Spring of 2018 will forever be remembered for the “Laurel” or “Yanny” battle. The four-second audio clip had families and friends in disbelief, unable to comprehend how anyone could hear anything different from what they heard.

The great debate, as repetitive as it was, shed light on the misleading nature of audio perception.

Now new research from the University of Kansas (KU) takes a deep dive into another form of misleading audio, the speech-to-song illusion.

“In the speech-to-song illusion a spoken phrase that is repeated many times begins to sound as if it is being sung, even though it is still being spoken,” said Michael Vitevitch, professor and chair of psychology at KU and lead researcher in the study.

Vitevitch and his team of undergraduate and graduate student researchers set out to find out why this illusion happens.

The findings

The team’s goal was to test if node structure theory, a model of language processing, could partly be responsible for our brain’s ability to hear spoken words as singing.

The researchers determined that fatigue in the brain’s word detectors was the primary reason behind the speech-to-song illusion.

“In node structure theory, there are detectors for words and detectors for syllables,” said Vitevitch. “The word detectors fatigue more quickly than the syllable detectors, sort of like sprinting muscles versus endurance muscles in your body.”

“The first few presentations of the spoken phrase are registered by the word detector as spoken words, but after a few more repetitions the word detectors tire out and don’t respond, so you lose the perception of the sounds as speech,” he continued. “The phrase is still being presented so the syllable detectors start to register that input. Syllables carry the rhythmic information of language, so since they are responding now your perception of the input shifts to something that is more rhythmic, music-like or song-like.”

The study

The team conducted six experiments to test their explanation.

They singled out phrases with many similar sounding words.

“We tried to take out words altogether by using Spanish words with non-Spanish speakers,” Vitevitch said in a statement.

“We tried focusing on the syllables and number of syllables,” he continued. “We looked at different characteristics, like is it the word that matters or the number of syllables?”

The researchers also created random lists of words, so that everyday speech couldn’t influence whether the subjects perceived something as music.

They tried everything they could to strip musicality from the phrases. They wanted to randomize the words so a phrase wouldn’t sound like a song the first time a subject heard it.

“When people hear it once, they said it didn’t sound musical at all,” Vitevitch said in a statement.

“The fact that we could get people to shift perception to something musical after several repetitions gives us confidence that we’re on the right track with the mechanism explaining the effect.”

Motivation for the study

Musicians, such as Steve Reich, have been using the speech-to-song illusion ever since the 1960s, but scientists didn’t start investigating it until the 1990s.

However, none of the scientists were ever able to give a reason to why we can hear repeated spoken word as song, until now.

“Some time ago I was reading through the table of contents of a recent journal and there was an article on the speech-to-song illusion which caught my attention because it was an auditory illusion, and one that was language-related, which is my main area of research,” said Vitevitch.

He continued to read articles and papers that demonstrated lots of interesting findings about the illusion.

Some explained that the illusion occurs in other languages, not just English.

In other studies, brain scans showed activity in the language areas when subjects heard speech, and activity was present in the music area when songs were perceived.

“But none of them gave an explanation about why the illusion happened in the first place,” said Vitevitch.

“Most of the previous studies were by music researchers, so as a language researcher I wondered if a model of language processing might be able to explain how the illusion occurred,” he continued. “That led to the studies in this paper which tested various parts of a language model to see if it was a viable explanation of the illusion.”

Conclusion

To most people, visual and auditory illusions, such as the “what color is the dress” debate in 2015 and the Laurel and Yanny battle, are just for fun. But, Vitevitch sees the illusions as great opportunities to further the understanding of music and speech perception.

“All scientists are trying to look inside of a black box to understand what’s going on inside,” Vitevitch said in a statement.

“We’re all trying to understand the universe or the brain or how atoms work. So, any opportunity to get a crack in the black box where you can look inside, you need to take. Things like illusions are often dismissed, but they’re unique opportunities to get another angle on what’s going on. Yes, they’re kind of fun and interesting and goofy and they get attention — but really they’re another opportunity to see what’s going on inside the black box.”