Written by Matthew O’Malley
Preface: The theory put forth below deserves consideration in its attempt to shed light on stuttering. Of course, it is possible the theory may not contribute to explaining the true nature of stuttering. However, the rationale that builds its case merits consideration and further exploration of the theory. It takes some time to unify the different parts of it. If you have an interest in stuttering, I believe it is worth taking the time to understand.
Part One: “Inner Speech” & Motor Signals
Have you heard of the term “inner speech” or subvocalization? Inner speech is simply when you talk to yourself in your head. It is audible thought in the form of words. As you read this silently, you are hearing yourself perform inner speech. It is the voice in your head that is reading these words. Inner speech is a term that was used by Lev Vygotsky, a Russian psychologist who died in 1934. Others since have viewed “inner speech” as having an important role in social development as well as in developing language and cognition.
As an influential figure in the field of developmental psychology, Vygotsky made assertions on the process of speech and language development in children. He believed that when children first learn to speak, they verbalize every word that comes to mind. As a result, children do not have inner speech in the early stages of speech development. However, as children develop, in response to social pressures, they begin inhibiting some overt speaking behaviors (speaking out loud) and internalizing them (inner speech). This results in inner speech. This means that every child in the early stages of speech and language development must transition from saying every word out loud to inhibiting words they do not want to say out loud. As most children who stutter begin to stutter early in their speech development, this period of transitioning from saying all speech out loud to inhibiting some speech and saying it internally, takes place around the same timeframe as when most people begin to stutter. As you will see, this inhibiting of “out loud” speech plays into the theory on stuttering I am explaining heavily.
Sidenote: To my knowledge, Vygotsky never linked the development of inner speech to stuttering.
More recent studies have revealed that when we perform “inner speech” (when we talk in our heads), we send neurological impulses like those that would be required to actually perform a speaking action. Stated differently, when we talk to ourselves, our brains and nervous system activate the same neurological systems required for motor action (movement) that are activated during actual speaking actions. Even more simply, when we talk in our heads, our brain sends signals to the muscles that control the vocal folds as well as the articulators (lips, tongue, jaw, etc.) as if we were physically performing a speech action (talking out loud).
To see more about this, here is a link to a New York University program using the above information about subvocal speech to develop new technologies. The article is not about stuttering but simply gives credence to my above statements. It states, “Speech begins in the mind. Whether we say something out loud or silently to our self, we are still saying something. We may not move the air to sound it out but the muscles used to vocalize the words still receive a signal from the brain. This subvocal speech can be perceived through electromyography by measuring the electrical potential created by muscle cells in the throat when they receive a speech signal from the brain. Electrodes on the neck capture the subvocal speech and the silent communication is converted into audible speech.”
Inner speech is not unique in the fact that electrical signals are sent to muscles despite not physically performing an act. In other words, talking in our heads is not the only action we perform in our imagination that causes neurological signals to be sent to corresponding muscles. This happens with any physical act we perform in our imagination. An example; if I imagine myself curling a dumbbell with my right arm, my brain will send impulses to my right bicep almost as if I am physically performing the action of curling a dumbbell.
In academia, the term for this is “motor imaging”. “Motor imaging” is simply a more technical term for “using your imagination”. Motor imaging means imagining yourself performing a physical act with your body. Here is a link to a research article on this concept published in “Neuroscience Letter” in 2008 if you are interested.
You might ask, “Well, if the motor signals are being sent to perform the movement, why aren’t the movements being performed?” I’ll begin answering that in Part Three.
Part Two: Speech is Movement
Speech is movement. When a person speaks, they begin moving their lips, tongue and jaw as well as the muscles that control the vocal folds. In addition, airflow through the lungs (which vibrate the vocal folds) is partly controlled by the contraction and relaxation of muscles.
The important concept to understand here for the theory I am explaining is simply that speech is movement. This is where people who stutter run into a problem. They go to speak and the proper movements don’t happen.
