MOTOR SPEECH DISORDERS
Apraxia of Speech and the Dysarthrias
Introduction
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No human movement patterns are as intricate, complex, or
intertwined with all the human activities of learning,
loving, and living as is speech.
Far more area of the brain is devoted to the control of
the tiny muscular adjustments of the tongue, lips, vocal
folds, and other speech articulators than to those muscle
groups needed for walking upright.
Far more coordination and synchrony are needed for
speech than for riding a unicycle, threading a needle,
rolling a log, or removing a thorn from the foot.
Introduction
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The amazing think is that the act of speech becomes so
automatic that we hardly think about it—until something
goes wrong.
Damage that disturbs the delicate sequence of sensorymuscular-integrative events can affect the function of the
respiratory mechanism, the tone-producing mechanism,
resonance system, and the articulatory system that shapes
and molds the sound stream into recognizable words.
Motor speech disturbances, such as apraxia of speech
(AOS) or the various dysarthrias, depend on the level of
motor control that experiences damage.
Speech Production
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As a fine motor skill, speech production is performed with
accuracy and speed; it involves motor flexibility in
achieving goals; and it improves with practice.
Speakers are capable of reaching highly precise
successions of vocal tract shapes at critical points in time,
with spatial-temporal goals being reached rapidly,
interactively, and automatically.
Velocity and directional changes in articulatory
movements are accomplished automatically in the upper
airway while constant subglottic air pressure is
maintained.
Speech is also goal-directed: the goal is to produce the
appropriate acoustic patterns via motor actions.
Speech Production
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Motor actions are afferently-guided by the speaker’s
internal referent of how it feels and sounds to produce
certain speech movements and acoustics.
Therefore, motor actions are not fixed movement routines
or stored patterns of muscle contractions.
Speakers are capable of generating functionally
equivalent motor sequences when needed.
Because speech requires adaptive control, it is one of the
most advanced examples of selective access to muscle
pattern generators for the purpose of spontaneously
creating novel motor acts.
Motor Control Processes
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Individuals who study the neurophysiology of speech
movement recognize several levels in the speech motor
control process.
The levels most commonly identified are planning,
programming, and execution.
Motor control process levels are thought to be distinct
operations occurring prior to or during the generation of
speech motor output.
While definitions of these contracts are often
operationalized, the terms are not always used uniformly.
Motor Planning
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Motor planning is considered an early process in speech
motor control, preceding programming.
Motor planning is seen primarily as a cognitive process
involving ideation and intention related to the formulation
of the spoken message.
The intended goals of speech production can be thought
of as linguistic units of phonemes, words, or phrases.
In the planning of speech utterances, such units are
represented or coded in terms of spatial, aerodynamic, or
acoustic targets.
Motor Planning
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The motor plan, then, represents an open-ended package
of motor actions for generating higher order linguistic and
phonological rules.
It involves selection of an appropriate movement strategy
in light of intended goals and prevailing physical
conditions (e.g., the “an” versus “a” for nouns; lip rounding
of /s/ in “soup” versus lip retraction in “see.”
It involves processes that cannot be easily automatized
because they involve the manipulation of unpredictable or
infrequent processing variables depending upon the
speaker’s discourse, selection of lexical items, and
syntactic/morphological decisions.
Motor Programming
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Motor programming entails provisional specification of
precisely how the motor plan is to be achieved: which
muscles are to contract, how much, and when.
Programming involves pretuning the excitability of various
sensory and motor pathways to be involved in the ensuing
movement process so as to provide for optimal use of
sensory information during execution.
The motor program is responsive to time constraints which
limits it to automatizeable processes operating on
predictable, highly repetitive, not-too-varied relationships.
Motor Programming
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Motor programming processes might include translation of
the action plan into a chain of executable and
coordinated articulatory movements, and adjustment of
the output to actual speaking conditions.
By making optimal use of sensory information, the motor
programmer organizes and assembles a set of muscle
command subroutines into an appropriate movement
configuration for action.
It does not detail movements and specific muscle
contractions; instead is assembles predictable movement
sequence templates, such as those needed to produce
stridency, or retroflexion, etc.
Motor Execution
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At the level of motoric execution, neural activation of
synergistic speech muscle groups for a given movement
sequence is initiated based on motor action programs.
Through the course of the execution process, the discharge
of motor neurons may be influenced to varying degrees
by numerous brain centers and sensory pathways.
It is at this final level of execution that speech motor
control, or the neuromotor-sensory mechanisms that direct
and regulate muscle contractions for speech production,
come into play.
