Conduction aphasia

In neurology, conduction aphasia, also called associative aphasia, is an uncommon form of difficulty in speaking (aphasia). It is caused by damage to the parietal lobe of the brain. An acquired language disorder, it is characterised by intact auditory comprehension, coherent (yet paraphasic) speech production, but poor speech repetition. Affected people are fully capable of understanding what they are hearing, but fail to encode phonological information for production. This deficit is load-sensitive as the person shows significant difficulty repeating phrases, particularly as the phrases increase in length and complexity and as they stumble over words they are attempting to pronounce. People have frequent errors during spontaneous speech, such as substituting or transposing sounds. They are also aware of their errors and will show significant difficulty correcting them.

For example:

"Clinician: Now, I want you to say some words after me. Say 'boy'. Aphasic: Boy.

Clinician: Home.

Aphasic: Home.

Clinician: Seventy-nine.

Aphasic: Ninety-seven. No ... sevinty-sine ... siventy-nice...

Clinician: Let's try another one. Say 'refrigerator'.

Aphasic: Frigilator ... no? how about ... frerigilator ... no frigaliterlater ... aahh! It's all mixed up!'''"

In 1970, Tim Shallice and Elizabeth Warrington were able to differentiate two variants of this constellation: the reproduction and the repetition type. These authors suggested an exclusive deficit of auditory-verbal short-term memory in repetition conduction aphasia whereas the other variant was assumed to reflect disrupted phonological encoding mechanism, affecting confrontation tasks such as repetition, reading and naming in a similar manner.

Left-hemisphere damage involving auditory regions often result in speech deficits. Lesions in this area that damage the sensorimotor dorsal stream suggest that the sensory system aid in motor speech. Studies have suggested that conduction aphasia is a result of damage specifically to the left superior temporal gyrus and/or the left supramarginal gyrus. The classical explanation for conduction aphasia is a disconnection between the brain areas responsible for speech comprehension (Wernicke's area) and that of speech production (Broca's area). This is due to specific damage to the arcuate fasciculus, a deep white matter tract. Aphasic people are still able to comprehend speech as the lesion does not disrupt the ventral stream pathway.

Signs and symptoms
Conduction aphasics will show relatively well-preserved auditory comprehension, which may even be completely functional. All cases are individualized and unique to their own extent. Speech production will be fluent, grammatically, and syntactically correct. Intonation and articulation will also be maintained. Speech often contains some paraphasic errors: phonemes and syllables will be dropped or transposed (e.g., "snowball" → "snowall", "television" → "vellitision", "ninety-five percent" → "ninety-twenty percent"). The hallmark deficit of this disorder, however, is in repetition. Aphasic people will show an inability to repeat words or sentences when asked by an examiner. After saying a sentence to a person with conduction aphasia, he or she will be able to paraphrase the sentence accurately but will not be able to repeat it. This is possibly because their "motor speech error processing is disrupted by inaccurate forward predictions, or because detected errors are not translated into corrective commands due to damage to the auditory-motor interface". When prompted to repeat words, the person will be unable to do so, and produce many paraphasic errors. For example, when prompted with "bagger", a person may respond with, "gabber". Recent summaries about the syndrome show similarities between defective speech and writing and their relatively good comprehension. The sudden speech of a conduction aphasic is fluent, yet it is lengthy and inadequately structured. Aphasic people have difficulty in finding words appropriate to context and in accurately pronouncing a word. Aphasic errors in naming, reading aloud, and repeating are recognized.

Individuals with conduction aphasia are able to express themselves fairly well, with some word finding and functional comprehension difficulty. Although people with aphasia may be able to express themselves fairly well, they tend to have issues repeating phrases, especially phrases that are long and complex. During an assessment of aphasia, the clinician usually examine the person's verbal fluency, comprehension, repetition, reading, writing, and naming. When asked to repeat something, the person will be unable to do so without significant difficulty, repeatedly attempting to self-correct (conduite d'approche).

However, aphasics recognize their errors and will repetitively try to correct them. Typically, an aphasic will make multiple attempts correcting errors until they are correct. This recognition is due to preserved auditory error detection mechanisms. Errors frequently fit a pattern of incorrect approximations. These common errors typically occur in morphemes that a) share one or more similarly located phonemes but b) differ in at least one aspect that makes the substituted morpheme(s) semantically distinct. This repetitive effort to approximate the appropriate word or phrase is known as conduite d'approche. Repetitive self correction is commonly used by Aphasic people of conduction aphasia. Due to their relatively preserved auditory comprehension, conduction aphasics are capable of accurately monitoring, and attempting to correct, their own errors in speech output. For example, when prompted to repeat "Rosenkranz", a German-speaking aphasic may respond with, "rosenbrau... rosenbrauch... rosengrau... bro... grosenbrau... grossenlau, rosenkranz,... kranz... rosenkranz".

Conduction aphasia is a mild language disability, and most people return to their normal lives. Broca's and Wernicke's aphasia are commonly caused by middle cerebral artery strokes. Symptoms of conduction aphasia, as with other aphasias, can be transient, sometimes lasting only several hours or a few days. As aphasia's and other language disorders are frequently due to stroke, their symptoms can change and evolve over time, or simply disappear. If the cause is a stroke, people can make a good recovery but may have persistent deficits. This is because the healing in the brain after inflammation or hemorrhage, leads to decreased local impairment. Furthermore, the plasticity of the brain may allow the recruitment of new pathways to restore lost function. For example, the right hemisphere speech systems may learn to correct for left-hemisphere damage. However, chronic conduction aphasia is possible, without transformation to other aphasias. These people show prolonged, profound deficits in repetition, frequent phonemic paraphasias, and repetitive self-correction during spontaneous speech.

