Auditory Illusions: A Virtual Barbershop and the McGurk Effect
Illusions of Sound: Do You Hear What I Hear?
Most people are under the impression that you hear with your ears. While it is true that the ears are the sense organs which allow a human to sense the presence of sound, the true ability to hear and listen to the world around us requires the human brain. Our brains interpret sound wave vibrations to create meaning: turning the complex tones of human speech into a message that carries meaning, and allowing humans to enjoy complicated melodies.
Since the human brain is what allows us to hear, it can often be tricked. Nearly everyone has heard of optical illusions, but few have heard of auditory illusions. Stereo headphones will be required to hear some of the auditory illusions, so have a set handy if you want to experience the “barbershop effect.”
Video of the McGurk Effect
The McGurk Effect: When Vision Affects Sound
Watch the video at the right with your eyes open. Listen carefully to the sound produced. Next, listen to the video with your eyes closed – what is the man saying now? If you are like 98% of adults, the sound you hear when your eyes are open will be “da – da- da.”
In actuality, the man is saying, “ba – ba – ba,” but the video of the man shows the lip movements of “ga – ga – ga.” The human brain takes the auditory and visual information into account, and fuses the two streams of information – producing a “da – da – da” sound in the brain.
You can watch the video sequence over and over again, and the effect will hold up (try closing your eyes halfway through the video stream and hear the change in the sound). The sound never actually changes, but the visual input from your eyes changes the way your brain perceives the sound.
This effect demonstrates how integral vision and hearing are to the understanding of human speech. Studies have shows the McGurk effect holds up at the sentence level, and is effective with people from all language backgrounds.
Virtual Barbershop Illusion (Headphones Required)
The Virtual Barbershop Auditory Illusion
The barbershop illusion requires stereo headphones. Before listening to the video on the right, make sure to wear stereo headphones. The barbershop illusion gives the listener the feeling that a barber is moving around the head, clipping hair. This illusion demonstrates the ability of human beings to localize sound: as the sound increases in intensity in one ear, we “feel” as if the barber is on that side of our head. This ability to localize sound gives us a great deal of information about our environment: which direction an emergency vehicle with a siren is coming from, for instance, or where a speaker is located.
What Is Elmo Saying?
Sometimes a string of nonsense will appear to form a coherent message. Several “babbling” toys have been accused of having subliminal or harmful messages, due to the human brain’s attempt to turn the meaningless sound into a meaningful message.
A phrase from the Sesame Street character Elmo hit the news after parents heard the phrase, “Who wants to die?” coming out of a toy. An example is in the video on the right: Potty Time Elmo appears to have a sinister message, though the saying is a construct of the mind. The character is not saying “who wants to die,” but “who wants to try?” Since the clarity of the sound is poor, our minds create a meaning from the unclear speech.
Another example is from a cuddly doll which appeared to state, “Islam is the light!” Store shoppers were stopped in their tracks by the Little Mommy Real Loving Baby Cuddle and Coo doll. The doll, however, said no such thing. The doll’s gibberish was construed as a message in the listener’s mind, though the actual babble was devoid of meaning.
Diana Deutsch, a researcher at the Unversity of California, San Diego, can reproduce the phantom word phenomenon in the laboratory. Two-syllable words were recorded and then played with the syllables alternating between the ears. The sounds heard by the listener were ambiguous, but nearly every listener managed to “create” a meaningful word from the noise. Women often heard the word “love,” while men often reported explicit words. Listeners who spoke a foreign language would hear words in their native language, though all the recordings were created from English words.
The background, beliefs, and emotional state of the listener all contribute to the meaning given to the phantom words.
Shepard Tones Video
Listen to the octave at the right: it appears to continually descend (or ascend) in tone. Roger N. Shepard noticed that when 12tones were played in a loop, the brain interpreted the tones as continually ascending – the tones appear to get higher and higher, though the same twelve tones are simply being played over and over again. Since each note is higher than the one before it, the brain constructs the sound of continually ascending pitch.
And equivalent optical illusion would be M.C. Escher’s continually escalating stairs, which couldn’t exist in the real world.
The Auditory Equivalent to M.C. Escher
Tritones are musical intervals consisting of three whole tones. The sound produced by tritons was considered to be evil, and the sound was termed diabolus in musica (the devil in the music) in the 1700’s.
When sequentially played Shepard tones are interjected with tritones, the tritone paradox occurs: some people hear the tones as continually ascending, while others hear the exact same tones as descending. Research has shown that native speakers of tonal languages hear the tritone paradox differently than those who speak non-tonal languages (specifically, native Vietnamese speakers heard the paradox differently from native English speakers).
Filling in the Gaps
When certain sounds of speech are omitted, the human brain will automatically fill in the blanks. Researchers have performed studies where certain phonemes (individual speech sounds) are omitted or replaced with noise. The listeners do not generally notice the missing phoneme, or are unable to identify which phoneme is missing. This effect is also known as the temporal induction of speech.
Falling Bells Illusion
While the pitch of the “falling bells” is actually getting higher through the sequence, the human brain hears the pitch get lower as the bells seem to fall through space. When the sound is looped, the illusion becomes apparent as the starting tone is much lower than the ending tone.
Stereo headphones are recommended for this illusion, as it creates the sensation that the bells are "falling through space."
Scale Illusion (Stereo Headphones Required)
Researchers have been able to induce something known as the “scale illusion,” where the brain changes the tone heard in each ear to create a melody. Typically, two scales are chosen (an ascending scale and a descending scale). The first note of the ascending scale is played in the right ear, for example, and then the second note of the descending scale is played into that same ear. The true sound presented is of alternating high and low tones. A similar scheme of notes is played into the left ear.
Instead of hearing alternating high and low notes, the listener’s brain often “groups” the high notes and low notes together. This causes the listener to hear an ascending – descending scale in one ear, and a descending – ascending scale in the other.
Handedness plays a part in the interpretation of the presented tones - left handed people tend to “hear” a low toned melody in the left ear and a high toned melody in the right (and the reverse is true for right-handed people).