AUDITORY APHASIAS
Auditory aphasias result from damage to a portion of the temporal lobe that includes part or all of six different cortical areas as defined on the basis of internal structure of the cortex, namely areas 41, 42, 22, 39, 21 and 37 of Brodmann. Studies of connectivity and behavioral involvement had led physiologists to group these fields into primary, secondary and tertiary auditory areas. Luria recognized three forms of auditory aphasia corresponding to the three functional areas.
He identified the primary auditory area in general terms as the "posterior one-third of the superior temporal gyrus" or Heschl's gyrus, and more specifically as the cytoarchitecturally defined anterior transverse temporal area 41 of Brodmann.
The secondary auditory area he identified generally as the "postero-superior portion of the temporal lobe", including most of the middle temporal gyrus with parts of the angular gyrus and supramarginal gyrus; more specifically as the cytoarchitecturally defined posterior transverse temporal area 42, the superior temporal area 22, and part of the angular area 39 of Brodmann.
The tertiary auditory area, considered grossly to be the "posteroinferior part of the temporal lobe" or inferior temporal gyrus, was identified cytoarchitecturally with the middle temporal area 21 and part of the occipitotemporal area 37 of Brodmann.
 |
|
|
 |
Psychophysiology of the Auditory Aphasias
Most of the results of damage to the primary, secondary, and tertiary auditory areas can be traced to the loss of "auditory analysis and synthesis." Roughly, this means that a patient with a posterior temporal lobe lesion loses the ability to, (1) segment the flow of speech into phonemic units and, (2) integrate auditory stimuli into patterns corresponding to the words of the language. Both of these defects are said to result from the loss or instability of the auditory patterns, or "auditory schemata", of phonemes, syllables, and words.
The instability of auditory schemata produces no loss of auditory acuity (pitch discrimination) but impairs phonemic discrimination. "Phonemic hearing" is a learned function and depends upon the language of the patient, who has learned to distinguish the primary sounds of the language (phonemes) on the basis of a small number of distinctive features characteristic of the language. Patients with temporal aphasia lost the ability to recognize the combinations of distinctive features that define phonemes and thus were unable to recognize them. They tended to confuse "oppositional" phonemes ie.,phonemes that differ by a single distinctive feature (e.g. "s" and "k"). In some cases, the "auditory schemata" of phonemes were retained, but the schemata for phoneme complexes (syllables) or syllable complexes (words) was unstable, so the patient could recognize phonemes, but not syllables or words respectively.
Luria proposed that instability of the auditory schemata of words leads to a dissociation between their auditory patterns and their meanings . A word that is sometimes perceived as "rug", sometimes as "lug", and sometimes as "dug" soon loses association with its original meaning.
The instability of auditory schemata can also affect expressive speech, reading, writing, and computation to the extent that those functions are mediated by auditory function or memory.
Signs and Symptoms of Auditory Aphasia
Disruption of Receptive Speech: The most prominent sign of auditory aphasia is loss of the ability to comprehend spoken speech; lack of comprehension is indicated by inability to carry on a conversation, fulfill verbal instructions, or carry out the following tasks:
1. Tests of phonemic hearing are the most specific probes for auditory aphasia; the patient must
- a. repeat, write or otherwise indicate recognition of phonemes,
syllables, and words;
- b. recognize incorrect pronunciation of words, e.g., detect which of the phonemic patterns "radio" or "nadio" is not a word;
- c. recognize words in a list read by the examiner that begin with a given sound.
2. Tests of word comprehension present difficulty for patients with auditory aphasia, but they are not specific for the syndrome. In such tests the patient is instructed to
- a. point at objects as named;
- b. point at objects that fit an abstract category, such as writing implements;
- c. recognize the names of objects not in the visual field, e.g., point at their own ear or indicate comprehension of the word "bicycle" by describing or drawing a bicycle;
- d. explain metaphors or identify their meanings in a multiple choice test.
Disorders of Expressive Speech
1. The predominant sign of an expressive speech disorder in auditory apahasias is nominal aphasia. Spontaneous speech is frequently interrupted by word-searching. Intonation and grammatical structure may be normal, but the speech consists largely of opening phrases, e.g., "Well, you see ...it's like this..." and interjections, e.g., "Oh hell, I don't know why I can't get that". Unlike the nominal aphasias that result from parietal or frontal lobe lesions, the nominal aphasia associated with temporal lobe lesions is is not alleviated by prompting.
2. Literal and verbal paraphasias occur. The articulation (motor aspect) of phonemes and words is intact, but the patient may substitute oppositional phonemes for one another, e.g., "p" for "b", or substitute words from the same sphere of meaning." e.g., "elbow" for "arm". Literal paraphasias are considered compensatory in nature. The attempt to utter the word "car" may result in such inhibition of the "auditory schema" of "car" that it is easier for the patient to say "automobile" instead.
Disorders of Reading and Writing
The degree of reading and writing difficulty in auditory aphasias depends on the extent to which these functions involve auditory analysis.
1. Patients may understand the meaning of a passage even though they cannot read it aloud. They only have difficulty reading words that are unfamiliar and that must be sounded out to be understood.
2. A patient is usually able to write his name and other familiar words on the basis of "kinestheic schemata" but has difficulty with words that he would ordinarily repeat to himself as he writes. He can copy written material without difficulty but makes phonemic substitutions in writing to dictation, and is virtually unable to write spontaneously.
Disorders of Computation
The ability to calculate aloud is lost in auditory aphasias, but the ability to read and write numbers is often intact, so the patient is able to calculate on paper.
Summary
Different signs of auditory aphasia predominate depending on whether the lesion involves more of the primary, secondary, or tertiary auditory area.
With lesions of the primary auditory area loss of phonemic hearing is pronounced.
With lesions of the secondary auditory area patients may recognize and be able to repeat individual phonemes but are unable to distinguish between phonemic sequences e.g.,. "pat" and "tap". Nominal aphasia and reading and writing defects may be severe.
With lesions of the tertiary auditory area the dissociation of the auditory aspect from the meaningful aspect of words is seen most clearly. Phonemic hearing may be intact, but auditory memory and visuo-auditory coordination are disrupted. A lesion in this area produces "amnesic aphasia", or "alienation of word meanings," i.e., the patient is able to repeat words but cannot recall their meanings. His native language may sound foreign to him.
