The World’s Oldest Optical Illusion

In the October 1892 issue of Fliegende Blätter, a German humor magazine, there appeared an image depicting an optical illusion. The image was a sketch of a rabbit’s head, or was it a duck’s head? Both images seemed to switch back and forth from being a duck then being a rabbit. Half a century later, the image appeared again in Ludwig Wittgenstein’s 1953 book Philosophical Investigations, where the famed philosopher used the example to illustrate how certain figures could be interpreted in more than one ways. Over the next seventy years, the duck-rabbit illusion would appear countless number of times in books, magazines and websites.

The duck-rabbit illusion is a classic one, but it’s certainly not the first.

Throughout human history, artists have used different techniques to deceive the audience, such as create an impression of depth in flat frescoes using a clever combination of brushstrokes and shadows, also known as the trompe-l'œil effect. Some evidence suggests that even Paleolithic artists who decorated cave walls with drawings of horses and bison made cunning use of the natural bulges of the rock to add a sense of volume and depth to their artwork. The prehistoric artworks found in the cave of Font-de-Gaume, in France, is especially noteworthy.

Prehistorian Duncan Caldwell, who has surveyed the Paleolithic art of several caves in France, has discovered that throughout the cave of Font-de-Gaume there appears engravings and drawings of wooly mammoths and bison that often share certain lines or other features, creating overlapping images that can be read first as one animal, then the other, just like the duck-rabbit illusion. Caldwell says that he found the mammoth-bison pair appearing so regularly throughout the cave that he it can’t be simply accidental. Indeed, Caldwell also found a small figurine that had been carved to give it the details of a bison on one side and those of a mammoth on the other. Caldwell posits that the physical similarities between the two animal shapes made them ideal candidates for prehistoric artists to superimpose their respective images in the same depiction.

The contours of a bison and a mammoth overlap in this sketch of prehistoric art, copied from the walls of the Font-de-Gaume cave by the cave art researcher Henri Breuil. Credit: Duncan Caldwell

Also read: A 19th century optical illusion at the Church of St. Ignatius

Another dramatic example of the mammoth-bison illusion appears on a carving from a spear-thrower from the site of Canecaude. In this piece, both the mammoth and the bison are shown sharing the same contour. In addition, there are two small details, as Caldwell sees it, that allows the entire image to be read as either of the two species, and seeing one causes the other to "disappear."

This carving from a spear-thrower features one image that can be seen two different ways. The eyes above and below the crescent, which serves both as a mammoth’s tusk (for the upper eye) and bison’s horn (for the lower eye) are highlighted in red in the photo on the right. Images courtesy of Duncan Caldwell

Andrew Howley writes in National Geographic:

The details in question are the eyes. Caldwell describes how there is "both an upper eye, which turns the crescent beneath it into a tusk, and lower eye, beside the front leg, that transforms the same crescent which we just interpreted as a "tusk", into a bison's overhead horn." Looking back and forth between the eyes then, we are able to see the entire shape transform from one animal to the other, an effect much more like the classic Gestalt shift of the duck-rabbit.

Howley writes that it’s hard to think of any other reasons other than that the delicately carved shapes were made intentionally, and are not just accidental markings. The details of the body of the animal, its tusk/horns, long hair, and legs, and are also realistically represented showing the artistic ability of its creator.

It’s possible, however, that the image is just that of a mammoth and not a mammoth-bison combo, especially since some pieces of the spear-thrower are missing. In the book Mammoths: Giants of the Ice Age, authors Adrian Lister and Paul Bahn propose that the mammoth's trunk would have looped back in the lower-left part of the carving, making the lower "eye" the curled up end of the mammoth's trunk.

Another example of an early optical illusion can be found in the Airavatesvara Temple in the South Indian state of Tamil Nadu. This illusion is unambiguous.

This bas-relief clearly shows two animals sharing a single head. Depending on which creature you focus, you should see an elephant or a bull. This temple was built by Chola emperor Rajaraja II in the 12th century, making it the oldest verified optical illusion in the world.