Paintings containing carefully constructed illusions can make your brain flip as it tries to make sense of what it sees
WHAT do you see when you look at the painting above? A semi-naked woman in the foreground? Check. A crumbling building in the background? Check. Nothing too unusual there. But what about the turbaned figures under the archway? Look now, and you might see them talking to two other characters in black and white. But look again, and they may instead be admiring a bust of the philosopher Voltaire.
This is Salvador Dali鈥檚 Slave Market with the Disappearing Bust of Voltaire, which includes an illusion that lets observers perceive two possible images. Just in case you can鈥檛 see it, the rear arch of the building becomes the forehead of Voltaire. The heads of the black-and-white figures are his eyes, and their clothes his cheeks and chin.
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, a visual neuroscientist at the Barrow Neurological Institute in Phoenix, Arizona, explains that the ambiguity arises because our brain鈥檚 perception of the world is a rough approximation of reality. 鈥淥ur brain has to fit within our cranium so it cannot process everything that is out there,鈥� she says. So the brain takes short cuts, sampling only the most significant parts of the scene, such as the contours, the edges and the corners of objects. The rest is typically built around our memories of past experience and our expectations of what should be there.
This is particularly noticeable when the images are vague, says Martinez-Conde. Compare the level of detail in the non-ambiguous figure of the woman on the left, who is richly depicted from the creases of her turban to the tendrils of her hair, with that of Voltaire, whose ears are missing and whose mouth cannot be made out. Similarly, the hands and necks are missing from the black-and-white characters. 鈥淭here is a lot of information which needs to be filled in, and the brain can fill it in in a number of different ways,鈥� she explains.
The brain鈥檚 expectations often feed into the visual system, determining how these brain regions fill in the missing details and group different parts of the image. It鈥檚 the same reason that trompe l鈥檕eil works (page 36), and it means your previous experiences might determine whether you see the figures or Voltaire鈥檚 face first.
But why does the painting seem to flip between the two interpretations at random? Previous fMRI brain scans have suggested that two separate pools of neurons code each of the possible interpretations (). This led and Kristine Krug at the University of Oxford to hypothesise that we see one interpretation when the corresponding set of neurons fires more strongly than the set representing the other image. Neural responses fluctuate, however, so eventually the other pool of neurons will gain the upper hand. 鈥淭he flip occurs when one of these two almost warring populations of neurons, for reasons probably quite random, becomes more active,鈥� says Parker. He has tested this theory in monkeys using a similar illusion ().
Why would the human mind have developed this strange trait? 鈥淲e evolved to make sense of partial visual details and to make out a coherent picture even in poor lighting,鈥� says Martinez-Conde. 鈥淚llusions are a by-product of this. They represent the dissociation between objective reality and subjective perception.鈥�
Read more: Six ways that artists hack your brain
