shifting segments away from the pattern centre (exploded condition) or displaying them out of concentric context (spiral condition) shuts down global processing. (c) Jittering segments radially disrupts both global and local processing. We conclude that RF patterns in the global processing range are analysed by detecting the points of maximum curvature and that, in this range, the visual system can only reliably process up to about 5 local curvature extrema.

Discriminating facial expressions of emotion and its link with perceiving visual form in Parkinson's disease

“…Perceiving visual form initially requires accurate encoding of local orientation information, and then integrating separate features into global forms and segregating them from their backgrounds [42,67,69]. RF pattern discrimination is thought to tap intermediate stages of visual form processing [41], where separate features are pooled into global forms.…”

Global Shape Processing Deficits Are Amplified by Temporal Masking in Migraine

“…Human sensitivity for detecting subtle deformations of circular shapes is in the hyperacuity range. 25 Psychophysical, [26][27][28] monkey physiology, 29 and human fMRI 30 studies support the view that the high sensitivity may be a result of processing at intermediate, extrastriate stages. 31 According to the reduced inhibition hypothesis, if masking was a consequence of inhibition, migraineur performance should be similar to controls without a mask, but better than controls when masked.…”

“…Consistent with psychophysical and imaging studies, the output of RF templates should depend on a number of factors, including RF contour contrast, 28 amplitude, 26,30,36,55 radial frequency, 26,28,55 size, 56 and orientation. 36 We only consider contrast (c and c m ) and amplitude (A and A m ) here as the other parameters did not vary in the experiments:…”

Aging and the integration of orientation and position in shape perception

“…The high sensitivity for discriminating amplitude modulations of radial frequency patterns provides strong evidence that shape perception is driven by a global mechanism that combines information across the circumference of the contour Habak, Wilkinson, & Wilson, 2006;Hess et al, 1999;Jeffrey, Wang, & Birch, 2002;Loffler et al, 2003;Schmidtmann et al, 2012;Wilkinson et al, 1998). This global pooling mechanism appears to be restricted to shapes with low radial frequency modulation (Schmidtmann et al, 2012;Wilkinson et al, 1998) and to be tuned to a specific type of shape and amplitude of radial modulations (Bell, Dickinson, & Badcock, 2008;Bell & Kingdom, 2009;Habak, Wilkinson, Zakher, & Wilson, 2004;Schmidtmann et al, 2012Schmidtmann et al, , 2013.…”

“…Shape perception is an intermediate-level process that appears to integrate orientation information across space using both local and global mechanisms (e.g., Achtman, Hess, & Wang, 2003;Bell, Gheorghiu, Hess, & Kingdom, 2011;Hess, Wang, & Dakin, 1999;Loffler, 2008;Loffler, Wilson, & Wilkinson, 2003;Schmidtmann, Gordon, Bennett, & Loffler, 2013;Schmidtmann, Kennedy, Orbach, & Loffler, 2012;Wilkinson, Wilson, & Habak, 1998). Studies examining whether shape perception is altered in older age have found mixed results (Habak, Wilkinson, & Wilson, 2009;Mayhew & Kourtzi, 2013;McKendrick & Battista, 2013;McKendrick et al, 2010;Rivest, Kim, Intriligator, & Sharpe, 2004;Y.…”