Speech function, stuttering, and genetics

 

Evolution could not have predicted the development of language (and speech), which developed very fast in evolutionary terms, and, it seems, quite accidentally. The brain was never meant to host language, but as its complexity evolved, it made this new function possible. In turn, language made possible abstract thinking and the higher cognitive functions associated with being human–and enabled us to become the dominant species.

It’s interesting to note that the brain processes nouns (object words) and verbs (action words) in different neural areas.25

Speech function is made possible by specialized areas in the brain devoted to this most complex of human neurologic functions. These brain areas process and generate words and issue motor output signals that direct and coordinate the sequential movements of the external speech articulators required to create words vocally. The external speech articulators include the mouth, tongue, and vocal cords, etc.

Since stuttered speech is a disorder of speech function, and since speech function is controlled by certain areas in the brain, isn’t the most likely cause of stuttered speech a dysfunction in these brain areas? Genetic and brain imaging studies (detailed later)  strongly suggest this is the case.

It’s important to understand the relationship of the brain to our external body parts. Without direction by the brain’s electrochemical signals, muscles are just motionless slabs of flesh. Fundamentally, we write with our brains, not our hands; we hear with our brains, not our ears; we see with our brains, not our eyes. We kick a ball with our brains, not our feet; and we speak with our brains, not our mouths. The external body parts are just the only thing the brain has to work with.

A common estimate is that about one out of every 100 persons stutter in all language groups. More males become chronic stutterers than females in about a 4:1 ratio. (Recent studies have found differences between males and females in brain areas that handle language function, which might turn out to explain why fewer females become stutterers.) Most stuttering begins between ages 2-6. About 75% of these children get over stuttering by adolescence. The other 25% go on to become adult stutterers.

This is around the critical age that laterality and brain hemispheric dominance are expressed, resulting in a favored right or left hand, foot, and eye, etc. Early limb movement–as the child passes through the normal developmental stages of crawling, creeping, and walking–appears to be intimately related to the development of the brain areas used for language function. Typical development usually results in one dominant hemisphere for speech and language. But nature deals in volume and this is not always the case.

Perhaps a genetic misstep might be responsible for the development of a type of mixed hemispheric dominance that gives rise to bilateral speech production circuitry. Signals to initiate speech are sent from two separate brain areas to speech articulator muscles at the same time, creating a “bottleneck” as mirror-image signals compete for the motor output pathway. The speech articulator muscles need one dominant or consolidated set of signals, but are bombarded with two in stutterers–resulting in the neurologic speech block. The greater the bottleneck, the more severe the speech block and stutter. Words get trapped in the brain before they ever get to the external speaking apparatus. As I continue to emphasize, words must get out of the brain before they can get out of the mouth.

This idea is not new–it’s been around since the 1920s. Recent studies (detailed below) support this concept and studies continue on it.

(An aside based on my own experience: Regular, patterned arm movements–such as in exercise workouts or weight training that involves both arms used together or each arm separately–may worsen stuttering in some PWS. This suggests that, in these cases, laterality issues could be a factor. Exercise involving primarily leg movement, such as jogging, may not have as telling an effect because the brain areas related to leg movement are less relevant than those for arm and hand movement.)

Researchers continue to pursue evidence that certain brain neurotransmitter systems might be involved in stuttering.

Stuttering tends to run in families. One study found that 60 percent of stutterers had at least one parent who stuttered. You’ll hear it suggested that stuttering could have been “learned” environmentally, rather than genetically. In my view, that’s unlikely. An impressionable child might mimic or pick up stutter-like symptoms temporarily, but unless the flawed biologic factors are in place, it shouldn’t last. It’s doubtful that you can “catch” or learn chronic stuttering from someone else any more than you learn dyslexia or Tourette’s disorder from someone else, family member or not.

You may have read or heard that everyone is dysfluent at times. That’s a merciful if not patronizing comment designed to make you feel better. Sure, it’s true, but the rare momentary dysfluencies of nonstutterers is not stuttering. Stuttering is a league apart. Momentary dysfluencies are normal. Stuttering is not. Most of us have seen a TV newscaster experience one of these rare, momentary dysfluencies, then laugh and perhaps slap his face and say something like, “I’ll spit it out in a minute, folks,” and move quickly right back into normal fluency without a hitch. That is not stuttering.

Studies continue to shed light on the neurologic/genetic basis of stuttering, including brain imaging studies showing differences in key brain areas of PWS compared to non-PWS. Genes are the blueprints for the structure and function of the brain, as with every other part of our bodies.

A study reported in the American Journal of Medical Genetics found three different genes affecting the dopamine neurotransmitter system in the brain. These genes appear to be associated with a spectrum of disorders including Tourette’s disorder, attention deficit hyperactivity disorder (ADHD), and stuttering. The genes have an additive effect: The more genes a person carries (from one to all three), the greater the severity of the disorder.18

More recent studies also suggest that genetics play a key role in developmental stuttering:

www.mnsu.edu/comdis/isad15/papers/drayna15.html

www.stutteringhelp.org/genetics-stuttering-new-developments

Pinpointing the neurologic/genetic basis may point the way toward possible treatment/prevention, and one day possibly–should we hope?–some type of “instant” cure.

If you don’t have a predisposition to stutter, it’s unlikely that you’ll become a chronic stutterer, regardless of what happens to you in childhood.

And if you do have a predisposition to stutter, studies suggest that the earlier in childhood stuttering is treated–with effective treatment–the greater the likelihood you can override the inborn flaw(s) and actually prevent chronic stuttering. The language/speech function areas, as all other brain areas, appear to be more pliant early in life. And early treatment in children would be simpler than that for adult PWS.