Auditory Verbal Agnosia
Author: Catherine Guilbeau
Auditory verbal agnosia, better known as pure word deafness (PWD), is an exceptionally
rare and specific type of auditory agnosia. Agnosias in general are defined as having
the inability to interpret and understand sensations. Like other agnosias, PWD is
not classified in the Diagnostic and Statistical Manual of Mental Disorders (DSM)
because it is not considered to be a psychological disorder. The primary symptom of
PWD is the inability to comprehend spoken words. PWD patients describe hearing spoken
language as meaningless noise as though the person speaking was talking in a foreign
language. Additionally, it has also been noted that these patients experience greater
difficulty perceiving consonants because they are temporally more dynamic stimuli
compared to vowels which are steady state stimuli (Slevc, Martin, Hamilton, and Joanisse,
2011). Interestingly, patients with PWD maintain the ability to hear environmental
sounds, speak, repeat spoken language, read, and write (Wirkowski, Echausse, Overby,
Ortiz, and Radler, 2006).
Research of PWD patients indicates that this particular type of agnosia is rarely
if ever present in someone from birth. Rather, PWD results from accidents involving
head injuries or traumas; consequently, PWD patients often suffer from additional
injuries as well. The addition of other injuries adds to the complexity of identifying
lesions specific to PWD in the brain. The scarce number of people diagnosed with PWD
also adds to this difficult endeavor: no patient displays damaged areas of the brain
in exactly the same places. Though difficulties in identifying the physical causes
of PWD have been encountered, two dominant areas of the brain, both involving damage
to the temporal lobe, have been identified as contributing to the cause of PWD (Zhu,
Lv, Shan, Xu, and Luo, 2010).
The most common group of areas damaged in cases of PWD are the temporal lobes, especially
the transverse temporal gyri also known as Heschl's gyri (Wirkowski et al., 2006).
Heschl's gyri are the first cortical structures to process incoming auditory information
and are unique in their position within the lobes as the only parts that run toward
the center of the brain. Difficulty in identifying lesioned areas of the brain associated
with PWD not only involves where the lesion is but when in the process of hearing
the lesion affects auditory functions. Because damage to the temporal lobes bilaterally
interrupts language comprehension early in the auditory process, it is unclear if
understanding language can be localized to Heschl's gyri or if auditory information
is being cut off before reaching the part of the brain that interprets language.
A second and less dominant group of damaged areas present in PWD patients involve
unilateral lesions in the left temporal lobe (Slevc et al., 2011). Previously, researchers
have identified the left hemisphere as the more dominant hemisphere for language functions
and processing rapid temporal aspects of sounds, an ability which is imperative for
identifying consonants. Though the right hemisphere also contributes to auditory processes,
it serves a more dominant role in processing environmental sounds or spectral aspects
of sound, faculties which are intact for PWD patients. Specifically, the superior
temporal cortex, which processes frequency of sound, and the planum temporal, which
is a triangular region around Wernicke's area, are areas in the left temporal lobe
that are damaged in PWD patients. Wernicke's area, a major area for language comprehension,
is included in both of these specified areas. Language comprehension is consequently
affected when it is damaged. Further, the planuum temporal is one of the most asymmetric
parts of the brain and is roughly ten times larger in the left temporal lobe compared
to the right. This asymmetry sheds light on the importance of the left temporal lobe
specifically for PWD patients and may explain why damage to only the right temporal
lobe does not result in PWD. Therefore, though important parts of the brain have been
recognized as damaged in some PWD patients, specific neurobiological causes of the
disorder have yet to be identified.
Treatment for PWD patients beyond compensatory treatment proves to be nearly impossible
at this time. In addition to little clarity in the locations of lesions resulting
in PWD, the neurological aspects of the brain are unaffected therefore making drug
treatment ineffective as of yet. Several variations of therapy have been used in an
effort to help the PWD patient 'relearn' the language using similar techniques used
when learning a foreign language (Slevc et al., 2011). Activities such as pairing
the sounds of the word 'chair' with a picture of a chair were used in hopes of creating
a connection for the patient between the sound of the word and the visual stimulus.
However, because patients did not forget the language for they can still read and
write it, they cannot relearn the language as though it is foreign. Rather, PWD patients
have lost the ability to understand spoken language and find meaning in spoken language.
The only therapy that has caused any improvement in PWD patients, phoneme discrimination
therapy, confronts the same problem. Phoneme discrimination therapy works with the
most basic parts of words, for example ba, -da, -la (Tessier, Weill-Chounlamountry,
Michelot, and Pradat-Diehl, 2007). PWD patients have shown particular difficulty in
discriminating between the sounds of phonemes. Therefore, the theory behind this therapy
is to increase patients' ability to hear minute differences between phonemes in order
to then combine the phonemes into identifiable and meaningful spoken words. Yet because
PWD patients no longer have the ability to meaningfully understand spoken language,
this technique did little to help improve understanding; it only improved phoneme
As a result of difficulty in identifying damaged brain structures as well as effective
treatments, PWD patients most often rely on compensatory treatment using the functions
they do have intact. The most important skill is the ability to read and write. With
these abilities, patients are able to communicate with others easily, though with
some inconvenience, due to the length of time it takes to write a word compared to
speaking a word. Skills often used by those who are completely deaf such as sign language
and lip reading are also used to communicate more effectively and efficiently. Therefore,
although PWD is a rare disorder in which treatment is scarce and the cause is ultimately
a mystery to researchers today, PWD patients often successfully adapt to their deficiencies
and are able to live functional and comfortable lives.
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