Amusia proves to be more complicated, for the appreciation of music has several aspects: the recognition of a familiar melody and the ability to name it (musicality itself); the perception of pitch, timbre, and rhythm; and the ability to produce, read, and write music. There are many reports of musicians who became word-deaf with lesions of the dominant temporal lobe but retained their recognition of music and their skill in producing it. Others became agnosic for music but not for words, and still others were agnosic for both words and music. According to Segarra and Quadfasel, impaired recognition of music results from lesions in the middle temporal gyrus and not from lesions at the pole of the temporal lobe, as had been postulated by Henschen. Many other studies implicate the superior temporal gyrus in these deficits. A loss of the ability to perceive and produce rhythm may or may not be associated. In any case, the temporal lobe opposite that responsible for language (i.e., the right) is implicated in almost all cases.
That the appreciation of music is impaired by lesions of the nondominant temporal lobe finds support in
Milner’s studies of patients who had undergone temporal lobectomy. She found a lowering of the patient’s appreciation of the duration of notes, timbre, intensity of sounds, and memory of melodies following right temporal lobectomy; these abilities were preserved in patients with left temporal lobectomies, regardless of whether Heschl gyri were included. Shankweiler had made similar observations, but in addition found that patients had difficulty in denominating a note or naming a melody following left temporal lobectomy.
More recent observations permit somewhat different interpretations. Tramo and Bharucha examined the mechanisms mediating the recognition and discrimination of timbre (the distinctive tonal quality produced by a particular musical instrument) in patients whose right and left hemispheres had been separated by callosotomy. They found that timbre could be recognized by each hemisphere, somewhat better by the left than by the right. Also, it was observed that lesions of the right auditory cortex impaired the recognition of melody (the temporal sequence of pitches) and of harmony (the sounding of simultaneous pitches). However, if words were added to the melody, then either a leftor right-sided lesion impaired its recognition (Samson and Zatorre). From functional imaging studies, it appears that the left inferior frontal region is activated by tasks that involve the identification of familiar music (Platel et al), as if this were a semantic test, but passively listening to melodies activates the right superior temporal and occipital regions (Zatorre et al).
By way of summary, Stewart and colleagues systematically reviewed the subject and were able to separate disorders of musical listening into the following categories: appreciation of pitch (including interval, pattern, and tonal structure), timbre, temporal structure, emotional content, and memory for music. The authors present clinical cases, mostly strokes that illustrate each defect. Taken together, these data suggest that the nondominant hemisphere is important for the recognition of harmony and melody (in the absence of words), but that the naming of musical scores and all the semantic (writing and reading) aspects of music require the integrity of the dominant temporal and probably the frontal lobes as well.
In essence, word deafness is a failure of the left temporal lobe function in decoding the acoustic signals of speech and converting them into understandable words. This is the essential element of Wernicke aphasia and is discussed in Chap. 22. However, word deafness can occur by itself, without other features of Wernicke aphasia. Other aspects of language such as reading, are not affected. The syndrome is sometimes seen as patients are improving from Wernicke aphasia. Also, as mentioned earlier, verbal agnosia may be combined with agnosia for sounds and music, or the two may occur separately.
Auditory Illusions and Hallucinations (See Also Chap. 14)