These 115 Photos Show How Much Perspective Can Change What You See (New Pics)

Illusions have a long history, dating back to ancient Greece.

In 350BC, Aristotle noted that “our senses can be trusted, but they can be easily fooled”.

He noticed that if you watch a waterfall and shift your gaze to static rocks, the rocks appear to move in the opposite direction of the water's flow, an effect we now call the “motion aftereffect” or the waterfall illusion.

Tracking the flow of the water seems to “wear out” certain neurons in the brain as they adapt to the motion. When you shift your gaze to the rocks, other competing neurons overcompensate, creating the illusion of movement in the opposite direction.

But the real boom in the study of illusions began in the 19th Century. A school of scientists who studied perception, among many other things, created simple illusions to shed light on how the brain perceives patterns and shapes, which kick-started the early theories on how our eyes can play tricks on our minds.

The Ebbinghaus illusion, for example, revealed that our brain makes judgments about size using adjacent objects, and this can be manipulated. (The orange circles in that picture are actually the same size.)

“[Early illusions] were of interest theoretically because they went against the prevailing view that you could understand vision if you understood the way in which an image is formed in the eye,” says illusion historian Nicholas Wade from the University of Dundee in Scotland.

“The phenomena were small but reliable; they were experimentally tractable and it generated this incredible boom of variations on simple figures.”

Yet this period also saw a series of misguided attempts to find a ‘unifying theory’ of illusions. The literature on illusions is “littered with over-interpretations”, Wade adds.

Today, illusion research is much more advanced. Thanks to modern technology, scientists can actually look inside our brains as we view images like the ones on this list. Functional magnetic resonance imaging (fMRI), for example, lets researchers see how our neurons respond to specific visual illusions.

Because of this, the way we understand perception has also shifted. For instance, one school of thought suggests that what we see is shaped by the brain’s constant effort to predict what will happen next. The idea is that many illusions exist because we’re always trying to stay one step ahead, compensating for the tiny delay between an event and our awareness of it. In other words, what you think you’re seeing is often your brain’s best guess, not a perfect snapshot of reality.

“Although our sensations feel accurate and truthful, they do not necessarily reproduce the physical reality of the outside world,” neuroscientists Stephen L. Macknik and Susana Martinez-Conde write.

“Of course, many experiences in daily life reflect the physical stimuli that send signals to the brain. But the same neural machinery that interprets inputs from our eyes, ears and other sensory organs is also responsible for our dreams, delusions and failings of memory. In other words, the real and the imagined share a physical source in the brain.”

So, they recommend taking a lesson from Socrates: “All I know is that I know nothing.”