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Case Reports
, 49 (2), 216-30

Speech Perception, Rapid Temporal Processing, and the Left Hemisphere: A Case Study of Unilateral Pure Word Deafness

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Case Reports

Speech Perception, Rapid Temporal Processing, and the Left Hemisphere: A Case Study of Unilateral Pure Word Deafness

L Robert Slevc et al. Neuropsychologia.

Abstract

The mechanisms and functional anatomy underlying the early stages of speech perception are still not well understood. One way to investigate the cognitive and neural underpinnings of speech perception is by investigating patients with speech perception deficits but with preserved ability in other domains of language. One such case is reported here: patient NL shows highly impaired speech perception despite normal hearing ability and preserved semantic knowledge, speaking, and reading ability, and is thus classified as a case of pure word deafness (PWD). NL has a left temporoparietal lesion without right hemisphere damage and DTI imaging suggests that he has preserved cross-hemispheric connectivity, arguing against an account of PWD as a disconnection of left lateralized language areas from auditory input. Two experiments investigated whether NL's speech perception deficit could instead result from an underlying problem with rapid temporal processing. Experiment 1 showed that NL has particular difficulty discriminating sounds that differ in terms of rapid temporal changes, be they speech or non-speech sounds. Experiment 2 employed an intensive training program designed to improve rapid temporal processing in language impaired children (Fast ForWord; Scientific Learning Corporation, Oakland, CA) and found that NL was able to improve his ability to discriminate rapid temporal differences in non-speech sounds, but not in speech sounds. Overall, these data suggest that patients with unilateral PWD may, in fact, have a deficit in (left lateralized) temporal processing ability, however they also show that a rapid temporal processing deficit is, by itself, unable to account for this patient's speech perception deficit.

Figures

Figure 1
Figure 1
Anatomical MRI images of NL’s brain. The left side is a rendered image of NL’s left hemisphere, and the right side shows axial slices in neurological convention with the left hemisphere on the left side.
Figure 2
Figure 2
a. Region of interest (ROI) used to define Heschl’s convolutions in NL’s right superior temporal gyrus. b. DTI pathways passing through the ROI shown in a. Images are shown in neurological convention with the left hemisphere on the left.
Figure 3
Figure 3
Spectrograms of stimuli used in Experiments 1 and 2. a. “easy” (maximally different 800 Hz vs. 1600 Hz onset frequency) /ba/ and /da/ stimuli (top) and their non-speech F2 analogues (bottom). b. “easy” (maximally different 1700 Hz vs. 2100 Hz steady state frequency) /be/ and /bi/ stimuli (top) and their non-speech F2 analogues (bottom).
Figure 4
Figure 4
NL’s performance on the entire 8 sessions (4 pre-treatment and 4 post-treatment) for: a. spectrally different stimuli, b. temporally different stimuli with short (20 ms) transitions, c. temporally different stimuli with medium (40 ms) transitions, and d. temporally different stimuli with long (60 ms) transitions. Solid green lines represent performance on the synthetic speech stimuli and dashed blue lines represent performance on the nonspeech analogues. Performance on easy (maximally different) pairs is represented with circles and performance on hard (minimally different) pairs is represented with triangles.
Figure 5
Figure 5
NL’s degree of improvement (pre-test minus post-test scores) collapsed across the individual pre- and post-treatment assessments for speech stimuli and their nonspeech analogues, and for easy (maximally different) and hard (minimally different) contrasts. Improvement in spectral discrimination is shown in solid blue, improvement in temporal discrimination is shown in hashed red, with fine hashing representing performance on stimuli with short (20 ms) transitions, medium hashing representing performance on stimuli with medium (40 ms) transitions, and wide hashing representing performance on stimuli with long (60 ms) transitions.

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