Blind ‘see’ size better in tests

Close your eyes and imagine a familiar object, such as a loaf of bread. Keep your eyes closed, and indicate the length of the loaf between your hands. Now, open your eyes.

Researchers at Otago University say you will probably have overestimated the size of the loaf - unless you are blind.

They concluded that people who are blind are more accurate than sighted people in representing the size of familiar objects because sighted people tend to have distorted visual memories.

The study's blind participants - 24 volunteers from the Foundation of the Blind - relied on manual representations less influenced by visual experience. Participants showed no overestimation, and were more accurate in estimating object sizes.

The study - Superior Performance of Blind Compared with Sighted Individuals on Bimanual Estimations of Object Size, by Melissa Smith, Liz Franz, Susan Joy, and Kirsty Whitehead - was published in Psychological Science, a journal of the American Psychological Society.

The accuracy of volunteers' visual memory was tested by asking them to imagine a set of familiar objects such as a soft-drink can, a loaf, and a carton of eggs, and then demonstrate the size of each object with their hands, without being able to see.

"Surprisingly, in over 100 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," Ms Franz said.

The researchers said in their paper that people who were blind remembered familiar objects based on manual, rather than visual, representations from handling such objects. Sighted people relied on visual memories.

The researchers argued that such visual representations might be less accurate because sighted individuals see objects every day in different orientations and from different distances.

"Sighted individuals, when asked to perform object estimations from memory, tend to overestimate the size of objects that are frequently seen and manipulated in the space around the body," the researchers reported.

Each object was tested 10 times for the comparisons.

Half of the blind people said they imagined holding the object while assessing its size. But sighted people most commonly imagined the object in relation to a body part.

- NZPA