Part Three: Understanding Neurological Processes That Both Enable And Prevent Physical Movement
There are a number of movement disorders that exist. A few of the more well-known ones are Parkinson’s disease, Ataxia, Dystonia, and Huntington’s disease. Due to the suffering that often accompanies living with these conditions, a significant amount of research has been conducted to understand them. As the above listed conditions are disorders of movement, a lot of light has been shed on the processes that underlie and enable movement.
Since different disorders get more focus and funding than others based on factors like prevalence and life impact, often researchers “piggyback” off of the findings of other disorders to cast light on the one they are studying. This is no different with stuttering. Parkinson’s disease research in particular has shed a significant amount of light on the neurological basis of movement and what can go wrong with it. Those looking to understand stuttering have borrowed significantly from the findings of research on Parkinson’s disease to better understand stuttering. Much of the theory I am about to discuss has become understood as a result of Parkinson’s disease research
Before continuing, let me preface this by saying the explanation of neurological function I am about to give is simplified. Following it are links to educational videos from the Khan Academy about some of what we know about the underlying neurological processes of movement. If you would like to gain a more complex understanding of the neurological basis of movement, I suggest you watch them. I could write out explanations that cover the same material as the videos, however, it would take you longer to read and your understanding of it still would not be as good as if you watch the videos. My written explanations should suffice for the purpose of this post. However, Khan Academy’s clear and concise explanations through videos that teach both visually and auditorily (as well as through story and analogy) trump learning through reading. Below is my simplified version; just enough to link each part of this post.
I would also like to share that leading researchers in the field look to this neurological system to explain stuttering. The information on movement explained below makes strong contributions to theories on the nature of stuttering for some prominent researchers in the field. This part of this theory I am explaining is not new. However, linking all of the parts (which I will do in the end) is.
Some background on movement:
Our neurological system works both to enable movement (muscular contraction) as well as inhibit movement (inhibit muscular contraction). Both of these elements of the system are necessary to control all movement we perform. If one element of this system is not functioning properly we get problems with movement. For example, if the part of the neurological system that inhibits movement isn’t working, the result is individuals in bodies who can’t stop them from moving; tremors etc. If the part of the neurological system that enables movement is not functioning properly the result is difficulty with initiating and sustaining movement; weakened movements or the inability to move.
Those well versed in stuttering theory have likely heard of the possible role of the basal ganglia in stuttering and some researchers’ interest in it. Those very well versed have likely heard of both the sensorimotor cortex and thalamus and their possible role in stuttering. Those extremely well versed in stuttering theory have likely heard of the direct loop and indirect loop.
Let’s provide a little clarity to these terms. All of the above terms relate to one another for the purposes of what we are talking about. The basal ganglia is simply the name given to a group of structures in the brain with a central role in movement. The sensorimotor cortex is the name given to another region of the brain involved in movement. The thalamus is the name given to another region of the brain involved in movement.
These structures work together to enable or inhibit movement:
All three of these structures (basal ganglia, sensorimotor cortex, thalamus) are what make up both the direct loop and the indirect loop. The two different loops take different routes through these structures. Primarily these structures working in conjunction with one another are responsible for whether a bodily movement ends up being performed or inhibited (not performed). Generally, the direct loop is responsible for enabling movement. The indirect loop is responsible for inhibiting movement.
Both of these loops must work in unison and in balance to give us control over the movements of our bodies. At any given moment, aspects of our neurology want to move virtually every part of our body all the time. On the other hand, aspects of our neurology want to prevent all movement at a given moment. Both of these aspects of our neurology and the structures related to them, keep each other in check, resulting in the body moving when you want it to and the body not moving when you don’t want it to.
When the above systems stop keeping each other in check properly, or there is damage or decay to the neurological structures involved in this system, we begin to see movement disorders like Parkinson’s disease.
The neurological system determines whether it performs a movement or inhibits a movement. This is very important for stuttering.
