Perceptual learning of dysarthric speech
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Perceptual learning, when applied to speech, describes experience-evoked adjustments to the cognitive-perceptual processes required for recognising spoken language. It provides the theoretical basis for improved understanding of a speech signal that is initially difficult to perceive. Reduced intelligibility is a frequent and debilitating symptom of dysarthria, a speech disorder associated with neurological disease or injury. The current thesis investigated perceptual learning of dysarthric speech, by jointly considering intelligibility improvements and associated learning mechanisms for listeners familiarised with the neurologically degraded signal. Moderate hypokinetic dysarthria was employed as the test case in the three phases of this programme of research.
The initial research phase established strong empirical evidence of improved recognition of dysarthric speech following a familiarisation experience. Sixty normal hearing listeners were randomly assigned to one of three groups and familiarised with passage readings under the following conditions: (1) neurologically intact speech (control) (n = 20), dysarthric speech (passive familiarisation) (n = 20), and (3) dysarthric speech coupled with written information (explicit familiarisation) (n = 20). Subsequent phrase transcription analysis revealed that the intelligibility scores of both groups familiarised with dysarthric speech were significantly higher than those of the control group. Furthermore, performance gains were superior, in both size and longevity, when the familiarisation conditions were explicit. A condition discrepancy in segmentation strategies, in which attention towards syllabic stress contrast cues increased following explicit familiarisation but decreased following passive familiarisation, indicated that performance differences were more than simply magnitude of benefit. Thus, it was speculated that the learning that occurred with passive familiarisation may be qualitatively different to that which occurred with explicit familiarisation.
The second phase of the research programme followed up on the initial findings and examined whether the key variable behind the use of particular segmentation strategies was simply the presence or absence of written information during familiarisation. Forty normal hearing listeners were randomly assigned to one of two groups and were familiarised with experimental phrases under either passive (n = 20) or explicit (n = 20) learning conditions. Subsequent phrase transcription analysis revealed that regardless of condition, all listeners utilised syllabic stress contrast cues to segment speech following familiarisation with phrases that emphasised this prosodic perception cue. Furthermore, the study revealed that, in addition to familiarisation condition, intelligibility gains were dependent on the type of the familiarisation stimuli employed. Taken together, the first two research phases demonstrated that perceptual learning of dysarthric speech is influenced by the information afforded within the familiarisation procedure.
The final research phase examined the role of indexical information in perceptual learning of dysarthric speech. Forty normal hearing listeners were randomly assigned to one of two groups and were familiarised with dysarthric speech via a training task that emphasised either the linguistic (word identification) (n = 20) or indexical (speaker identification) (n = 20) properties of the signal. Intelligibility gains for listeners trained to identify indexical information paralleled those achieved by listeners trained to identify linguistic information. Similarly, underlying error patterns were also comparable between the two training groups. Thus, phase three revealed that both indexical and linguistic features of the dysarthric signal are learnable, and can be used to promote subsequent processing of dysarthric speech. In summary, this thesis has demonstrated that listeners can learn to better understand neurologically degraded speech. Furthermore, it has offered insight into how the information afforded by the specific familiarisation procedure is differentially leveraged to improve perceptual performance during subsequent encounters with the dysarthric signal. Thus, this programme of research affords preliminary evidence towards the development of a theoretical framework that exploits perceptual learning for the treatment of dysarthria.