Considering Dysarthria: A Speech Disorder 'On the Margins'
Considering Dysarthria: A Speech Disorder 'On the Margins'
Henrike BlumenfeldThe goal of this paper is to portray dysarthria, a language impairment, as a disorder that is "on the margins" of the category of speech disorders. The argumentation will be that since dysarthria shares common underlying neurological causes with motor diseases rather than with other language impairments, it is set apart from other language impairments and evidence for the overlap of the motor modality with the language modality.
Language is arguably one if not the most complex functions produced by the human brain, and one that is all but transparent as to the underlying neurological structures and processes in that so much is going on at the same time that it is hard to tell what is what. What we do know is that there are different areas of representation for different aspects of language. An area in the left hemisphere above the perisylvian fissure in the frontal lobe anterior to the motor cortex roughly controls production and fluency of speech; another area, in the temporal lobe of the same hemisphere roughly controls comprehension and word retrieval. Other regions next to and between these areas carry related functions, and lesioning to them results in variations of the symptoms caused by lesioning one of the central regions described above(1)..
These areas are known to us as the "language areas." However, they can be completely intact in a person that is suffering from severe language impairment. Are there other "language areas" then? Strictly speaking, no, loosely speaking, yes. Language is more than a mental function where structures are established, words are retrieved and sound associations are made. The final execution of language is purely a question of complex motor control.
Disruption of this complex interaction of motor signals towards the formation of speech is known as dysarthria (2). It is sometimes confused with impairment resulting from injury to the "language areas" (1), illustrating the point that the process of producing language is multi-layered, and that it is often hard to tell what is what.
The symptoms of dysarthria may be slurred speech, nasality of sound, or quiet and slow speech (3).. These different impairments reflect the sub-sections in the 'motor symphony' that produces speech: The lips, tongue, teeth and jaw shape specific sounds; the soft palate channels air into either the nasal or oral cavity, producing different sound qualities; the larynx (voice box) makes the air vibrate through the vocal folds to create voice; and finally, the respiratory system powers this process (3).
If dysarthria is only a motor impairment, but in the strict sense not a language impairment, what kind of neurological damage causes it? Dysarthria is caused by lesions to the motor cortex or the cranial nerves (2). More specifically, damage to the upper motor neurons, leading from the brain to the spine, causes spastic dysarthria, marked by excessive and uncontrolled tone. Further, if the lesion is unilateral, the impairment is characterized by drooping of one (the opposite) side of the face, slow tongue movement, and a difficulty meeting the weak side of the mouth. If it is bilateral, the tongue and thus the range of possible sounds, are more greatly affected, palate mobility is reduced, resulting in more nasal speech, and there are chewing and swallowing problems (4).
Damage to the lower motor neurons, leading from the spine to the muscles, results in flaccid dysarthria, which affects all aspects of speech production (5).: the articulators, soft palate, larynx and respiratory system. Flaccid dysarthria is often seen together with swallowing problems, or dysphasia (5).
The motor control of speech is interesting because, as the movements occur along a central vertical line, the task requires a high degree of coordination from both hemispheres. This becomes especially clear in the case of spastic unilateral dysarthria, described above, which has asymmetrical symptoms. This is an example of the high degree of coordination between the hemispheres. Reilly and Marian (6) point out that a good amount of this coordination is present in the child as early as 40 hours post-natally - the infant can protrude its tongue in imitation of someone else. This poses interesting questions as to the amount of myelination (specifically of the corpus callosum) at birth. It could be worth it to pursue this point further, seeing whether tongue protrusion and other facial movements are really universally present in young infants, and to what extent they are free from asymmetric characteristics (so that the claim that both hemispheres are involved can legitimately be made).
It seems to be a distinct possibility that, as dysarthria is characterized by the dis-coordination of motor signals from the two hemispheres, or general impairment of their functions, it can occur even in pre-linguistic infants. However, it would not be detected as such, because it is officially categorized as a language disorder. In this sense, it seems beneficial to undertake close studies of the development of facial movement in infants, in order to detect and take therapeutic steps against dysarthria before it becomes a language disorder.
A further point that makes the dysarthrias interesting is that the form resulting from impairment to the upper motor neurons (spastic dysarthria) and the form resulting from impairment of the lower motor neurons (flaccid dysarthria) share some common symptoms. This suggests that functions are to a good extent distributed across the neural network: there is more than one way to lesion the network and with that impair a single function.
To conclude, dysarthria could well be viewed as a language disorder "on the margins", as its underlying neurological causes are not specifically linked to the brain areas specific to language, but rather to another modality: movement. Thus dysarthria is in one category in terms of symptoms, but in another in terms of underlying cause. This status also makes it a common side effect to other diseases that affect the motor neurons, such as multiple sclerosis (7). Cerebral Palsy (8), or ALS (9). .In this sense, dysarthria is strong evidence for the claim that output-to-representation of function mappings in the brain are not necessarily one-to-one, but rather that different functional modalities overlap to some extent. In simpler terms, the motor modality contributes to the language modality, so that injury to a specific aspect of the motor modality means injury to the language modality. This effect of 'modalities' on each other, then, clearly has to be taken into consideration as research is conducted into the question of 'what is what' in the language process.
WWW Sources1) The Gale Encyclopedia of Medicine: Aphasia , A description of aphasia and related speech disorders
2) Dysarthria , a brief definition
3) Dysarthria , A longer description of dysarthria and the sub-systems it affects
4) Center for Speech Language and Occupational Therapy , A more technical description of the different dysarthrias
5) Flaccid Dysarthria , A lecture slide listing some characteristics of Flaccid Dysarthria
6) Neonatal Imitation: An hypothesis and model , Describes infant's early development of voluntary facial muscle movements.
7) How MS Affects Speech , Describes the frequent co-occurrence of dysarthria with MS
8) Gale Encyclopedia of Childhood and Adolescence: Cerebral Palsy , Describes CB, mentioning that dysarthria can be a side-effect.
9) Phonatory Instability in ALS dysarthria: a case study , Describes the specific form of dysarthria in a single patient with ALS.
Comments made prior to 2007
This article is a bit confusing, since the title says: a SPEECH
disorder "on the margins" but throughout the text it is discussed,
whether dysarthria is a LANGUAGE disorder or not. It clearly is no
language disorder, but a speech disorder.Dysarthria is defined as:
"Dysarthria comprises a group of speech disorders resulting from
disturbances in muscular control. Because there has been damage to the
central or peripheral nervous system, some degree of weakness,
slowness, incoordination, or altered muscle tone characterizes the
activity of the speech mechanism." (Darley et al. 1975:2)
A language disorder such as aphasia is regarded as a higher level cortical disorder with a ìdeficit in the comprehension and/ or expression of language involving one or more of the primary modalities of language function (listening, reading, speaking, writing, or gesturing) (Love 1995:30). Dysarthria and aphasia can co-occur, but in speakers with a pure aphasic-phonologic disorder the speech production is generally well articulated and not distorted. Furthermore neither writing nor the comprehension of reading and writing is affected in speakers with dysarthria.
Therefore there are differences between speech and language disorders, even if similar regions in the brain may but not have to be affected. Ataxic dysarthria for example is due to damage of the cerebellum, which is responsible for motor control ... Jess, 10 December 2006