Medical school might be a long, slow slog, but once doctors have their training, they can often make diagnoses in a matter of moments. New research suggests that doctors actually identify an abnormality in less than two seconds—not much longer than it takes them to name an animal or a letter of the alphabet.

Twenty-five radiologists submitted to having their brains scanned while performing visual diagnoses of chest x-rays. Mixed in with images of abnormal chest x-rays were clean ones on which the outline of an animal or consonant had been superimposed to test the speed with which doctors recognized familiar objects. The researchers, led by Marcio Melo, of the Laboratory of Medical Informatics at the University of São Paulo, found that the same regions of the brain were active when doctors correctly identified any of the three objects. The findings were published online Wednesday in PLoS ONE.

"Diagnostic hypotheses in medical practice are often made by physicians in the first moments of contact with patients; sometimes even before the report of symptoms," Melo and the team wrote in their paper. Indeed, the doctors in the study—who had been prepared to see the variety of images—named the type of abnormality—such as cavitation or cardiomegaly—in an average of 1.33 seconds. Animals got named in 1.23 seconds.

The well-trained brains still had to work a little harder to come up with the medical terms for the conditions than for the more common visual clues, according to the study. The fMRI (functional magnetic resonance imaging) scans showed that although the diagnoses called on the same collection of areas around the brain as did animals and letters, they also put a much higher demand on the left inferior frontal sulcus and posterior cingulate cortex, which are areas of higher cognitive processing. The researchers stuck mostly with familiar conditions for the medical images so that the subjects wouldn't have to puzzle over a rare manifestation or term. Even so, many of the doctors recalled considering more than one name within the short time it took them to decide and offer an answer.

These quick diagnostic decisions might not come as much of a surprise given that radiologists are expected to have this sort of visual expertise and knowledge base. But ramifications for an accurate chest x-ray reading versus the importance of someone else correctly identifying a drawing of a camel are obviously a bit different.

"Clearly, a formal and definitive diagnosis cannot be made in seconds but its core cognitive process, the generation of diagnostic hypotheses which were the names of lesions in our study, is crucial for a final correct diagnosis," the researchers wrote. And knowing more about what's happening in doctors' brains as they diagnose patients—and the importance of snap judgments—might help improve medical training and decrease misdiagnoses.