Motor Control Summary
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The “plan” is the cognitive strategy for achieving spoken
message formulation.
The “program” is the actual neural code of motor
processing instructions needed to perform a sequence of
motor actions.
Motor execution involves the direct activation of motor
neurons, muscle contraction, and movement.
Neurology of Speech Motor Behavior
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Production of speech requires the complex interaction of
cortical, subcortical, and peripheral levels of the nervous
system.
The cerebral cortex contains motor and sensory areas that
are important in skilled voluntary movement.
The cortical motor areas in the frontal lobe plan and
sequence the organization of motor activities.
They continually transmit and receive updated information
on motor and sensory activity from various other cortical
and subcortical areas.
Neurology of Speech Motor Behavior
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The neurons in the lower portion
of the primary motor cortex
control the execution of motor
commands to the structures used
in articulation, phonation, and
resonation.
Just in front of this motor strip,
the premotor area supplements
the functioning of the motor strip
by integrating and refining oral
motor output and also
participating in planning.
Neurology of Speech Motor Behavior
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The supplemental motor cortex,
lying beneath the premotor
cortex, receives input from the
occipital, parietal, and temporal
lobes and passes this
information on to the primary
motor cortex.
The direct motor pathways from
the primary motor cortex, the
corticobulbar tracts, descend
through the subcortex and
brainstem to connect with the
cranial nerves that control the
oral mechanism.
Neurology of Speech Motor Behavior
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An indirect motor system
loops through various
subcortical structures and
the cerebellum to regulate
and coordinate the neural
impulses of the motor and
premotor cortex.
Specifically, the cerebellum
functions as an error control
device by comparing motor
output from the motor
cortex with sensory signals
from joints and muscles.
Neurology of Speech Motor Behavior
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Because it is furnished with information about all levels of
motor activity, it is an integrating center for coordinated
movement, muscle tone and strength, and posture and
position of the body.
The cranial and spinal nerves make up the peripheral
nervous system (PNS).
The cranial nerves originate in the brainstem and receive
motor execution signals from both the right and left motor
cortices to activate the muscles for speech production.
They also provide sensory feedback by way of the
indirect motor system to the premotor cortex for movement
modification.
Motor Speech Disorders
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As a neuromotor-sensory process, speech requires
complex neural integration and rapid coordination of
several physiological systems: respiration, phonation,
resonation, and articulation.
When any of the underlying structural or physiological
components of speech processing are disrupted through
neurological injury or disease, there are certain
predictable consequences.
Neurologically based disturbances in the selection,
sequencing, and coordinated production of speech sounds
have been labeled oral apraxia, verbal apraxia,
phonemic paraphasia, literal paraphasia, oral-verbal
apraxia, anarthria, dysarthria, apraxic-dysarthria,
cortical dysarthria, and phonetic disintegration.
Apraxia of Speech, Dysarthria, Aphasia
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The differential classification of dysarthria from apraxia
is less problematic, as such differentiation is currently
syndrome based (Croot, 2002).
However, there is still quite a bit of controversy when it
comes to the labels of speech apraxia and phonemic
paraphasia.
Because the jury is still out on the correct label for these
entities, for the purposes of our discussion, we will define
speech apraxia as primarily a phonetic-motoric disorder
and phonemic paraphasia as primarily a linguisticphonemic disorder.
Apraxia of Speech, Dysarthria, Aphasia
Remember, that motor planning is the process in which
movement goals for speech articulators are specified and
synchronized whereas motor programming involves
commands for particular muscle groups to realize motor
plans.
 Disruption to linguistic processes prior to the computation
of motor plans and programs is defined as aphasia.
 Disruption subsequent to the computation of motor plans
and programs is defined as dysarthria.
 It is likely that AOS represents a disruption in both the
computation of the motor plan and the motor program,
resulting from cortical and white matter lesions of the
dominant hemisphere.
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Apraxia of Speech
The most popular cognitive model for distinguishing AOS
from aphasia and dysarthria proposes that AOS is the
appropriate diagnosis when a person is able to complete
the linguistic processing required for speech production but
is hindered in his ability to produce the articulatory
sequences of retrieved phonological representations.
 In other words, someone with ASO has the conceptual,
semantic, and grammatical formulation of the message to
be communicated and can retrieve the abstract
phonological representation for the intended words, but he
is not able to transform retrieved phonological
representations in specifications for articulation (Code,
1998; Miller, 1991; and Itoh & Sasnuma, 1984) .
 These articulatory specifications seem to overlap the motor
plans and motor programs.
