Document Detail

The precision of size constancy.
MedLine Citation:
PMID:  1455718     Owner:  NLM     Status:  MEDLINE    
The precision of objective size judgments, made when target disparity changed at random from trial-to-trial, was compared to the precision of angular size judgments made under the same condition. Subjects judged incremental changes in the vertical distance separating a pair of horizontal lines. For the objective judgments (in cm), the angle subtended by the target separation decreased with increasing depth consistent with the natural geometry of physical objects. For the angular judgments (in arc min), the angular separation did not change with disparity. For separations subtending an angle < 10 arc min, objective thresholds were considerably higher than angular thresholds, indicating that size constancy does not function well at small scales. At larger scales (> 20 arc min), the Weber fractions for angular and objective thresholds were nearly equal (approximately 6%) for two of the three subjects. These same two subjects also learned to judge "objective size" when angular subtense systematically increased with increasing depth in an exact inversion of the natural relationship. Although their "anti-constancy" judgments were less precise (approximately 9%) than their constancy judgments, the fact that subjects could learn this task with little practice suggests that constancy itself may be a learned response. Angular thresholds for targets presented only in the fixation plane were significantly lower than the angular thresholds measured with random changes in disparity, showing that observers with normal stereopsis do not have direct access to information about the angle subtended at the retina.
S P McKee; L Welch
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Publication Detail:
Type:  Journal Article; Research Support, U.S. Gov't, Non-P.H.S.; Research Support, U.S. Gov't, P.H.S.    
Journal Detail:
Title:  Vision research     Volume:  32     ISSN:  0042-6989     ISO Abbreviation:  Vision Res.     Publication Date:  1992 Aug 
Date Detail:
Created Date:  1993-01-06     Completed Date:  1993-01-06     Revised Date:  2007-11-14    
Medline Journal Info:
Nlm Unique ID:  0417402     Medline TA:  Vision Res     Country:  ENGLAND    
Other Details:
Languages:  eng     Pagination:  1447-60     Citation Subset:  IM    
Smith-Kettlewell Eye Research Institute, San Francisco, CA 94115.
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MeSH Terms
Depth Perception / physiology
Discrimination (Psychology) / physiology
Sensory Thresholds / physiology
Size Perception / physiology*
Vision Disparity / physiology
Grant Support
P-30-EY-01186/EY/NEI NIH HHS

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