Causes
Conduction aphasia is caused by damage to the parietal lobe of the brain, especially in regards to the area associated with the left-hemisphere dominant dorsal stream network. The arcuate fasciculus, which connects Broca's area and Wernicke's area (important for speech and language production and comprehension, respectively), is affected. These two areas control speech and language in the brain. The arcuate fasciculus is a thick band of fiber that connects the two areas and carries messages between them. When this area is damaged, the person experiences damage to the auditory-motor integration system. This results in disruption to the delayed auditory feedback network, causing the individual to have difficulty correcting themselves on speech repetition tasks. Additionally, recent evidence suggests that conduction aphasia can also be caused by lesions in the left superior temporal gyrus and/or the left supramarginal gyrus.

Conduction aphasia can also be seen in cases of cortical damage without subcortical extensions.

Pathophysiology
Recent research has pointed to multiple different explanations for conduction aphasia, which is based on newer models suggesting language is facilitated by "cortically based, anatomically distributed, modular networks."

Diagnosis
Several standardized test batteries exist for diagnosing and classifying aphasias. These tests are capable of identifying conduction aphasia with relative accuracy. The Boston Diagnostic Aphasia Examination (BDAE) and the Western Aphasia Battery (WAB) are two commonly used test batteries for diagnosing conduction aphasia. These examinations involve a set of tests, which include asking person to name pictures, read printed words, count aloud, and repeat words and non-words (such as shwazel). Neuro-imaging should also be used to look for a stroke, tumor, infection, or another pathology in the setting of conduction aphasia. This can be done through a CT or MRI or the brain; these are the first imaging modality of choice.

Treatment
Speech and language therapy are typically used as a treatment. There are no medical or surgical treatments. Treatment for Aphasia is generally individualized, focusing on specific language and communication improvements, and regular exercise with communication tasks. Regular therapy for conduction aphasics has been shown to result in steady improvement on the Western Aphasia Battery. The Western Aphasia Battery assesses neurological disorders to discern the degree and type of aphasia present. The test also discerns a person's strengths and weaknesses, which can be used to treat the person better. Therapists should customize their treatment for each patient. The main focus for during speech therapy for conduction aphasia person is to strengthen correct word usage and auditory comprehension. A major goal is to focus on repetition.

History
In the late 19th century, Paul Broca studied person with expressive aphasia. These person had lesions in the anterior perisylvian region (now known as Broca's area), and produced halting and labored speech, lacking in function words and grammar.

For example:  Clinician: What brought you to the hospital?

Patient: yes ... ah ... Monday ... ah ... Dad ... Peter Hogan, and Dad ... ah ... hospital ... and ah ... Wednesday ... Wednesday ... nine o'clock and ah Thursday ... ten o'clock ... doctors two ... two ... an doctors and ... ah ... teeth ... yah ... and a doctor an girl ... and gums, an I. Comprehension is generally preserved, although there can be deficits in interpretation of complex sentences. In an extreme example, one of his person could only produce a single syllable, "Tan".

Meanwhile, Carl Wernicke described person with receptive aphasia, who had damage to the left posterior superior temporal lobe, which he named "the area of word images". These person could speak fluently, but their speech lacked meaning. They had a severe deficit in auditory comprehension. For example, "Clinician: What brings you to the hospital?

The two disorders (expressive and receptive aphasias) thus seemed complementary, and corresponded to two distinct anatomical locations.

Wernicke predicted the existence of conduction aphasia in his landmark 1874 monograph, Der Aphasische Symptomenkompleks: Eine Psychologische Studie auf Anatomischer Basis. He was the first to distinguish the various aphasias in an anatomical framework, and proposed that a disconnection between the two speech systems (motor and sensory) would lead to a unique condition, distinct from both expressive and receptive aphasias, which he termed Leitungsaphasie. He did not explicitly predict the repetition deficit, but did note that, unlike those with Wernicke's aphasia, conduction aphasics would be able to comprehend speech properly, and intriguingly, would be able to hear and understand their own speech errors, leading to frustration and self-correction.

Wernicke was influenced by Theodor Meynert, his mentor, who postulated that aphasias were due to perisylvian lesions. Meynert also distinguished between the posterior and anterior language systems, leading Wernicke to localize the two regions. Wernicke's research into the fiber pathways connecting the posterior and anterior regions lead him to theorize that damage to the fibers under the insula would lead to conduction aphasia. Ludwig Lichtheim expanded on Wernicke's work, although he labeled the disorder commissural aphasia, to distinguish between aphasias tied to processing centers.

Sigmund Freud would argue in 1891 that the old framework was inaccurate; the entire perisylvian area, from the posterior to the anterior regions, were equivalent in facilitating speech function. In 1948 Kurt Goldstein postulated that spoken language was a central phenomenon, as opposed to a differentiated and disparate set of functionally distinct modules. To Freud and Goldstein, conduction aphasia was thus the result of a central, core language breakdown; Goldstein labeled the disorder central aphasia.

Later work and examination of brain structures, however, implicated the arcuate fasciculus, a white matter bundle connecting the posterior temporoparietal junction with the frontal cortex. Norman Geschwind proposed that damage to this bundle caused conduction aphasia; the characteristic deficits in auditory repetition were due to failed transmission of information between the two language centers. Studies showed that conduction aphasics had an intact 'inner voice', which discredited the central deficit model of Freud and Goldstein. The Wernicke-–Lichtheim–Geschwind disconnection hypothesis thus became the prevailing explanation for conduction aphasia. However, recent reviews and research have cast doubt on the singular role of the arcuate fasciculus and the model of spoken language in general.