You know can't always believe your eyes, as optical illusions—or "brain failures," as Neil deGrasse Tyson calls them—make clear. It turns you can't always believe your ears either. Recently the internet went nuts over a four-second audio clip that sounded like "Yanny," "Laurel," or both. Audiologists contend that the clip has two distinct tracks laid on top of each other at different frequencies. Scientists call this an aural illusion—and it's not the only one. Here are six others that will make you doubt what you hear.
1. BRAINSTORM OR GREEN NEEDLE
The Illusion: Twitter users bored with the Yanny/Laurel question have been sharing this equally divisive clip. Some people think the garbled recording says "brainstorm," while others hear "green needle." Many have discovered that their thoughts can change the outcome. If you repeat the phrase "green needle" in your head, that's exactly what you'll hear when you listen to the clip. But if you've got "brainstorm" on your mind, then "brainstorm" is the term your ears are going to pick up.
How It Works: The video is a clip from a 2014 YouTube toy review. Uploaded by critic DosmRider, it's about a plastic space station from the Ben 10 collectibles line. The playset comes with a loading dock for action figures that trigger different sounds when they get plugged in. A crab-like character called Brainstorm is represented by one of these models. Put him on the station, and his name blares from the speakers. While listening to the soundbite, many people thought the toy was saying "green needle."
The clip contains a variety of different acoustic patterns—some of which are consistent with the term "green needle" while others match "brainstorm." Your expectations of which words you'll hear—coupled with the low-quality audio—do the rest. "When faced with an acoustic signal which is somewhat ambiguous because it is low-quality or noisy, your brain attempts a 'best fit' between what is heard and the expected word," Valerie Hazan, a professor of speech sciences at University College London, told The Telegraph.
2. SHEPARD TONES
The Illusion: In the above video, you hear what sounds like a single, perpetually swelling tone. A common fixture in the movie scores of composer Hans Zimmer, whose work you've heard in films like Dunkirk and Interstellar, this effect makes us believe that we're hearing the impossible: sounds whose pitch seems to rise endlessly without ever peaking or actually getting louder.
How It Works: The clip is in fact three separate sounds being played together—what are called Shepard tones. Each of these is an octave higher than the one beneath it. When separated into individual tones, as this Vox video explains, you can hear that the highest tone fades in volume, the middle one remains constant, and the lowest one increases. Because we're constantly hearing two upward-moving waves, we convince ourselves that the three-layered sound (taken as a whole) is growing higher and higher at a steady pace. It works for tones moving down in octaves as well.
3. CIRCLES, BEEPS, AND SENSORY CONFUSION
The Illusion: The opening 15 seconds of this video contain two multisensory displays. In the first, a lone black circle flashes onto the screen. This is accompanied by one high-pitched beep. You will then see the exact same thing happen again, with another solitary black circle popping into view. But this time, there will be two beeping sounds instead of one. Even though the animation is identical in both runthroughs, some viewers think they can see two flashing circles in that second display.
How It Works: Dubbed the sound-induced flash illusion by its discoverers, the trick plays on the fact that your brain sometimes consults other senses to figure out what your eyes are seeing. That's how the back-to-back beeps can fool you into mistaking a single flash for two separate ones. Some people might be especially vulnerable to the illusion. A 2012 study found that in a pool of 29 volunteers, nearly everyone reported seeing the second flash in at least a few trial runs. However, participants with small visual cortexes—a region of the brain which deciphers optical signals—saw it way more often than their peers did.
4. THE MCGURK EFFECT
The Illusion: In the previous entry, sound may have changed what you saw. In this one, seeing might change what you hear. A man says "bah" over and over. Or does he? Turn off the sound and see the shape his mouth makes as he speaks. He's actually saying "fah."
How It Works: First documented in the 1970s by researcher Harry McGurk [PDF], the McGurk Effect involves an incongruence between audio information and visual information. The brain's desire to reconcile these incongruent inputs is so strong, it can change what you hear to align with what you see.
5. SPEECH TO SONG
The Illusion: Diana Deutsch, who teaches at the University of California, San Diego, is an authority on the psychology of music. One day in 1995, Deutsch was editing an audio lecture she'd recorded. The sentence fragment "sometimes behave so strangely" was playing on a loop in her office. As she heard repeated over and over again, the phrase began to sound less like talking (which it was) and more like singing. It's had the same effect on other people. In the above video, notice how, after a certain point, this spoken-word recording picks up a musical quality, even though the speaker never actually sings.
How It Works: It's a phenomenon Deutsch has named the speech-to-song illusion. Repetition is a core component of all music, and it seems our brains try to create little melodies out of statements or sounds repeated to excess. How or why this occurs isn't completely understood. As future experiments dissect the illusion, psychologists may learn new things about how the mind organizes and processes the things it perceives [PDF].
6. PHANTOM WORDS
The Illusion: Once you click play on the video above, some bombastic, repeating syllables are going to hit your eardrums. For best results, place yourself between two speakers, but a decent set of headphones should also do the trick. Amidst this aural onslaught, your mind will probably identify some recognizable words or phrases. Test subjects who've listened to this have reported hearing words such as "no brain," "window," "raincoat," "mango," and "Broadway."
How It Works: Have you ever looked at a bowling ball and thought the three holes on its side resembled a human face? That's called pareidolia. Something like that is going on here. We're hard-wired to seek out patterns, both visually and aurally. There are two tracks in this audio clip, with each containing an ambiguous word or two. These sounds mix together in the air and then reach your ears as an unrecognizable racket. Listen long enough, and sooner or later you'll begin to hear "phantom words"—words or statements that aren't really being said. Since humans crave patterns, we force ourselves to hear them.
This experiment was another brainchild of Diana Deutsch's. She's found that the phantom words a person hears are liable to reflect their current mood. For example, weight-conscious test subjects might hear food-related terms.