They were made to delve into the depths of the mind and reveal its darkest secrets through the dancers, butterflies and occasional blood-soaked carcass that people saw when they looked at the patterns.

Nearly a century after Hermann Rorschach invented the ink blot test, the controversial assessment has all but vanished from psychiatric clinics. But even if the blots did not live up to their diagnostic promise, they have remained a source of intrigue to some scientists.



Key among the mysteries is why people see so many shapes and figures in the Rorschach blots, and how some of the strange patterns seem to conjure up more images than others. Writing on Tuesday in the journal Plos One, a group of scientists claim to have an answer.



Researchers led by Richard Taylor at the University of Oregon analysed the 10 Rorschach ink blots and found that the five black and white patterns varied in their fractal complexity. The less complex the fractals in the blots, the more images people tended to see.



Rorschach’s Blot Seven is shown at the top (a). Note the tell-tale fractal signatures of irregular curves or shapes at the edges of the symmetrical image. Some people see a woman’s head with a ponytail. Below (b) the inkblot has been altered with the fractal borders removed. The ability to see hidden patterns is reduced. Photograph: Courtesy of Richard Taylor

Fractals are patterns that repeat themselves over different size scales. The most familiar ones appear in nature, in the branching of trees, the edges of clouds, and the contours of coastlines. When the ratio of fine to coarse details is high, scientists say the image has a high fractal complexity.

“Your eyes are amazing pattern detectors, but why are they getting fooled? It’s almost as if they are getting trigger happy. There are seeing things that aren’t there,” said Taylor, whose work on fractals began with studies of Jackson Pollock’s spectacular drip paintings



Rorschach, a Swiss Freudian psychiatrist, came up with the ink blot test in 1921 when he published ten cards, five in colour and five in black and white. The blots were made by pouring ink onto cards, folding them, and pressing them tightly to produce the characteristic symmetrical patterns.



Though Rorschach died a year later, the blots were used widely in psychiatry to assess people’s personalities and mental health. In the Nuremberg trials, Hermann Göring, the founder of the Gestapo, said that blot number two resembled two men in a “fantastic dance”, while Rudolf Hess, appointed deputy führer to Adolf Hitler, saw in it a cross-section of an insect dotted with blood.



By the end of the 20th century, many psychiatrists realised that they could not read too much into the images people reported seeing. “Today, they are more seen as an indication of a person’s creativity. It’s not saying they’ve got a mental disorder because one person sees a hippopotamus and another person sees a naked woman,” said Taylor.



Taylor drew on records of the images people saw in different blots to show that as fractal complexity fell, the number of objects they saw rose. Blot one, the least complex, evoked nearly 300 different images, while the most complex prompted only 170.



With only five blots to base their results on, the scientists went on to create asymmetrical patterns of varying fractal complexity on a computer. Again, people reported seeing more images in the patterns when the fractal complexity was lower.



If the intriguing discovery is backed up by further work, Taylor believes it could have important implications for understanding the human visual system, and for building artificial “bionic” eyes.



But Mark Georgeson, who studies the human visual system at Aston University in Birmingham, believes the study raises new questions, such as why less complex contours should lead people to see more shapes, what features in the patterns are critical for the images they perceive, and whether other, non-fractal, differences in the images might strongly affect the types and shapes of images people see.



“That’s the remaining mystery,” Taylor said. “it’s got a lot of profound implications, not just for how we detect patterns, but for how our visual system gets fooled.