Stuttering: A Theory Involving Mirror Neurons

Written by Matthew O’Malley

Preface: The theory put forth below deserves consideration in shedding light on the nature of stuttering.  I am aware of the possibility that the theory may not actually contribute to explaining the true nature of stuttering.  However, the rationale that builds its case merits both consideration and further exploration.  If you have an interest in stuttering, I believe it is worth taking the time to understand.

Any theory that attempts to explain the nature of stuttering must be able to adequately address its phenomena.  One of the most well-known and puzzling phenomena in regards to stuttering is the fact that many people who stutter are fluent when they are alone.  Viewing this objectively, you see that a change in the environment (alone vs. with others) of a person who stutters affects their ability to control their speech.  Puzzling, huh?

That being said, being alone or being in the company of others is not the only change in environment that causes a change in an individual’s stuttering severity.  Many changes in the environment can either decrease the fluency of a person who stutters or increase the fluency of a person who stutters.

Speaking is complex.  It involves the formulation of language.  However, the physical performance of speech (“speaking out loud”) is a motor task.  It is movement of the articulators (lips, tongue etc.) as well as breath control and vocal fold vibration.  People who stutter know what they want to say.  The problem arises when they go to make the speaking movements to “say out loud” what they want.  Quite simply, the movements don’t happen and they experience a “speech block” or stutter.  Something goes wrong with the motor execution.

So, how is it that a motor ability (speaking) can be so dramatically affected by changes in the environment?  How is it possible that a person can speak quite fluently in a room on their own, and the moment another person enters that same room, they become unable to perform the exact same motor ability (speaking)?  How is it that speech movements are inhibited (speech blocks) in the presence of another person but free to flow when alone?  How does environment which is “outside of us” dramatically affect a voluntary motor movement (speaking) of our bodies which is controlled “inside of us”? Quite puzzling indeed.

However, there actually is a link between these two seemingly separate entities (environment and motor movement). It is a fairly recent discovery.  The link is “mirror neurons”.  They are called mirror neurons because they mirror the environment motorically.  Because of mirror neurons, environmental stimuli that is observed by an individual resonates and activates the motor systems that control movement.  Mirror neurons provide a direct link between changes in the environment and changes in activity in the neurological motor system.  In essence, changes in the environment affect the human motor system significantly.

For example, if you observe a person raise their hand in class, your neurological system has rehearsed this same movement itself through mirror neurons.  If you watch a person grab an apple, you have rehearsed the same movements in your neurological system through mirror neurons.

Important sidenote:  Mirror neurons are not their own independent type of neuron.  There was not a discovery of a new distinct type of neuron.  However, the discovery of mirror neurons was simply a discovery of a new function in neurons that were already known to exist.  This function is mirroring the observed environment motorically.  Neurons that have this as one of their functions are called “mirror neurons”.

So, even if you sit perfectly still and simply observe people in the environment around you, your neurological system that is responsible for movement is active.  It is mimicking what it would need to do to perform the same actions you are observing others doing.  This means that when you are simply observing others move, the parts of the brain that are responsible for your own movement are active; the spine receives neurological impulses from the brain to perform movement; the parts of the body which you are observing being moved are receiving movement impulses.  In essence, when you observe another person perform an action, your neurological system rehearses the same action.

So, you might ask, if your neurological system is mimicking the movements of others, why aren’t you yourself performing the same movements?  Good question, and an important one in explaining stuttering.  It’s because there are systems in place to prevent you from physically performing the movements.  After all, it would not be helpful if everyone was walking around mimicking every movement they observed.  Therefore, there are systems in place that inhibit/block the movement your neurological system is rehearsing.  So despite the neurological system “acting out” movements in a sense, the physical result is an absence of movement.  This is because the mechanisms that determine whether a movement gets physically performed or blocked/inhibited has determined they should inhibit the movement.

Sidenote:  To learn more about the underlying neurology of movement go here, to part 3.

If you are a person who stutters, does having a movement get “blocked” sound familiar?  When a person who stutters goes to speak, the movements seem to get blocked.  As a matter of fact, that is the exact word people who stutter use to describe a stutter; a “block”.

To very briefly summarize the above, when an individual observes another individual performing a physical act, the observing individual rehearses the physical act they are observing in the part of their neurological system that is responsible for motor movement.

