Speech planning: How our brains prepare to speak

In a fascinating exploration of speech planning, research led by the NYU Grossman School of Medicine reveals how our brains prepare to speak before we actually verbalize our thoughts.

The study, conducted among individuals undergoing surgery for epilepsy treatment, is pivotal for advancing our understanding of speech production. It unveils significant insights into the brain’s mechanisms behind speech planning.

The brain’s blueprint for speech

The study delves into the roles of the inferior frontal gyrus and motor cortex – two regions situated in the outer layers of the cerebral cortex. These areas are pivotal for controlling the muscles involved in speech production, influencing our selection of words and sounds.

The researchers examined brain-mapping recordings from 16 patients, aged 14 to 43, at NYU Langone Health over the period from 2018 to 2021. These individuals were undergoing pre-surgical evaluations for epilepsy.

The process involved stimulating brain regions to pinpoint and conserve areas critical to speech while targeting seizure-inducing tissue for removal.

Importantly, a key innovation of this research lies in measuring the brief time intervals, less than two seconds, between brain stimulation and its impact on speech. This approach offers fresh perspectives on the cortex’s role in speech planning.

Exploring speech production

The researchers have unveiled new insights into the organization of speech within the brain. They found that the delays before speech disruption, known as latencies, vary across different brain regions. This discovery highlights the brain’s complex role in speech production and its elaborate mechanisms.

“Our study adds evidence for the role of the brain’s motor cortex and inferior frontal gyrus in planning speech and determining what people are preparing to say, not just voicing words using the vocal cords or mouthing the words by moving the tongue and lips,” stated Dr. Heather Kabakoff, a speech pathologist at NYU Langone.

Furthermore, Dr. Adeen Flinker, the study’s senior investigator and a neuroscientist, explained: “Our results show that mapping out the millisecond time intervals, or latencies, between electrical stimulation in parts of the brain to the disruption or slurring of words and eventual inability to speak can be used to better understand how the human brain works and the roles played by different brain regions in human speech.”

Dr. Flinker emphasized the clinical implications of these findings. Additionally, he suggested that this research could pave the way for improved surgical techniques to protect speech functions during brain surgeries.

The next frontier in speech and brain research

The team is now expanding their focus, setting their sights on broader horizons. Consequently, they aim to unravel the roles of other brain parts in speech and auditory processing.

The researchers are studying real-time brain corrections of speech errors to enhance our understanding of speech control and modification. This endeavor marks another step forward in decoding the complexities of human communication.

The process of speech planning

The process of speech planning in the brain is a complex, multi-step process involving various brain regions working in coordination. It includes several key stages:

Conceptualization

This is the initial stage where the intent or idea of what one wants to communicate is formed. It involves abstract thinking and decision-making processes in the prefrontal cortex, where the brain decides on the message it wants to convey.

Lexical selection

Once the concept is formed, the brain selects the appropriate words to express the idea. This involves the temporal lobe, particularly the left temporal lobe for most people, where language comprehension and vocabulary are managed.

Syntactic processing

After selecting the words, the brain organizes them into a grammatically correct structure. This involves Broca’s area, located in the left frontal lobe, which is responsible for speech production and the grammatical aspects of language.

Phonological processing 

This stage involves planning the sounds that need to be produced to articulate the words. This involves the interaction between Wernicke’s area, which is involved in language comprehension and the processing of phonological (sound) information, and Broca’s area for the motor aspects of speech production.

Motor planning

Before speech occurs, the brain must plan and coordinate the specific movements of the mouth, tongue, vocal cords, and lungs that produce speech. 

This involves the motor cortex, which controls voluntary muscle movements, and the cerebellum, which coordinates the timing and precision of these movements.

Execution

Finally, the motor cortex sends signals through the nervous system to the speech organs, executing the planned movements and producing speech. This involves intricate coordination of muscles and breathing to articulate words and modulate voice.

Throughout this speech planning process, the brain also relies on feedback mechanisms involving auditory and somatosensory systems to monitor and adjust speech production in real-time. This ensures accuracy in articulation and intonation.

The exact neural pathways and interactions between these areas are still a subject of ongoing research, as the brain’s processes for managing speech are incredibly complex and vary from person to person.

The study is published in the journal Brain.

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