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An optical illusion is making dentists drill holes that are too big

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There's an optical illusion where a small, enclosed area appears larger when surrounded by a larger area. This visual anomaly, called the Delboef illusion, is making the holes that dentists cut in preparation for fillings appear smaller than they really are.

This illusion was first documented by Franz Joseph Delboeuf, a Belgian philosopher and expert on hypnosis. He noticed that, if the same circle is placed inside two different concentric circles, its size will appear to vary. It's the same effect that causes people to put too much food on a large plate.


But in this case, it's causing dentists to drill drill oversized holes in your face.


Professor Robert O'Shea, an Australian visual perception expert, came to this surprising conclusion after teaming up with dentistry researchers from New Zealand. He asked them to prepare 21 teeth for root canals by drilling cavities in each. But after taking a look at their work, O'Shea discovered that all of the dentists drilled larger holes than necessary. The critical aspect for the illusion, it appears, is the ratio between the size of the tooth and the size of the small area.

This is a problem because removing more of the tooth than what's required can lead to the tooth splitting or cracking.

"We do not know if dentists are aware of this," noted O'Shea. "We think that dentists think, either consciously or unconsciously, after they have made a hole of a particular size: 'That looks rather small — I need to make that hole bigger'."

Awareness of the problem, he says, is key. In consideration of the new research, dentists, should consider measuring the area that needs to be drilled, instead of relying on their eyes to determine how big the hole needs to be.


Read the entire study for free at PLoS One: "Dentists Make Larger Holes in Teeth Than They Need to If the Teeth Present a Visual Illusion of Size."

Images: PLoS One/O'Shea et al.