Speech production is a complex process involving neural circuits in the brain. There are three processes of speaking: cognitive-linguistic, sensorimotor planning and programming, and motor execution. These processes all work seamlessly in producing speech unless there is the impact of neurodegenerative diseases or stroke. These impediments can affect different aspects of communication and complicate the speech production process.

Listen to me say this: Hello, my name is Julie Liss and I’m a professor at Arizona State University. Seems easy enough, right? I just verbally introduced myself to you, and my words came out smoothly and pleasantly and you easily understood what I said. But the ease with which we speak belies the extraordinary complexity of the neural circuits involved in producing speech.

Let’s take a look under the hood and take a look at my brain. When I decide I want to communicate an idea, I first have to select the words I will use to convey that idea. These words then must be assembled in strings that comply with the rules of the language. These words must then be translated into something not unlike a musical score–a sensorimotor score, if you will, that instructs all of the muscles involved in speaking to work together to produce the sounds that make up the words we want to say. Finally, all of the muscles must be activated at the right time and in the right patterns to produce the acoustic signal that is perceived as our words. 

We can think about speaking as involving three processes:

First, the cognitive-linguistic process, in which we decide we want to say something, select the right words to convey that message, and assemble the words in strings that represent our message. This process involves higher order parts of the brain, such as the language areas in the frontal and temporal lobes; as well as structures that mediate memory and emotion, such as the hippocampus and amygdala. Even before a word is uttered, the brain is busy formulating the message.

Second, the sensorimotor planning and programming process, in which we translate this string of words into a score of finely orchestrated muscle commands. Again, even before a word is uttered, the “instructions” for producing the word must be loaded into the cue. 

Third, the motor execution process, in which we activate that sensorimotor score, moving the muscles of respiration, phonation, resonance, and articulation to produce the acoustic signal of speech. Finally time to utter the word, unpack the motor execution process, describing the functions of respiration, phonation, resonance, and articulation and how they are orchestrated in their movements to produce speech. This would involve describing breath as the “power source” for speaking, setting the vocal folds into vibration, which produces the sound of our voice “phonation”; and this voice is filtered through the oral nasal cavities where it is shaped into the sounds of speech by our articulators, which include the tongue, lips, and soft palate.

For most of us, this complex system seems simple. After all, it’s something we began to grasp from before we can remember. But when a wrench is thrown into the mix, such as a stroke or other neurodegenerative disease, the impact on speech when even one of these processes is affected becomes clear. Someone may have no trouble producing words, but due to difficulties with their cognitive-linguistic processes have trouble finding the right one. If motor execution is affected, a person may have no trouble forming the sentence mentally and understanding speech, but struggle to converse due to muscle weakness and issues with coordination.