Delboeuf illusion causes dentists to drill bigger holes

The Delboeuf illusion is an optical illusion whereby a large context makes a small, enclosed area appear smaller than it really is. Researchers have found that this illusion is having an impact in the field of dentistry, with some dentists making larger holes in teeth than they really need to.

Professor Robert O’Shea, a psychology expert in visual perception from Southern Cross University, teamed up with two researchers in dentistry from New Zealand’s University of Otago - Associate Professor Nicholas P Chandler and Dr Rajneesh Roy. Their research paper has been published in the journal PLOS ONE.

“Healthcare depends, in part, on the ability of a practitioner to see signs of disease and to see how to treat it,” the researchers said in the journal. “Visual illusions, therefore, could affect healthcare. Yet there is very little prospective evidence that illusions can influence treatment.”

The researchers sought such evidence by supplying eight specialist dentists (endodontists) with at least 21 isolated teeth, each containing holes.

“We asked the endodontists to cut cavities in preparation for filling,” the researchers said. “Each tooth presented a more or less potent version of a visual illusion of size, the Delboeuf illusion, that made the holes appear smaller than they were.”

Professor O’Shea said the finding was “quite a surprise”; under some circumstances, dentists remove more of a tooth than necessary. He said this has negative implications such as “prolonging the operation … exposing patients to the sound of the cutting instrument for longer than necessary … [and exposing] patients to increased risk of complications after the procedure, such as cracking or splitting of the tooth”.

Associate Professor Chandler added, “These complications may require the tooth to be removed, leading to more expense for the patient.”

The researchers noted, “The size of cavity endodontists made was linearly related to the potency of the Delboeuf illusion.” Professor O’Shea stressed that the cavity size was not just related to the size of the tooth, as, “The critical aspect for the illusion is the ratio between the size of the tooth and the size of the small area.

“We found that dentists make larger holes in small teeth too, if the ratio was in the range that produces the illusion,” he added.

Professor O’Shea theorised, “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.’”

Associate Professor Chandler suggested that knowledge of the illusion could be incorporated into the early stages of clinical training. Professor O’Shea added that the research could be relevant to all providers of healthcare.

“When operating, healthcare providers generally try to save as much healthy tissue as possible,” he said. “It is important for them to know that their eyes can deceive them into removing more healthy tissue than necessary.”