The Neuroscience on the Web Series:
CMSD 642 Neuropathologies of Swallowing and Speech

CSU, Chico, Patrick McCaffrey, Ph. D


Chapter 8 Apraxia: Definition and Description: Site of Lesion


Before we get into apraxia of speech (AOS) you will need to review your neuroanatomy notes on the upper motor and lower motor systems. The motor system is present at all levels of the nervous system. It includes "efferent connections of the cortex, especially the frontal lobes; the basal ganglia, cerebellum, and related CNS pathways; descending pathways to motor nuclei of cranial and spinal nerves; efferent fibers within cranial and spinal nerves, and striated muscle" (Duffy, 1995, p. 21). In addition, you must remember that the planning/programming associated with motor speech is thought by many to occur in Broca's area on the third frontal convolution, although a recent study has found that the Island of Reil was involved in 25 stroke patients who had a disorder involving the motor planning of speech (muscle movements) (Dronkers, 1996). Apraxia of speech (AOS) is believed by most rersearchers to result from a Broca's area lesion. According to Duffy (1995), who is with the Mayo Clinic, AOS as the primary speech pathology accounted for 9% of patients with motor speech disorders seen at the clinic between 1987 and 1990. In addition it is seen as a secondary diagnosis with aphasias and other neurological disorders. Remember, it is not a neuromuscular disorder. It is a planning/programming problem. According to Duffy, 1995, vascular lesions are the most prevalent cause of AOS. In addition he states that most cerebral vascular accidents (cva's) occur in the middle cerebral artery of the left hemisphere.

I like the neurological model of communication developed by Meitus and Weinberg, 1983, It enables one to clearly see the steps involved in oral communication from the development of the concept to the production of speech.

Meitus & Weinberg (1983, p.227) have four ordered steps or levels in their neurological model of oral communication. They are:

  • Ideation - when the concepts we wish to express are generated
  • Symbolization - when these concepts are put into a symbolic system congreunt with the rules of the speaker's language;
  • Translation - when these linguistic units or symbols are translated into neuromotor commands that result in the innervation of the motor nerves.
  • Execution - the actual movements of the speech mechanism.

According to Meitus & Weinberg (1983), impairments at each of these levels will result in a different disorder:

  • Impairment at the ideation level will result in mental- verbal dysfunction (or mental confusion, dementia, confused language)
  • Impairment at the level of symbolization will result in aphasia
  • Impairment at the level of translation will result in apraxia of speech
  • Impairment at the level of execution will result in dysarthria.

What Is Apraxia of Speech?

According to Halperin (1986 in Chapey, 1986, p.422), "apraxia of speech is an articulation disorder that results from impairment due to brain damage, of the capacity to order the positioning of speech musculature and the sequencing of muscle movements for volitional production of phonemes and sequences of phonemes; but it is not accompanied by significant weakness, slowness, or incoordination of these same muscles in reflex and automatic acts." There is a great difference between motor speech programming and the neuromuscular execution of speech.

With an acquired motor disorder of speech (as is dysarthria), basic language processes are intact, (although some would consider apraxia to be a type of phonoligical aphasia) but the mechanical production of speech is impaired because of nervous system damage.

Apraxia of speech is a disruption of the capacity to program the skilled oral movements necessary for speech. The problem is with the programming associated with incorrect neural commands at higher, more central levels.

"Apraxia" comes from the Greek word "praxis," which means action.

In the past, apraxia was often classified as a type of articulation disorder and was in fact called central dysarthria by some. Now, it is considered a motor planning/programming deficit. According to Wertz (1984), apraxia of speech "is a neurogenic phonological disorder, resulting from the sensorimotor impairment of the capacity to select, program and execute coordinated movements of the speech musculature for the production of voluntary speech." In other words, the part of the brain that generates the motor programs for speech/phonology is damaged. Darley et.al. 1975 have referred to the motor planning aspect of language/speech as the motor speech programmer (MSP). Kearns and Simmons (1989, in Northern, 1989) reported that research using spectographic analysis of voice onset time (VOT) supports the programming position. The following physiological studies support the spectrographic results: fiberoptic observation, electromyography, voicing measures, and muscular movement.

