Local and global contributions to shape discrimination

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 asses...

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Bibliographic Details
Published in:Vision research (Oxford) 2003-03, Vol.43 (5), p.519-530
Main Authors: Loffler, Gunter, Wilson, Hugh R., Wilkinson, Frances
Format: Article
Language:English
Subjects:
Biological and medical sciences
Circularity
Discrimination (Psychology) - physiology
Form Perception - physiology
Form vision
Fundamental and applied biological sciences. Psychology
Global processing
Humans
Pattern discrimination
Pattern Recognition, Visual - physiology
Perception
Photic Stimulation - methods
Psychology. Psychoanalysis. Psychiatry
Psychology. Psychophysiology
Psychophysics
Sensory Thresholds
Shape perception
Vision
Visual Pathways - physiology
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Summary: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.
ISSN:0042-6989
1878-5646
DOI:10.1016/S0042-6989(02)00686-7