Ponzo Illusion

Two bars on a railway line

Two horizontal bars of exactly the same length lie across a pair of lines that converge towards the top, like railway tracks running off into the distance. The bar near the top, where the lines are closer together, looks longer than the one lower down.

Distance cues and size constancy

Converging lines are one of the strongest depth cues there is. Parallel edges such as railway tracks, corridors and roads appear to come together as they recede, and your visual system has learned that pattern so well that it applies it automatically, even to a simple diagram like this one.

Once the lines are read as distance, something called size constancy kicks in. This is the visual system's habit of keeping objects at their true size as their distance changes: a person walking away from you doesn't seem to shrink, because your brain scales them up to compensate. The top bar is read as being further away, yet it produces the same image on your retina as the "nearer" bar, so your brain concludes that it must be physically longer. Scott Murray and his colleagues found that this scaling shows up as early as the primary visual cortex. An object that looked bigger because it seemed further away activated a larger area of V1, even though its image on the retina hadn't changed at all (Murray, Boyaci & Kersten, 2006).

The Müller-Lyer illusion also distorts length, but it depends on the shapes at each end of a line. The Ponzo illusion is a more direct demonstration of size constancy, driven by a sense of depth across the whole scene.

How I built the test

The test runs eight trials in a random order. In three of them you adjust the bottom bar to match the top one, and in another three you adjust the top bar. That way, a habit of always setting the bar a little too long or too short cancels out instead of being mistaken for the illusion. The last two trials take the converging lines away, and they measure how accurate your eye is anyway. That gives the other six trials something to be compared against: if the rails shift you by 12% while bare bars only put you 3% out, the difference is the illusion.

The adjustable bar starts at a random length each time, and I've shifted both bars sideways by different amounts so you can't simply check whether their ends line up. I also gave the bars square ends instead of rounded ones, so the length you see is exactly the length being measured.

Your Ponzo score and your Ebbinghaus score

When Samuel Schwarzkopf, Chen Song and Geraint Rees measured how strongly people experienced the Ponzo illusion and the Ebbinghaus illusion, they found that both were stronger in people whose primary visual cortex (V1) has a smaller surface area. Yet the two scores didn't predict each other: someone strongly fooled by one could be barely affected by the other (Schwarzkopf, Song & Rees, 2011). That suggests the two illusions, which look so similar on paper, rely on different parts of the brain's size machinery. The Ebbinghaus page has the same kind of test, so you can compare your own two scores.

The same trick, everywhere

You'll come across the Ponzo effect well outside the lab. It's thought to contribute to the moon illusion, where the moon looks bigger on the horizon than it does high in the sky. Photographers and film-makers use it on purpose, too: placing someone near the vanishing point of a corridor or a road makes them look larger and more imposing than they would without those lines. Forced perspective takes the same idea out into real, three-dimensional space.

Ponzo's lines

The Italian psychologist Mario Ponzo described the illusion in the early 1910s. It's usually dated to 1911, although his paper appeared in 1912 (Ponzo, 1912), and he was interested in why the moon looks bigger near the horizon. Herschel Leibowitz and his colleagues later showed that the illusion grows stronger when the converging lines are part of a real photographed scene with plenty of depth cues, which supports the size-constancy explanation (Leibowitz et al., 1969).