Other Apraxias

Limb apraxia is another planning deficit that impairs the voluntary movements of the arms, hands, legs and feet. Many tests used to diagnose receptive aphasia require patients to respond to questions by pointing at items or following commands involving limb movement like "scratch your nose." For this reason, speech pathologists must be aware that a person's poor scores on some sections of an aphasia battery may be due to limb apraxia rather than aphasia. lesions that cause limb apraxia are usually in the pre-motor area, Brodmann's area 6. According to Duffy, 1995, limb apraxia is a disorder of the dominant hemisphere, i.e. the left hemisphere for most..

As with apraxia of speech, voluntary movements of the limbs are affected by this disorder, but often involuntary movements remain intact. A patient with this problem would be unable to follow the command "scratch your nose" but could perform this action with ease if his/her nose were itching.

Oral apraxia is an inability to make voluntary, non-speech oral movements. A person with this problem would be unable to stick out his tongue if told to do so, but could perform this action without struggle if given an icecream bar to eat. As the auditory comprehension sections of some aphasia batteries include commands involving oral structures like "lick your lips," oral apraxia may also be mistaken for aphasia.

Apraxia of gait refers to difficulty with programming the motor movements involved in walking.

Apraxia of the swallow refers to a patients inability to swallow volitionally.

Back to AOS also called Verbal Apraxia

According to Darley (1978, as cited in Meitus & Weinberg, 1983, p. 266), the characteristics of apraxia of speech are as follows:

The patient struggles to avoid errors and to correct articulatory positioning, leading to prosodic alterations (slow rate, equalization of syllabic stress, etc).

Substitution errors predominate, but some prolongations, additions, and repetitions occur. Distortions and omissions are less frequent . Recent studies using instrumentation and narrow transcription reveal that distortions are actually the predominant error, but using broad transcription and listening, substitutions errors seem most common (Wertz, et al., 1984, p. 53).

Sound or syllable transpositions may be present.

Sometimes, articulatory errors are complications rather than simplifications.

Errors are often inconsistent off-target approximations. There is variability in type of articulatory errors, and instances of error-free fluent speech on some words, phrases, and occasionally on sentences.

Anticipatory errors may be present. The patient anticipates a speech sound which will occur further on in the word or sentence.

There is greatest difficulty in initiating speech, as noted by hesitations, and nonfluencies, visible searching, groping movements of the articulators, numerous trials, and false starts (this resembles stuttering).

Speech comprehension and word recognition abilities are often disproportionately better than speech production abilities.

Patients recognize their articulatory errors.

The number of errors increases as a function of word length.

More errors occur on consonants, and sounds which require precise articulatory adjustments, e.g. fricatives and affricates. Consonant clusters are most difficult of all. Palatal and dental phonemes are associated with a high rate of errors (Halperin, 1986 in Chapey, 1986).

Greater difficulty is noted during purposeful speech than during the production of reactive, automatic, or emotional speech.

There are increased errors in the production of words such as nouns, verbs, adjectives and adverbs, which carry more linguistic weight (Meitus & Weinberg, p. 268).

Accuracy on frequently occurring phonemes is higher (Wertz et al. p. 62).

Accuracy is better with meaningful speech compared to nonsense stimuli (Wertz et al. p. 63).

Fewer errors are made during unstructured spontaneous language than in word repetition or imitation tasks (Chapey, 1981). However, there is debate about this, with Wertz et al., 1984, (p. 66) believing the opposite to be true.

Patients do better with stimuli presented by a visible examiner (auditory-visual mode) than with stimuli presented on a tape recorder (Halperin, 1986).

Patients do better given repeated trials of a word rather than with increased stimulus presentations. In other words, one stimulus presentation with several opportunities to imitate it is better than the converse. (Johns and Darley, 1970, in Halperin, 1986) This has important implications for therapy!!

Some report no benefit when using the following:

having patients observe themselves in a mirror,

introducing masking noise to prevent patients from hearing their own speech,

delaying the patient's imitative response,

varying the instructional set given to the patient as to the task's difficulty,

using a metronome (Halperin, 1986).

Apraxia of speech is often, but not always, accompanied by oral motor apraxia, or an inability to volitionally move the mouth, tongue, lips, cheeks, larynx, and pharynx as directed (Halperin, 1986).

