Public Release: 

Blind more accurate at judging size than sighted

Association for Psychological Science

Close your eyes and imagine a loaf of bread. With your eyes still closed, estimate with your hands the size of that loaf of bread. Do you think your mental representation is an accurate one? Specifically, how accurately have you gauged its size? According to researchers from the University of Otago in New Zealand, you probably overestimated the size of the bread. That is, unless you are blind.

Researchers Melissa Smith, Elizabeth A. Franz, Susan M. Joy, and Kirsty Whitehead, of the University of Otago in New Zealand, found that "blind individuals were more accurate than sighted individuals in representing the size of familiar objects."

Their findings are presented in the study, "Superior Performance of Blind Compared with Sighted Individuals on Bimanual Estimations of Object Size," in the January 2005 issue of Psychological Science, a journal of the American Psychological Society.

This research sought to examine the accuracy of memory representations by instructing participants to imagine a set of familiar objects (e.g., a can of soda, a loaf of bread, a carton of eggs) and then demonstrate the size of each object with their hands, without being able to see. Participants included both sighted individuals (who closed their eyes for the task) and blind individuals.

"Surprisingly, in over one hundred participants with normal vision, a marked overestimation in object size was demonstrated, suggesting that the visual-memory representations in sighted individuals might not be accurate after all," Franz said. Meanwhile, blind participants showed no overestimation and were more accurate in estimating object sizes.

The researchers argue that in people who are blind, the memory of familiar objects relies only on manual (not visual) representations that are based on their experiences holding the actual objects. In sighted individuals, however, memory of familiar objects relies on visual-memory representations. The authors believe visual representations may be inaccurate in size because "sighted individuals see objects everyday in different orientations and from different distances."


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