So, when a person who stutters is alone and speaking, the environment does not have another person in it for the neurological system to mimic the movements of.  As a result, the neurological motor system is not interfered with.  Once another person enters the person who stutter’s environment, the neurological motor system of the person who stutters becomes active as it mimics the movements of the person who has entered the environment.  So, the neurological motor movement system is significantly affected by the presence of another person as opposed to the presence of nobody.  This is possibly a significant revelation in the understanding of stuttering.

As you will read later in the post, there is a sub-class of mirror neurons called “echo neurons”.  Echo neurons activate motor systems based solely on sound.  So, if I hear somebody crack their knuckles, the neurological motor system required for me to crack my knuckles rehearses these movements.  More importantly for stuttering, when we hear another person speak, we rehearse these speaking movements in our own neurological system.

Of course, we do not perform any of these movements.  We do not physically say out loud every word another person says to us.  We do not crack our knuckles when we hear another person crack their knuckles.  We do not perform these movements/actions because there are mechanisms that suppress these actions and inhibit them, as I mentioned.

Academics who have researched mirror neurons talk about these mechanisms that inhibit movement. Rizzolatti & Craighero (2004) state, “the absence of any overt movement during such situations must involve inhibitory mechanisms that allow smooth transitions from execution to observation.”, and, “”We suggest that the mirror neuron system is involved in the withholding of unwanted movement during action observation. Mirror neurons are differentially recruited in the behaviour that switches rapidly between making your own movements and observing those of others.”  Vigneswaran, Philipp, Lemon, & Kraskov (2013) adds, “Suppression neurons reversed their activity pattern and were actually facilitated during execution.”

Side note: If you have read my piece on the relationship between “inner speech” and stuttering you are likely seeing some parallels in that theory and this one.  The two theories coexist and combine to make a larger, more comprehensive theory.  I will talk a bit more about this at the end.  That piece is here if you are interested.

The problem in people who stutter is over-inhibition of speech movement.  The neurological system is constantly “deciding” which movements to inhibit/block and which movements to enable/perform.  As a person who stutters has a conversation with someone and is turn-taking, the neurological system is suppressing the movements that it is rehearsing through mirror neurons as the person who stutters listens to their conversational partner talk.  As the conversational partner is talking, the neurological system of the person who stutters is rehearsing the movements they would need to perform to say what their conversational partner is saying.  However, these movements are suppressed and not performed.

The problem arises when it is the person who stutter’s turn to talk in the conversation.  The neurological system that was inhibiting movement when the conversational partner was talking is now supposed to enable speech and perform the speech movements.  However, it continues to inhibit speech actions as though they should be suppressed and not said out loud.

In adding to this, when a person is alone, their motor system is much less active because mirror neurons have nobody to mimic the movements of.  As a result there are less competing movement signals.  The neurological system has less signals to inhibit when a person is alone, therefore it has less trouble enabling a person who stutter’s attempts to speak out loud.  When another new person enters the environment, however, the neurological system begins mimicking this new person’s movements covertly (including their speech movements).  The person who stutter’s neurological system must then inhibit all of the movements of the other person they are covertly performing as a result of mirror neurons.  While inhibiting these new signals, the neurological system is still asked to enable the speaking actions the person who stutters wants to say out loud.  However, when a new person is in the environment, there are more signals competing and more signals for the neurological system to have to determine which ones to perform and which ones to inhibit.  The problem is, the neurological system determines it should inhibit speech that the person who stutters is trying to say out loud.  This is stuttering.  This is blocking.

A bit more on mirror neurons:

Mirror neurons are the subject of discussion across a large number of academic disciplines.  They were accidentally discovered in the 1990’s by Dr. Giacomo Rizzolatti, an Italian neurophysiologist.  While Dr. Rizzolatti was doing research on the neurology of motor movement in monkeys, he observed something unexpected.  He noticed that when the monkeys were completely still (no movement), yet observing human experimenters performing physical actions, corresponding neurons in the neurological systems of the monkeys activated.  These neurons were in the area of the brain/neurological-system responsible for movement.  So, despite the monkeys not physically moving, their observation of the experimenters moving, caused activation in the parts of the brain/neurological-system responsible for movement.  In this experiment, neurological activity was noticed in the frontal and premotor cortex which have a role in motor execution (movement).  He called these newly discovered neurons “mirror neurons” and since that experiment, much effort has been put into gathering more information on them and understanding their function.  Here is a link to an interview with Dr. Rizzolatti as well as more information on this discovery.

