Auditory verbal agnosia

Auditory verbal agnosia, also known as Pure Word Deafness, is caused by bilateral damage to the posterior superior temporal lobes or disruption of connections between these areas. It exhibits itself as inability to comprehend the meaning of speech, but (in most cases) still being able to hear, speak, read, and write. Individuals that exhibit pure word deafness are usually also able to comprehend non-verbal sounds. Sometimes this agnosia is preceded by complete cortical deafness of varying duration, however this is not always the case. Researchers have documented that in most patients exhibiting auditory verbal agnosia the differentiation of consonants suffers more than that of vowels, but as with most neurological disorders there is variation among patients.

Presentation
Auditory verbal agnosia is a rarely diagnosed disorder in its pure form of being incapable of phonemic discrimination. Often individuals diagnosed with auditory verbal agnosia are also incapable of discriminating between non-verbal sounds as well as speech. The underlying problem seems to be temporal in that understanding speech requires the discrimination between specific sounds which are closely spaced in time. Note that this is not unique to speech; studies using non-speech sounds closely spaced in time (dog bark, phone ring, lightning, etc.) have shown that those with auditory verbal agnosia are unable to discriminate between those sounds in the majority of cases, though a few putative examples of speech-specific impairment have been documented in the literature. Historically, those with pure word deafness are not deaf - they can (in the absence of other impairments) hear sounds, including speech (so it has nothing to do with words). As such, "pure word deafness" is something of a misnomer. "Pure word" is intended to mean that only words are affected (not accurate for many cases); "deafness" is intended to specify that these patients have an inability to hear (not accurate).

Auditory verbal agnosia can present as the result of acute damage or chronic, progressive degeneration over time. Cases have been documented that result from severe acute head trauma resulting in bilateral temporal lobe damage. In contrast, auditory verbal agnosia has also been documented to present progressively over several years. In one such case, the patient exhibited progressive word deafness over a 9 year period but did not exhibit any other cognitive of mental deterioration. This patient was found, using MRIs, to have cortical atrophy in the left superior temporal lobe region.

Auditory verbal agnosia is unique among other language perception disorders in that it has a high degree of specificity. This specificity suggests that there is a separation between speech perception, nonspeech auditory processing, and central language processing.

In a study examining a female patient presenting with bilateral temporoparietal destruction was diagnosed with auditory verbal agnosia. It was found that she was unable to comprehend auditory sounds even when the rate of speech was reduced, however temporal resolution was improved when the length of a nonverbal sound was increased. This same patient was also unable to perceive and reproduce rhythm on a rate-dependent scale.

In extremely rare cases, auditory verbal agnosia has been known to present as a symptom of neurodegenerative disease, such as Alzheimer’s disease. In such cases auditory verbal agnosia is a symptom that is typically followed by more severe neurological symptoms typical of Alzheimer’s disease.

Causes
Auditory verbal agnosia is often associated with lesions to the left posterior superior temporal lobe, but no such unilateral case has yet been documented without damage to the white matter tract connecting superior temporal lobes bilaterally or bilateral damage to the superior temporal lobe. In cases where unilateral damage to the left superior temporal lobe has been documented, patients exhibited problems processing both speech and non-speech sounds (in other words, not typical of auditory verbal agnosia). These facts, in combination with the existence of cases of damage to these white matter tracts without detectable cortical damage, in combination with cases of pure word deafness resulting enlargement of the III ventricle alone suggest that the disorder results from damage to the left-right superior temporal circuit rather than the superior temporal area on one hemisphere or another.