Each of the following sounds contains a pair of tones. Your one and only job is to vote on whether the second tone is higher or lower in pitch than the first.
The brain is a tricky little troublemaker! Not everyone perceives these sounds the same. All four tones are examples of what is known as the "Tritone Paradox."
The Tritone Paradox was first demonstrated by psychology of music researcher Diana Deutsch in 1986.
Each pair consists of two "Shepard tones" separated by half an octave (also known as a tritone). A Shepard tone is a computer-produced tone that is composed of six sine waves that are separated by octaves, she told BuzzFeed Science.
There is no right answer, Deutsch says.
"There is no correct answer as to which tone is higher and which is lower," she explained, "because the other components of a complex tone that are needed to define a tone's height are missing."
She goes on to explain that a harmonic complex tone, which is what many musical instruments produce, consists of the fundamental frequency and different multiples of that frequency (if the pitch is 100 Hz, then the other components of the noise would be 200 Hz, 300 Hz, 400 Hz, etc). "The brain determines the pitch of the complex tone by considering all the components," she said.
Shepard tones, in contrast, consist only of tones that are separated by octaves (which means that the different waves are related in a 2:1 pattern). Since the superimposed sound waves are related differently, your brain can't actually perceive the pitch height of a tone. Essentially, your brain improvises on the spot. As a result, Deutsch explained, "there is no correct answer to the question of which tone is higher and which is lower."
Here's something just as crazy: Where you grew up and what language you learned first is actually a pretty good predictor of how you perceive these sounds!
In one study by Deutsch, for example, a significant percentage of subjects from California heard a pattern as ascending when subjects from the south of England heard the identical pattern as descending, and vice versa.