Page 16 - Winter 2020
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These findings indicate that, like Wernicke’s aphasics, Broca’s aphasics display a deficit in the selection of sounds and their features. However, unlike Wernicke’s aphasia, their deficit also affects articulatory implementation of those features, reflecting the role of the frontal areas of the brain in the motor planning and production of speech.
ofspeech.Soundsdistinguishedbyonefeatureareharderto discriminate than sounds distinguished by more than one feature, whether discriminating nonsense syllables or words. This pattern emerges across languages, aphasia syndromes, and lesion site (Basso et al., 1977; Blumstein et al., 1977a), suggesting that speech perception recruits both anterior and posterior brain structures and that the properties of sound are organized, as in production, in a network-like architecture based on articulatory and acoustic similarity.
It is clear from these results and those from the foreign
accent syndrome that a listener’s ears cannot always identify
the nature of speech production deficits. Such is also the
case for sound substitution errors, such as producing Although the pattern of errors is the same in
[da] as [ta]. Indeed, there has been considerable debate in the literature about whether these errors reflect the mis- selection of a sound followed by its correct production or a misperception by the examiner that the produced sound was a substitution error (Buckingham and Yule, 1987). Here again, acoustic analysis can provide an answer.
Kurowski and Blumstein (2016) examined the acoustic characteristics of sound substitution errors between [s] and [z]. They compared the acoustics of [s]-[z] substitutions with correct productions of these sounds. The question was whether the perceived correct productions and the substitution errors were acoustically the same or different. Results showed that [s]-substitution errors were more [z]-like than correct productions of [s], and [z]-substitutions were more [s]-like.
These findings support the network-like architecture of the sounds of language described in Speech Production Deficits in Aphasia. Sounds are “connected” to one another in the network not only in their selection but also in their implementation. A selected sound partially activates and hence coactivates a sound similar to it in the network. As a consequence, a sound substitution contains a trace or acoustic residue of its coactivated neighbor.
The Perception of Speech
A first step in considering speech perception deficits is examining the extent to which those with aphasia can discriminate differences between speech sounds. An impairment in distinguishing sounds like [p] and [b] would not only indicate a deficit in perceiving the sounds of speech but could affect the discrimination of words like “pear” and
“bear,” potentially influencing language comprehension.
Similar to production, perception is influenced by the articulatory-acoustic feature distance between the sounds
discriminating nonsense syllables and real words, there are more errors discriminating nonsense syllables, even though the same sound contrast is compared (Blumstein et al., 1977a). For example, discriminating [pa] versus [ba] results in more errors than discriminating “peak” versus “beak.” These results are not surprising. Words can be discriminated based on differences in their sounds as well as their meaning. Nonwords can only be discriminated based on their sounds.
One important difference has emerged between Broca’s and Wernicke’s aphasia. The number of speech discrimination errors was substantially greater in Wernicke’s aphasia. Again, neuroanatomy provides an explanation. Primary auditory areas are located in the temporal lobe (see Figure 1). Here, the acoustic input is transformed into the acoustic features of speech (Turkeltaub and Coslett, 2010; Mesgarani et al., 2014). Hence, speech perception is more vulnerable with damage to the temporal areas than to the frontal areas.
The poor performance of patients with Wernicke’s aphasia raises the possibility that their speech perception impairment underlies or contributes to the severity of their auditory comprehension deficits. This possibility was first noted by Luria and Hutton (1977) who proposed that the poor auditory comprehension in Wernicke’s aphasia reflected a deficit in “phonemic hearing.” In this case, misperception of similar sounds could result in perceiving thewrongwords,leadingtocomprehensionfailures.
This hypothesis was tested by giving subjects an auditory word-picture matching task. Here, subjects point to the picture of an auditorily presented word from a group of pictures that includes a target word, such as “pear,” a word perceptuallysimilartothetarget,“bear,”andawordrelated in meaning to the target such as “apple.” (Both pear and
16 Acoustics Today • Winter 2020

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