How Your Brain Tells Speech and Music Apart
Threads Involved: Music
| From | johnwunder |
|---|---|
| To | Johnny |
| Date | 20241005152004 |
| Headline | How Your Brain Tells Speech and Music Apart |
| Source | Scientific American |
Simple cues help people to distinguish song from the spoken word
Analysis: It's all about the rhythm Web References https://www.scientificamerican.com/article/how-your-brain-tells-speech-and-music-apart/
People generally don’t confuse the sounds of singing and talking. That may seem obvious. But it’s actually quite impressive—particularly when you consider that we are usually confident that we can discern between the two even when we encounter a language or musical genre that we’ve never heard before. How exactly does the human brain so effortlessly and instantaneously make such judgments?
Scientists have a relatively rich understanding of how the sounds of speech are transformed into sentences and how musical sounds move us emotionally. When sound hits our ear, for example, what’s actually happening is that sound wavesare activating the auditory nerve within a part of the inner ear called the cochlea. That, in turn, transmits signals to the brain. These signals travel the so-called auditory pathway to first reach the subregion for processing all kinds of sounds, and then to dedicated music or language subregions. Depending on where the signal ends up, a person comprehends the sound as meaningful information and can distinguish an aria from a spoken sentence.
That’s the broad-strokes story of auditory processing. But it remains surprisingly unclear how exactly our perceptual system differentiates these sounds within the auditory pathway. Certainly, there are clues: music and speech waveforms have distinct pitches (tones sounding high or low), timbres (qualities of sound), phonemes (speech sound units) and melodies. But the brain’s auditory pathway does not process all of those elements at once. Consider the analogy of sending a letter in the mail from, say, New York City to London or Taipei. Although the letter’s contents provide a detailed explanation of its purpose, the envelope must include some basic information to indicate its destination. Similarly, even though speech and music are packed with rich information, our brain needs some basic cues to rapidly determine which regions to engage.
The question for neuroscientists is therefore how the brain decides whether to send incoming sound to the language or music regions for detailed processing. My colleagues at New York University, the Chinese University of Hong Kong, and the National Autonomous University of Mexico and I decided to investigate this mystery. In a study published this spring, we present evidence that a simple property of sound called amplitude modulation—which describes how rapidly the volume, or “amplitude,” of a series of sounds changes over time—is a key clue in the brain’s rapid acoustic judgments. And our findings hint at the distinct evolutionary roles that music and speech have had for the human species.
Past research had shown that the amplitude modulation rate of speech is highly consistent across languages, with a rate of four to five hertz, meaning four to five ups and downs in the sound wave per second. Meanwhile the amplitude modulation rate of music is consistent across genres, at about 1 to 2 Hz. Put another way: when we talk, the volume of our voice changes much more rapidly in a given span of time than it does when we sing.
Given the cross-cultural consistency of this pattern in past research, we wondered whether it might reflect a universal biological signature that plays a critical role in how the brain distinguishes speech and music. To investigate amplitude modulation, we created special white noise audio clips in which we adjusted how rapidly or slowly volume and sound changed over time. We also adjusted how regularly such changes occurred—that is, whether the audio had a reliable rhythm or not. We used these white noise clips rather than realistic audio recordings to better control for the effects of amplitude modulation, as opposed to other aspects of sound, such as pitch or timbre, that might sway a listener’s interpretation.
Across four experiments with more than 300 participants, we asked people to listen to these audio files and tell us if they sounded more like speech or music. The results revealed a strikingly simple principle: audio clips with slower amplitude modulation rates and more regular rhythms were more likely to be judged as music, and the opposite pattern applied for speech. This suggests that our brain associates slower, more regular changes in amplitude with music and faster, irregular changes with speech.
These findings inspire deeper questions about the human mind. First, why are speech and music so distinct in their amplitude over time? Evolutionary hypotheses offer some possible answers. Humans use speech for communication. When we talk, we engage muscles in the vocal tract, including the jaw, tongue and lips. Generally, a comfortable speed for moving these muscles for talking is around 4-5 Hz. Interestingly, our auditory perception of sound at this speed is enhanced. This alignment in speed, production and perception is likely not a coincidence. A possible, though still untested, explanation is that humans talk at this neurophysiologically optimized fast speed to ensure efficient information exchange—and this fast talking could explain the higher amplitude modulation rate in speech versus music.
On the other hand, one hypothesis about the evolutionary origin of music is that it effectively builds social bonds within a society by coordinating multiple people’s activities and movement , such as through parent-infant interactions, group dancing and work songs. Studies have shown that people bond more closely when they move together in synchrony. Therefore, it’s possible that for music to serve its evolutionary function, it needs to be at a speed that allows for comfortable human movement, at 1 to 2 Hz or below. Additionally, a predictable beat makes the music more appealing for dancing in a group.
There are still many questions to explore. More studies are needed to understand whether the brain is able to separate music and speech using acoustic modulation from birth—or whether it relies on learned patterns. Digging into such questions could have therapeutic potential. Understanding this mechanism could help patients with aphasia, a condition that affects verbal communication, understand language via music with carefully tuned speed and regularity. Our evolutionary hypotheses, too, warrant further investigation. For example, many different hypotheses exist around the evolutionary origins of music and speech, which could spur other investigations. And more cross-cultural research could ensure these ideas really hold up across all communities.
Ultimately, as to the mystery of how the brain separates music from speech in the auditory pathway, we suspect there is more still to uncover. Amplitude modulation is likely just one factor—one line, perhaps, on the addressed envelope—that can help explain our brain’s amazing auditory discernment.