Some patients who suffer from apraxia of speech have impaired oral sensation and perception (Wertz et al., 1984 p. 75).

The following taken from Halperin (1984), who himself draws from numerous sources, is somewhat redundant but it may help you better understand apraxia of speech:

Involuntary speech is relatively preserved.

Because apraxia involves problems with motor planning for voluntary or purposeful speech, involuntary speech or the recitation of common, over-learned phrases and sayings remains remarkably intact. A patient with apraxia may struggle and appear to "grope" for sounds when trying to produce simple sentences or even single words during a conversation but might be able to recite the Lord's prayer or the Pledge of Allegiance clearly and easily.

For apraxics, imitative responses are even more difficult to produce than spontaneous speech, possibly because this kind of speech is especially "voluntary."

Articulatory errors increase as the complexity of the motor adjustment required of the articulators increases.

Therefore, it is easier for the apraxic patient to produce vowels than consonants. Single consonants are more readily produced than blends or clusters. Fricatives and affricates are the hardest manner of production for apraxics to master. As for place of production, palatal and dental phonemes seem to be the most difficult sounds. In general, phonemes that occur with high frequency in the language reproduce more accurately than those that occur less frequently.

Articulation errors will increase as the length of words increases. When saying a multi-syllable word, an apraxic may produce the syllables out of order.

Some errors are anticipatory.

Like stutterers, apraxics will sometimes realize that they are going to have difficulty producing an up-coming sound and hesitate. Such pauses make apraxic speech dysfluent and impairs prosody.

Some errors are perseveratory.

The apraxic patient will have difficulty virtually every time he/she tries to produce certain sounds or certain words. According to Deal (1974, as cited in Halpern, 1984), apraxic patients demonstrate a "consistency" effect when asked to read the same passage several times. That is, they make errors in the same places during all trials.

Errors are inconsistent.

While apraxics will error repeatedly on the same word, the specific off-target approximations produced will vary. For example, if an apraxic patient has difficulty saying the word "telephone" he/she may first err by saying "pelephone" but then mispronounce the target as "kelephone" or "felephone" on the next trial.

Initial consonants are misarticulated more frequently than are consonants in other positions.

This is another way in which apraxic speech resembles stuttered speech. Initial consonants may be more difficult for apraxics because they are more affected by the anticipation of errors. Also, it may be that once the apraxic has started a word and produced a vowel sound, this helps them to ease into the production of the remaining consonants.

Articulatory errors occur more often on words that carry a lot of psychological or linguistic weight and are therefore most essential for communication.

This is yet another way that apraxic speech resembles stuttered speech.

Errors are often off-target approximations of the desired articulatory production.

Errors are more likely to differ from targets in terms of place and manner of articulation rather than voicing.

Although omissions, distortions, additions and transpositions all occur, substitutions are the type of error most frequently made by apraxics.

According to Johns and Darley (1970, as cited in Halpern, 1981), patients are more likely to produce an articulatory target correctly when they are given several opportunities to do so.

Repeated presentation of the target by the clinician, on the other hand, is not helpful. This means that the clinician should present a stimulus once and then give the patient four or five chances to repeat it.

The modality in which a stimulus is presented influences the articulatory success of apraxic patients.

The repititions of an individual with apraxia are more accurate when they are able to watch a clinician produce the target (speech reading the clinician) than when they only hear or read the stimulus.

Finally, it should be noted that there are several different types of apraxia. The form of the disorder that affects speech should really be called apraxia of speech or verbal apraxia to distinguish it from these other types

Site of lesion

There is some controversy but apraxia of speech appears to result from damage to the inferior or third frontal convolution of the dominant hemisphere (the region of Broca's area). This region is also known as Brodmann's area 44/45. Frequently Broca's aphasia and apraxia co-occur. Some feel that the basal ganglia may have programming functions similar to those attributed to Broca's area.

Such lesions can be produced by stroke or traumatic brain injury (or other types of brain damage) tumors, metabolic encephalopathy and etc. According to McNeill and Kent, 1990 (in Duffy, 1995), apraxia of speech may be an integral part of the Broca's aphasia syndrome. Many others disagree.


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Other courses in the Neuroscience on the Web series:
CMSD 620 Neuroanatomy | CMSD 636 Neuropathologies of Language and Cognition

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