The goal of the above system is to inhibit involuntary movements so they do not get performed and to enable voluntary movements so they do get performed.
To gain a more complex understanding of this system, the below videos are great resources:
Part 4: Linking the Previous Three Parts Together
In Part 2, I covered that speech is movement. It is the contraction and relaxation of muscles. In Part 1 I covered “inner speech” or subvocalization, and that despite not speaking out loud, we are still sending similar neurological signals to the muscles responsible for speaking out loud. In part 3, I covered how our brain and neurology filter out some movement signals so these movements do not get performed. I also covered how our brain and neurology enable other movement signals and cause those movements to get performed.
Now let’s link all this to explain stuttering. When we perform “inner speech”, we are hearing ourselves produce audible words in our heads. At the same time, the muscles responsible for producing speech are being sent signals from the brain like those that would be needed to perform the speaking action and talk out loud. However, these movements do not get performed. Why?
The neurological system I explained that determines whether a movement gets inhibited or enabled has determined that it should inhibit (not perform) this speaking action (inner speech). The result is you do not speak out loud. This, of course, is great. If you spoke every word you heard in your head, maintaining social appropriateness would present some serious challenges, to say the least. So, in this scenario (where you are thinking to yourself in your head, performing inner speech), it is an absolute blessing that you do not perform all the speaking actions that correspond to your inner speech. This is what you want in this scenario.
However, the problem for people who stutter comes when they actually want to say something out loud. When the person who stutters goes to speak out loud, the neurological system treats it as though it is “inner speech” or subvocalization. As a result the muscles required for speech are inhibited as the neurological system has determined that this “speaking action” is one that it should inhibit, therefore it does not get performed. The result is “blocking”. The result is inaction (because the speech action has been inhibited by the neurological system) which is the same as blocking.
Now, as the person who stutters goes to speak, the neurological system is “confused” for lack of a better term. At certain moments, it determines that it should perform the speaking action the person who stutters is trying to perform (treats it as speaking out loud) and at other moments it determines it should inhibit the speaking action (treats it as “inner speech”). This is why the person who stutters can block/stutter when initiating speech; then they get a couple words out and then again they end up in a block. What happened in that sequence is when the person who stutters goes to initiate speech, the neurological system determines that it should not perform the action so they block/stutter (treats it as inner speech). As the person who stutters continues to try to say the words voluntarily, the neurological system at some point “realizes” that it should enable this speaking action and perform it (determines it is out loud speech). So, it does, and words start flowing. A couple words into speaking, the neurological system again “believes” it should inhibit the speaking action (treats it as inner speech). So it does and again the person is blocking/stuttering. Etc.
This is where the person who stutters feels completely at a loss in regards to controlling their speech. A neurological process that is not voluntary is intermittently determining that it should inhibit speaking actions (causes blocks) and it should enable speaking actions (when speech flows).
My theory for why the neurological system has difficulty with speaking out loud is because of “inner speech” or subvocalization. The neurological system confuses speech you want to say out loud with inner speech. As a result it inhibits it; it does not perform the action. This is why people stutter. This is what a “block” is.
How often do we perform inner speech? It likely varies a bit from person to person, but I do not know anyone who is not experiencing some level of dialogue in their head for most of their waking hours. As I said earlier, “inner speech” is a form of motor imaging. To refresh, motor imaging is imagining yourself doing an action with your physical body. As a person motor images, impulses are sent to the muscles they are using in their imagination, despite not moving at all. Inner speech is no different than other motor imaging in this way. As you perform inner speech, impulses are being sent to the speech muscles.
On that note, there is not another activity that we “motor image” nearly as often as inner speech. From a very young age, our heads become filled with inner speech until the day we die. There is no other task we “motor perform” that is even in the ball park of frequency with inner speech. This frequency could play a role in the neurological system treating “speaking out loud” like inner speech.