Neuroscientists and academics from a variety of disciplines theorize that this neurological activity of mirror neurons serves a very important developmental function.  As a child/person is growing and developing, they are able to begin learning motor skills simply through observation.  As a child watches their parent smile or walk around, they are performing a covert neurological rehearsal of these movements.  A very familiar slogan that applies here is “monkey see, monkey do”.  Mirror neurons make that saying even more applicable, because as a monkey is watching another monkey perform an action, their neurology is rehearsing that same action covertly.

It is also suggested that mirror neurons play an important role in intention understanding.  As we watch another person do something, we can better understand their intentions and rationale as we are covertly performing the action ourselves.

To bolster and confirm some of the my assertions, let’s take a look at some articles from peer-reviewed academic journals.  I am going to number and list each source and provide relevant quotes from each one.  Following my list of sources and relevant quotes, I will continue with more thoughts about mirror neurons in general and their possible relationship to stuttering.

Source 1Archarya & Shukla (2012)

“Essentially, mirror neurons respond to actions that we observe in others. The interesting part is that mirror neurons fire in the same way when we actually recreate that action ourselves.”

“In another experiment, they showed the role of the mirror neuron system in action recognition, and proposed that the human Broca’s region was the homologue region of the monkey ventral premotor cortex. Subsequently, a study by Ferrari Pier Francesco and colleagues described the presence of mirror neurons responding to mouth actions and facial gestures.”

“In both humans and monkeys, the mirror system also responds to the sound of actions.”

“The fact that the observation of actions like hand grasping activates the caudal part of IFG (Broca’s area), neuroscientists proposed that the mirror mechanism is the basic mechanism from which language evolved.”

Source 2Kraskov, Phillip, Waldert, Vigneswaran, Quallo, & Lemon (2014)

“We suggest that the mirror neuron system is involved in the withholding of unwanted movement during action observation. Mirror neurons are differentially recruited in the behaviour that switches rapidly between making your own movements and observing those of others.”

“The discovery that PTNs have mirror neuron properties raises at least two important issues. First, it demonstrates that even the executive components of the motor cortex are involved in situations in which we observe the actions of others, and second, it emphasizes that the absence of any overt movement during such situations must involve inhibitory mechanisms that allow smooth transitions from execution to observation.”

Source 3:  Vigneswaran, Philipp, Lemon, & Kraskov state (2013)

“► PTNs in primary motor cortex show mirror properties ► These neurons project to the spinal cord, but no overt muscle activity was detected ► Compared with execution, mirror PTNs reduce or abolish discharge during observation ► Disfacilitation of mirror PTNs during observation may prevent unintended movement”

“Suppression neurons reversed their activity pattern and were actually facilitated during execution.”

Source 4Rizzolatti & Craighero (2004)

“Is there any evidence that humans possess an echo-neuron system, i.e., a system that motorically “resonates” when the individual listens to verbal material? There is evidence that this is the case.”

“Taken together these experiments show that an echo-neuron system exists in humans: when an individual listens to verbal stimuli, there is an activation of the speech-related motor centers.”

Source 5Lotto, Hickok & Holt state (2009)

“Moreover, a sub-class of mirror neurons – echo neurons – responds both to executing an action and to the sound resulting from such an action (e.g. the action and sound of crushing a peanut).”

The Mirror-Neuron, Inner-Speech, Automoticity Theory

I combine my explanations above on the role of mirror neurons in stuttering with my explanations on the role of inner speech (here) in stuttering.  I assert that these two theories illuminate significant elements of the core of the stuttering condition.  I also combine these theories with assertions I’ve made regarding interference with the automoticity of speech due to a person who stutters attempts to control speech (here). The above listed assertions make up the Mirror Neuron-Inner Speech-Automoticity Theory.

The Mirror-Neuron, Inner-Speech, Automoticity Theory concisely summarized:

Stuttering is the result of the over-inhibition of the musculature involved in speech production by the neurological system which prevents the movement necessary for speech production.  A significant factor in this over-inhibition is the difficulty the neurological system of the person who stutters has in differentiating between voluntary speaking actions and other motor rehearsals whose movement inhibition is desirable.  These motor rehearsals include inner speech, motor imaging, and mirror/echo neuron rehearsals due to interaction with, and observation of the environment.  As a result of the person who stutters experiencing this over-inhibition of speech movements, they begin attempting to consciously control their speech, which is naturally an automated process.  This attempt to control speech further complicates the stuttering condition.  In addition, the totality of the stuttering condition expands further as it is stigmatized due to its nature being misunderstood.  The attempt to control speech as well as the person who stutters interacting in an environment that stigmatizes the stuttering condition, results in a variety of other manifestations that often co-exist with the core stuttering condition (secondary behaviors, social anxiety, etc.).