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Apraxia of Speech
Because the discipline’s cognitive model of articulatory
planning and programming is still “under construction,” it is
not yet clear to what extent motor plans and programs are
retrieved from a precompiled store or computed online,
take account of feedback, or apply to phoneme-sized
segments or larger units such as syllables, words, or
phrases.
 Presently, the recommendation is to continuing diagnosing
a disorder as AOS when the hypothesized cognitive
disruption occurs in the translation of phonological
representation into specifications for articulation.
 Behaviors that are symptomatic of AOS, whether as a
relatively pure presentation, or in association with aphasia
or dysarthria, or both include:
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Apraxia of Speech
 Errors
in the production of speech segments, including
distortions, perceived phoneme omissions, substitutions,
additions, and exchanges;
 Poor transitionalization between segments of syllables of
speech (reduced coarticulation);
 Reduced speech rate;
 Equal syllable stress altering normal prosody; and
 A range of other behaviors or qualities of speech production
including articulatory groping, struggle or search,
effortfulness, and difficulty initiating speech.
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In contrast with the phonological errors in aphasia, apraxic
sound errors are more consistent in both location and type
than phonological errors.
Apraxia of Speech
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Moreover, successive attempts at the target and greater
target accuracy occur less often in AOS than they do in
phonological disruption.
AOS is highly likely to co-occur with aphasia, or
dysarthria, or both.
Many aphasic, apraxic, and dysarthric disorders occur
as the result of extensive lesions that impair multiple
cognitive systems, resulting in “aphasia with AOS” or
“AOS with dysarthria.”
Dysarthria
Dysarthria is primarily a problem with motor execution.
 Depending upon the location of damage to the CNS or
PNS, multiple system difficulties underlying speech
production will be realized.
 Speech characteristics, oral mechanism abnormalities,
lesion site, and etiology all help to distinguish the six major
kinds of dysarthria from AOS.
 The six types of dysarthria are described by the
predominant impairment to motor execution.
 The descriptor flaccid is applied to movements which
reflect a lack of muscle tone, or evidence of muscle
weakness or paralysis.
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Dysarthria
The descriptor spastic is applied to movements which
reflect reduced range and speed of motion, and
increased muscle tone.
 Ataxia refers to impaired movement coordination and
control.
 The descriptor hyperkinetic is applied to a pattern of
excess movement due to lack of motor inhibition.
 In contrast, the descriptor hypokinetic is applied to lack
of appropriate movement and muscle tone, control, and
posture.
 Combinations of two or more of these movement
abnormalities results in a mixed designation.
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Dysarthria Lesion Sites
Dysarthrias are also classified by their site of lesion.
 If the lesion occurs in the cortex and pyramidal tracts—the
upper motor neurons (UMNs)--the errors seen will primarily
be ones of programming function.
 If the lesion occurs in the extrapyramidal system, including
the cerebellum, coordination of laryngeal function may be
affected, especially control of pitch and loudness.
 If lesions occur in the brainstem or the cranial nerve
itself—lower motor neuron (LMN) disorders--the result may
be a specific kind of movement paralysis, or a change in
muscle tone.
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Dysarthria Classification
Type
Site of Lesion Cause
Speech
Flaccid
Cranial Nerve(s)
Belly's Palsy shallow breathing, breathy
voice, hoarseness, reduced
MG
pitch/ loudness, hypernasality,
imprecise articulation
Spastic
Motor Pathway
Stroke, TBI
drooling, chewing, swallowing
difficulties; observable facial
or articulatory weakness, slow,
jerky consonant production,
harsh vocal quality, emotional
lability
Dysarthria Classification
Type
Site of Lesion Cause
Speech
Ataxic
Cerebellum
Cerebellar
damage
articulatory inaccuracy,
inappropriate loudness
modulation, and poor
pitch control
Huntington’s
sudden forced
inspiration or expiration,
voice stoppages, periods
of breathiness, harsh
voice quality, excessive
loudness variations,
intermittent
hypernasality, distortion
of speech sounds
Hyperkinetic Basal Nuclei
Dysarthria Classification
Type
Site of Lesion
Cause
Speech
Hypokinetic Basal Nuclei
Parkinson’s
reduced loudness and pitch
variability, imprecisely
formed consonants, short
rushes of speech, hoarse and
breathy voice
Mixed
ALS; MS
ALS: labored and slow rate,
short phrasing with lengthy
silences, severely impaired
articulation; hypernasality
(flaccid/spastic)
MS: impaired loudness,
harshness, defective
articulation, impaired stress
(spastic/ataxic).
Multiple Areas