In addition, when people who stutter speak to another person, the amount of inner speech that is going on outside of the content they are speaking out loud is often significant. This means that when you are speaking out loud to another person, which requires neurological signals to be sent to activate speaking muscles, you are also performing a significant amount of inner speech simultaneously. As explained, inner speech also sends neurological impulses to the speaking muscles, however, these are to be inhibited. In other words, as a person who stutters speaks to another person out loud (sending signals to activate speaking muscles), they are also performing intense inner speech (also sends signals to speaking muscles, but these signals should be inhibited). See a problem? Conflicting signals are being sent. Inner speech should be inhibited and not performed. Speaking out loud should be enabled and performed. These two processes are happening simultaneously. And, yes, people who don’t stutter also have some inner speech going on while they talk out loud. However, it is likely not as much and their neurology did not ever show patterns of difficulty differentiating the two.
The above (speaking out loud while having inner speech as well) would be like playing the piano while hearing a different melody than the one you are playing in your heads. Performing inner speech while talking out loud would be like sprinting while imagining yourself running backwards.
At this point it seems likely to me that the neurological processes of movement play a central role in stuttering. I hypothesize that subvocal speech plays a large role in causing the neurological system in people who stutter to deviate from its desired performance. The neurological system in the person who stutters, often treats speech which they desire to say out loud as inner speech. As a result, the speaking actions are inhibited, resulting in blocking and stuttering.
Other thoughts and speculations:
I combine this theory with a theory I have explained on the role of mirror neurons in stuttering. They coexist and strengthen each other. If you are interested in reading that, my proposed role for mirror neurons in stuttering is here. I assert that the above theory and the role of mirror neurons contribute heavily to the core of the stuttering condition. However, I also combine these theories with the theory that people who stutter understandably begin to try to control their speech, as a result of stuttering experiences. This interferes with a process that is supposed to be automated (speaking), causing further complications of the condition. My assertions on speech’s automatic nature and the person who stutter’s attempts to control it are here. I call the entirety of this theory on stuttering the Mirror Neuron-Inner Speech-Automoticity Theory.
Tourette’s syndrome is the exact opposite of stuttering. While only a fraction of people with Tourette’s involuntarily say or shout profanities, it is a part of the disorder. What happens in Tourette’s is the neurological system of a person with Tourette’s determines that it should perform “inner speech”. It is unsuccessful in suppressing inner speech. So, when a person with Tourette’s says some version of profanity in their heads (just like everybody else does occasionally), the neurological system determines it should perform this action and say it out loud. While the person with Tourette’s is simply saying this profanity as part of their “inner speech”, it ends up being said out loud because that’s what the neurological system determined it should do. In this way, it is the opposite of stuttering.
If the sequence of Vygotsky’s theory in the development in inner speech is incorrect, this theory still makes sense. Receptive language in a child is always more developed than expressive language. This simply means that a child can understand more than they can say. As a result, “inner speech” may develop before expressive speech (speaking out loud). Since most people who stutter begin stuttering at around 2.5 years old or later, this theory I have put forth still makes sense. They have been talking in their heads (therefore inhibiting speech) much more than they have expressed (speaking out loud).
Researchers in the field who are focused on the basal ganglia as a possible causal area for stuttering talk about how it is responsible for sending “go” signals for each movement we perform. If the “go” signal is not sufficiently strong, the action will not be performed. One theory is “go” signals sometimes are not strong enough.
The basal ganglia and the direct and indirect loop are influenced by other brain systems (emotional systems and others). As a result the strength of “go” signals can likely be influenced by environmental factors as well as attitudes of the speaker.
The evolution of motor control did not involve the amount of rehearsal of a motor ability that humans perform with inner speech. Inner speech is an interesting subject in itself. Some elements of its nature could interfere with neurology and normal motor function.
Mindfulness and meditation are gaining a lot of traction as tools in in the treatment of stuttering. Meditation and mindfulness are often about emptying the mind. The goal of some meditation is to get to where you have silenced all inner dialogue. With the theory I have put forth, silencing inner chatter certainly makes sense in treating stuttering.