Other thoughts and speculations:

  • There was a study investigating the role of mirror neurons in the fluency inducing effect of choral speech which is another puzzling phenomenon surrounding stuttering. The choral speech effect is basically how people who stutter become fluent when speaking in unison with a group. An example would be people saying a prayer in unison in church. In this scenario, people who stutter become fluent. The choral speech effect makes perfect sense with the mirror neuron model. During choral speech, the people in one’s environment are performing the exact same action that the person who stutters is performing. As a result, there is no movement signal to inhibit. The action the neurological system is mimicking as well as the action the person who stutters wants to overtly perform (speaking out loud) are the exact same action. Therefore there is nothing to inhibit. As a result, there is no stuttering nor blocking. In the study, Kalinowski & Saltuklaroglu (2003) state, “We suggest that stuttering may best be ameliorated by reengaging mirror neurons via choral speech or one of its derivatives (using digital signal processing technology) to provide gestural mirrors, that are nature’s way of immediately overriding the central stuttering block.”
  • Source 2 above is an important one.  I mentioned their quotes in the body of this piece.  Source 2 talks about how a person must transition from observation of another’s action and the inhibition of movement to the performance of one’s own action and the enabling of movements.  It discusses the existence of a mechanism that must smoothly transition from inhibition of movement to the enabling of movement rapidly back and forth.  This is likely an area of interest in stuttering as the neurological system seems to inhibit speech movement that the speaker wants to overtly perform.  In essence, the neurological system is over-inhibiting movement.  Instead of only inhibiting neurologically rehearsed movements, it is also inhibiting movements the speaker wants to voluntarily perform causing “speech blocks” and stuttering.
  • The activation of mirror neurons is very similar to what happens neurologically during motor imagery.  When we imagine ourselves performing an act, we also neurologically rehearse these movements.  This is motor imaging.  As explained in this post, when we observe another person perform an action, we rehearse the action neurologically.  Therefore, observing another person perform an action is very similar to visualizing ourselves performing the same action.  Of course, both of these (motor imaging and observing others perform acts) also share a lot of similarity to overtly performing an action.
  • “Inner speech” or, speaking in our heads, is similar to listening to someone else speak.  In both cases, neurological motor systems become active and rehearse processes that would be required to overtly perform the same speaking actions.  My post on the role of inner speech and stuttering is here.
  • The intensity of neurological rehearsal due to the observation of another’s actions (speaking, walking etc.) may be partly dependent upon the level to which you pay attention to it.  For example, if someone is walking in the corner of your peripheral vision, you may not rehearse their walking neurologically, or the level to which you rehearse it is minimal.  Similarly if you watch intently every move someone makes, you may be neurologically rehearsing their movements with more intensity.
  • By no means am I convinced of this, but it is possible the neurological system is only inhibiting the muscles required for vocal fold vibration when a person who stutters goes to overtly speak.  In other words, a person who stutters is able to move their articulators (lips, mouth, tongue) when they go to overtly speak, however, the neurological system is “confused” as to whether or not to inhibit the vocal folds or enable them (because it treats the speaking actions as though they are rehearsals from the environment or inner speech).  As a result, the person who stutters sometimes goes to speak and the vocal folds are inhibited, yet they can move their articulators.  People often surmise that there is a motor timing issue in stuttering.  However, it is possible that vocal fold vibration is inhibited due to the theories I have explained and this results in a person who stutters inability to “sync” articulatory shaping/movement with both the onset and the continuation of vocal fold vibration, resulting in blocks and stuttering.  Metronome’s solve this and possibly enable the neurological system to “determine” that it should activate both the articulators and vocal folds simultaneously.  The problem could also be some other combination of inhibition of some speech muscles and activation of others.
  • Sound and motor systems likely have a unique and deeply connected relationship.  The fact that people feel compelled to tap their foot, bob their head, or even dance when music is playing is revealing.  Echo-neurons likely play a significant role in this.
  • Based on the intensity of neurological rehearsal which may be based on how astutely one pays attention could explain other variations in stuttering severity.  When a person who stutters interacts with a figure of authority, the attention they pay to them and thus the intensity of motor rehearsal may increase.  In addition, when a person who stutters interacts in groups, there are more people there speaking and performing actions for the person who stutters to mimic neurologically.  Does this cause higher levels of movement inhibition, thus causing speaking actions the person who stutters wants to overtly perform to be inhibited/blocked?
  • As I said in my post about the relationship between inner speech and stuttering, meditation and mindfulness seem plausible for improving stuttering based on these theories (inner speech and mirror neuron theories).  Stuttering is a problem with movement.  Meditation is about bringing the body to complete stillness.  Meditation is also about both quieting inner chatter as well as viewing inner chatter as nothing more than stimulus; not as inherent truth.  In addition, meditation and mindfulness are about being less reactive to environmental stimuli.  The reasons are many to believe that mindfulness and meditation could be useful tools in improving stuttering based on the two theories I explained.  These theories might explain why they are useful tools as many profess they are.
  • I will be spending some time contemplating other treatment ideas based on these theories.
  • The stuttering condition is complex especially in people who have lived with the condition for a while.  The brain attempts to adapt.  People develop lifestyles to cope with it.  People implement strategies to speak more fluently that are aberrant from normal speaking.  The above factors likely complicate the condition further which could partly explain a variety of manifested symptoms.
  • Mirror neurons make a strong case for why environment is extremely powerful in shaping people.  We know that children are dramatically affected by the environment around them.  The knowledge that a child is rehearsing neurologically everything in their environment, sheds new light on the power of environment.  Possibly, when parents hug, the child experiences this internally as it performs this action neurologically.  On the contrary, if a child observes a physically abusive relationship between parents, part of the negative impact on the child is likely due to the fact that they are rehearsing the abuse.  This has the chance to shed light on generational trauma and why children often end up so much like their parents for better or worse.
  • Some postulate that the understanding of language itself is motor based.  The claim is largely that our understanding of language is dependent upon symbols resonating in our motor system.  There are factions that oppose this theory.
  • In the research quotes above, there are a couple references to Broca’s area in the brain.  This area of the brain is known to contribute heavily to speech and language.  It has long been a focal point for the understanding of stuttering.
  • Also in the quotes above, mirror neurons were shown to be activated by facial expressions and mouth movements; certainly applicable to face to face communication and stuttering.
  • Mirror neurons also propose a partial explanation for why the listener of a person who stutters can become uncomfortable.  If they are observing the person who stutters struggling to speak ooften accompanied with anxiety etc., the listener is mimicking this neurologically.
  • One’s experience when interacting with a person that is charismatic is the result of mirror neurons.  When a person is speaking in passionate and positive ways, these very actions are resonating motorically in the listener’s neurological system.  The listener feels almost as though they are performing the actions and are thus very moved by a charismatic speaker.  They may even begin to “own” the views themselves.

