Speech processing in the brain goes beyond words

Slight changes in pitch, also called prosody, shape meaning in everyday conversation. New research suggests that even the smallest tonal variations, such as a speaker’s emphasis or intent, can significantly guide the listeners’ understanding of language.

In a multidisciplinary effort involving researchers from multiple institutions, scientists found that a specific part of the brain, known as Heschl’s gyrus, is responsible not only for basic sound processing but also for interpreting these subtle pitch shifts as meaningful information. 

The study reveals that this region doesn’t just process sounds, but rather transforms subtle changes in pitch – known as prosody – into meaningful linguistic information which structures how humans understand emphasis, intent, and focus in conversations.

Assumptions about speech processing

For decades, it was generally believed that prosody was mainly handled by the superior temporal gyrus, a brain region long associated with speech perception. 

According to co-principal investigator Bharath Chandrasekaran, a scientist at Northwestern University, these results “redefine our understanding of the architecture of speech perception.” 

“We’ve spent a few decades researching the nuances of how speech is abstracted in the brain, but this is the first study to investigate how subtle variations in pitch that also communicate meaning are processed in the brain,” Chandrasekaran said.

Speech processing and brain activity

At the heart of this discovery was a group of 11 adolescent patients undergoing neurosurgery for severe epilepsy, each with electrodes implanted deep in a crucial language-processing region of the cortex. 

Taylor Abel is the chief of pediatric neurosurgery at the University of Pittsburgh School of Medicine. 

“Typically, communication and linguistics research rely on non-invasive recordings from the surface of the skin, which makes it accessible but not very precise,” said Abel. 

“A collaboration between neurosurgeon-scientists and neuroscientists, like ours, allowed us to collect high-quality recordings of brain activity that would not have been possible otherwise, and learn about the mechanisms of brain processing in a completely new way.”

These implanted electrodes offered a rare, direct look at neural responses to subtle pitch changes in speech. To explore how the brain interprets the “melody” of speech, participants listened to an audiobook recording of Alice in Wonderland. 

During these sessions, scientists recorded brain activity from different regions in real time, providing detailed data on how auditory information was processed.

Early encoding of speech melody

Using this intracerebral monitoring, the researchers discovered that Heschl’s gyrus encoded changes in pitch – prosodic cues – as linguistic units, distinct from the raw acoustic signals that make up words. 

Study co-first author G. Nike Gnanataja is a professor at University of Wisconsin-Madison’s Department of Communication Sciences and Disorders. 

“Our study challenges the long-standing assumptions how and where the brain picks up on the natural melody in speech – those subtle pitch changes that help convey meaning and intent,” said Gnanataja.

“Even though these pitch patterns vary each time we speak, our brains create stable representations to understand them.”

By seeing how quickly these prosodic patterns registered in Heschl’s gyrus, the team realized that the process happens earlier than once assumed. 

Rather than being filtered extensively in higher-level brain areas, the natural rise and fall of speech gets broken down into meaningful contours at an earlier stage in the auditory pathway.

Acoustic signals and speech processing

A related line of research compared non-human primates’ ability to process the same acoustic signals. 

While these animals could detect the sounds themselves, they did not seem to abstract any linguistic or meaning-laden information from those pitch changes. This contrast highlights a uniquely human capacity to transform tiny shifts in tone into communicative signals.

By revealing this hidden layer of speech, the experts discovered how the brain processes pitch accents, which has profound implications for various fields. 

According to the researchers, this work could lead to novel rehabilitation methods for speech and language disorders. For instance, early detection and tailored interventions might help individuals with conditions such as autism or those recovering from strokes reestablish more effective communication strategies.

“Our findings could transform speech rehabilitation, AI-powered voice assistants, and our understanding of what makes human communication unique,” Chandrasekaran said. 

Improved knowledge of prosody might enable voice-recognition systems to better interpret user intent and emotional tone, bringing us closer to natural language processing that mirrors how people actually speak.

A hallmark of human language 

The discovery also underlines the significant role played by linguistic experience. Because non-human primates cannot form abstract categories from pitch accents, scientists now have new evidence that our ability to interpret and manipulate prosodic cues represents a hallmark of human language.

As research continues to uncover how these vocal subtleties are perceived and encoded in the brain, fields ranging from neurolinguistics to artificial intelligence will likely reap the benefits. 

By demonstrating that Heschl’s gyrus takes on advanced speech-interpretation tasks much earlier than once believed, this study encourages a reevaluation of the traditional hierarchy of speech processing – and highlights the complexity behind the timeless adage, “It’s not what you say, it’s how you say it.”

The study is published in the journal Nature Communications.

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