See Inside August/September 2006

The Quirks of Constancy

Even when we consciously know two lines are the same length, why can't we help seeing them as different?

ILLUSIONS are anomalies that can reveal clues about the mysterious workings of the brain to neuroscientists in much the same way as the fictional Sherlock Holmes can solve a crime puzzle by homing in on a single out-of-the-ordinary fact. Think of the phrase “the dog that did not bark” (in Sir Arthur Conan Doyle's short story “Silver Blaze”) or of the missing dumbbell (in Conan Doyle's Valley of Fear).

Perhaps the most famous examples of such visual tricks are the geometric optical illusions. In the Ponzo illusion (a), first demonstrated by Italian psychologist Mario Ponzo in 1913, one horizontal line looks shorter than the other one, although they are identical. In the Mueller-Lyer illusion (b, on opposite page), created by German psychiatrist Franz Mueller-Lyer in 1889, the line bounded by the diverging arrowheads looks shorter than the one with converging arrowheads—although they, too, are identical.

This is only a preview. Get the rest of this article now!

Select an option below:

Customer Sign In

*You must have purchased this issue or have a qualifying subscription to access this content

It has been identified that the institution you are trying to access this article from has institutional site license access to Scientific American on
Click here to access this article in its entirety through site license access.

Share this Article:


You must sign in or register as a member to submit a comment.
Scientific American Holiday Sale

Scientific American Mind Digital

Get 6 bi-monthly digital issues
+ 1yr of archive access for just $9.99

Hurry this offer ends soon! >


Email this Article


Next Article