Works Cited

Vigneswaran, G., Philipp, R., Lemon, R. N., & Kraskov, A. (2013). M1 Corticospinal Mirror Neurons and Their Role in Movement Suppression during Action Observation. Current Biology23(3), 236–243.

Kraskov, A., Philipp, R., Waldert, S., Vigneswaran, G., Quallo, M. M., & Lemon, R. N. (2014). Corticospinal mirror neurons. Philosophical Transactions of the Royal Society B: Biological Sciences369(1644), 20130174.

Acharya, S., & Shukla, S. (2012). Mirror neurons: Enigma of the metaphysical modular brain. Journal of Natural Science, Biology, and Medicine3(2), 118–124.

Wilson, S. M., Saygin, A. P., Sereno, M. I., & Iacoboni, M. (2004). Listening to speech activates motor areas involved in speech production. Nature Neuroscience Nat Neurosci, 7(7), 701-702.

Lotto, A. J., Hickok, G. S., & Holt, L. L. (2009). Reflections on mirror neurons and speech perception. Trends in Cognitive Sciences13(3), 110–114.

Rizzolatti, G., & Craighero, L. (2004). The Mirror-Neuron System. Annu. Rev. Neurosci. Annual Review of Neuroscience, 27(1), 169-192.

Kalinowski, J., Saltuklaroglu, T. “Speaking with a Mirror: Engagement of Mirror Neurons via Choral Speech and Its Derivatives Induces Stuttering Inhibition.” Medical Hypotheses 60.4 (2003): 538-43.

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