Spinning Dancer Illusion

Which way is she turning?

A black silhouette of a dancer, one leg raised, turns steadily on the spot. Ask the person next to you which way she's spinning and you may well disagree: one of you sees her turning clockwise, the other anticlockwise, and you're both seeing the animation correctly. Keep watching and the direction may even flip for you, often the moment you stop concentrating.

The dancer in the demo isn't a copy of the original animation. It's a simple 3D model that your browser turns and flattens into a silhouette, which is all it takes to recreate the effect. The model only ever turns one way, and the "depth cue" checkbox proves it: shade the nearer parts lighter and the ambiguity vanishes.

Why both answers are right

The trick is what the picture leaves out. A silhouette has no shading, no face, no shadow and no perspective, nothing to say which side of the figure is towards you. A real turning figure gives the eyes a steady stream of depth cues: the nearer arm catches more light, limbs change shape as they turn, and two eyes see slightly different views. Remove all that and the flat shape on screen fits a figure turning one way just as well as its mirror image turning the other. The ambiguity is mathematically real, not a lapse of attention, and like the Necker cube, the dancer is a bistable figure: perception settles on one reading, then sometimes switches (Sterzer, Kleinschmidt & Rees, 2009).

Which way do visitors see her turn?

Most people who watch Nobuyuki Kayahara's original animation see the dancer turning clockwise: in an online poll of about 1,600 people that Nikolaus Troje and M. McAdam cite, roughly two-thirds did. Troje and McAdam tested whether that reflected a built-in preference for clockwise motion by re-creating the silhouette and rendering it from a range of camera heights. Across all their trials, clockwise and anticlockwise answers came out almost exactly even, so there was no clockwise bias. Instead, viewers favoured whichever reading put them above the figure looking down, and the pull got stronger the further the camera was from level (Troje & McAdam, 2010). In the original animation, the clockwise reading is the one that puts you above the dancer, which is what tips most people towards it.

The dancer on this page is rendered from exactly level, with no camera elevation at all, so neither reading puts you above her and that tie-breaker is gone. If Troje and McAdam are right, this site's visitors should split roughly evenly between clockwise and anticlockwise, unlike the original's lopsided result. The poll under the demo is there to find out: answer with the direction you saw first, before using the depth cue.

Not a left-brain, right-brain test

The dancer went viral in the late 2000s with a claim that the direction you see reveals whether you're "left-brained" (analytical) or "right-brained" (intuitive). There's no evidence for that. Which way you see her turn has nothing to do with brain hemispheres, an idea that oversimplifies how the brain divides its work anyway. What does affect it is the viewing angle, as Troje and McAdam showed, plus what you happened to be looking at and plain chance.

Steering the spin

You can influence the direction with some effort. Many people can flip it by imagining that the supporting leg is the left one or the right one, by following the raised foot and deciding whether it's passing in front or behind, or by glancing away and back while holding the other direction in mind. As with the Necker cube, the figure never changes; only your brain's current guess about its 3D shape does.

Kayahara's dancer

The Japanese web designer Nobuyuki Kayahara made the animation in 2003. It spread widely a few years later, usually with the brain-hemisphere claim attached, which probably helped it travel far further than a purely scientific illusion would have. Vision scientists have since used it and similar silhouettes to study how the brain recovers 3D shape from ambiguous movement. For still images that seem to move, see the rotating snakes.