Speech as a Motor Function, Not a Measure of Intelligence

​Many children with speech challenges, such as apraxia, stuttering, poor articulation, and non-verbal communication, are often misunderstood by those around them—family members, teachers, therapists, and even doctors. Their speech abilities are frequently misinterpreted as indicators of their overall cognitive function, leading to incorrect assumptions about their capacity to learn, reason, and express their thoughts. Equating a child's speech abilities with their cognitive abilities is as misguided as linking someone's skill in gymnastics to their intelligence.

Speech production is an intricate and highly refined motor skill, unique to humans, requiring the precise coordination of numerous muscles and structures within the body. These include the lips, tongue, cheeks, jaw, and both the hard and soft palates, all of which are supported by the diaphragm's control of breath. The complexity of speech is such that it engages multiple cranial nerves, including the vagus, glossopharyngeal, hypoglossal, trigeminal, facial, and accessory nerves (Image 1). Any weakness or dysfunction in these nerves can result in diminished or completely absent oral motor function, further complicating speech production.

Image 1. Source: Bailey, Regina. "The Names, Functions, and Locations of Cranial Nerves." ThoughtCo, Aug. 11, 2024, thoughtco.com/cranial-nerves-function-373179.

In addition to the peripheral mechanisms, deeper injuries within the brain, particularly in regions like Broca's and Wernicke's areas, which are pivotal for speech processing, can significantly impact speech. However, it's crucial to note that damage to these areas doesn't necessarily affect cognitive ability (Stoler, 2020). Such injuries can arise from various causes, including stroke, traumatic brain injury, tumors, infections, and certain deficiencies in B vitamins.

Furthermore, dopamine, a neurotransmitter essential for motor control, also plays a vital role in speech production (Alm, 2021). Its influence on motor control is so profound that low levels of dopamine in the brain have been linked to various speech issues, including stuttering, soft or monotone speech, abnormal prosody, reduced facial expressiveness, breathiness, hoarseness, and imprecise articulation. These symptoms are prominently observed in conditions like Parkinson's disease but can also manifest in other neurological disorders.

Dopamine deficiencies can result from inadequate energy supply to specific neurons in the brain (Morris et al, 2018), or from low levels of tetrahydrobiopterin (BH4), a cofactor necessary for the tyrosine hydroxylase (TH) enzyme.(Vancassel, 2021). Vitamin B6 (PLP) is also essential for the synthesis of dopamine from L-DOPA (Nova-Mesa, 2019). Deficits in any of these areas can impair speech development and disrupt other motor functions of the body.

Image 2. Role of PLP (vitamin B6) and BH4 (tetrahydrobiopterin) in dopamine synthesis

Additionally, cerebral folate receptor autoantibodies (FRAA) that result in low levels of folate in the brain have been shown to impact speech, behavior and neurological function in children with autism (Quadros, 2021). Treatment for the presence of these antibodies includes high dose folinic acid (Leucovorin). Improvement in verbal communication has been reported in children with autism in as little as 12 weeks when taking high dose folinic acid (Quadros, 2018). Folinic acid treatment has also been reported to improve spasticity and gait disturbance in a 12 year old girl who also had speech difficulties. Her motor challenges began at 3.5 years old. She was not treated with folinic acid until she was 12 years old when her condition declined. She received 15 mg/day of folinic acid which resulted "in an amazing effect after less than one week". Her dose was increased at 14 years old when her spasticity increased. "Her Isovorin (calcium levofolinate, a form of folinic acid) dose was doubled to 30 mg/day, again with amazing results. Her gait and stability improved and her speech is now near normal at the age of 14." (Hansen​, 2005). This girl's experience underscores how motor challenges can significantly impact speech. Addressing the root cause of these motor difficulties led to noticeable improvements in her speech.

Understanding the complex interplay between these neurological and biochemical factors is crucial for accurately diagnosing and treating speech disorders, ensuring that children receive the support they need without unwarranted assumptions about their cognitive abilities.