Auditory Verbal Agnosia

Auditory Verbal Agnosia

auditory verbal

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 discrimination.

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.

Bibliography

Ela, W., Nancy, E., Chris, O., Orlando, O., & Linda, R. (n.d). Clinical communication: I can hear you yet cannot comprehend: A case of pure word deafness. Journal Of Emergency Medicine, 3053-55.

Gibbons, C., Oken, B., & Fried-Oken, M. (2012). Augmented input reveals word deafness in a man with frontotemporal dementia. Behavioural Neurology, 25(2), 151-154.

Poeppel, D. (2001). Pure word deafness and the bilateral processing of the speech code. Cognitive Science, 25(5), 679.

Slevc, L., Martin, R. C., Hamilton, A., & Joanisse, M. F. (2011). Speech perception, rapid temporal processing, and the left hemisphere: A case study of unilateral pure word deafness. Neuropsychologia, 49(2), 216-230.

Tessier, C., Weill-Chounlamountry, A., Michelot, N., & Pradat-Diehl, P. (2007). Rehabilitation of word deafness due to auditory analysis disorder. Brain Injury, 21(11), 1165-1174.

Wirkowski, E., Echausse, N., Overby, C., Ortiz, O., & Radler, L. (2006). I can hear you yet cannot comprehend: A case of pure word deafness. Journal Of Emergency Medicine, (1), 53.

Zhu, R., Lv, Z., Shan, C., Xu, M., & Luo, B. (2010). Pure word deafness associated with extrapontine myelinolysis. Journal Of Zhejiang University - Science B, (11), 842.

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