Page 13 - Winter 2020
P. 13

 aphasia syndromes and the details of their underlying neuropathology (see Goodglass, 1993; Damasio, 1998). Rather, I describe here Broca’s and Wernicke’s aphasia because most of the research exploring speech deficits in aphasia has focused on these two syndromes.
Figure 1 shows a lateral view of the left hemisphere of a human brain and identifies some (but not all) of the neural structures implicated in Broca’s and Wernicke’s aphasia (Damasio, 1998; Bates et al., 2003). Lesions in the frontal areas of the brain, including the inferior frontal gyrus, the insula, and often motor areas, typically produce Broca’s aphasia. In contrast, lesions in the posterior areas, including the superior and middle temporal gyri and often extending into the parietal areas that include the supramarginal and angular gyri, produce Wernicke’s aphasia. That said, there is considerable variability in the areas of brain injury that give rise to the classical aphasias, including Broca’s and Wernicke’s aphasia.
The language deficits in these two types of aphasia are qualitatively different (Goodglass, 1993). Individuals with Broca’s aphasia have difficulty speaking: speech quality tends to be slow and labored and often includes sound errors and sound distortions. Additionally, a distinguishing characteristic of this disorder is agrammatism, which means that sentences tend to be grammatically simple and difficulties emerge in the use of grammatical words. For example, “the” may be omitted as well as grammatical endings such as plurals, “books” may be produced as “book.” Comprehension, on the other hand, although not perfect, is relatively good.
In contrast, language production in Wernicke’s aphasia is fluent and easily articulated. However, it is often empty of semantic content such as “I did it because it was so and I am.” Speech errors occur as well as substitutions of words with those that may be semantically related, such as saying “mother” for “father.” Additionally, those with Wernicke’s aphasia have severe auditory language comprehension impairments.
That Broca’s aphasia largely presents with a production impairment and Wernicke’s aphasia with a comprehension impairment is not surprising given the neural architecture of the brain. Namely, motor and associated neural structures are located in the frontal lobe, and auditory and associated neural structures are localized in the temporal lobe (see
Figure 1). However, despite good auditory comprehension, those with Broca’s aphasia display speech perception deficits, and despite a fluent, well-articulated speech output, those with Wernicke’s aphasia display speech production deficits. This provides the first indication that neither speech perception nor speech production is localized in one circumscribed area of the brain. Rather, the speech systems underlying production and perception are broadly represented in the left hemisphere.
Speech Production Deficits in Aphasia
Speech production errors commonly occur in aphasia regardless of the aphasia syndrome. Sounds may be substituted for each other, “pot” may be produced as “bot”; sounds may be produced in the wrong order, “dummy” is spoken as “muddy”; or they may be deleted, “train” becomes “tain,” or added, “same” becomes “stame.”
The most common error type for those with aphasia is sound substitution. Analyses of sound substitutions in Broca’s and Wernicke’s aphasia show both striking similarities and also differences (Lecours and Lhermitte, 1969; Blumstein, 1973; Haley et al., 2013). First, to the similarities. For individuals with either Broca’s or Wernicke’s aphasia, the occurrence of sound substitutions is variable, and it is not possible to predict if and when a substitution error will occur. Importantly, however, there is a pattern to these sound substitutions. When they occur, the substituted sound and target sound are
“similar” to each other. What does this mean?
Speech sounds in language differ from one another in systematic ways. They can be broken down into a smaller set of dimensions or features that reflect how the sounds are articulated and their associated acoustic correlates (Jakobson et al., 1952). For example, as shown in Figure 2, consonant sounds like [b], [d], and [t] may be broken down into features that indicate how they are produced (their manner of articulation), where in the mouth they are produced (their place of articulation), and what the vocal cords are doing during the production of the sound (voicing). [b] is a consonant produced with a closure in the mouth, the closure is at the lips, and the vocal cords are vibrating during production. It is distinguished from [d] by one feature, place of articulation, and it is distinguished from [t] by two features, place of articulation and voicing; in the production of [d], the vocal cords are vibrating as the consonant is produced
Winter 2020 • Acoustics Today 13

   11   12   13   14   15