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Poster B77, Wednesday, November 8, 3:00 – 4:15 pm, Harborview and Loch Raven Ballrooms

Phoneme Perception Deficits from Unilateral Left Hemisphere Stroke: A Voxel-Based Lesion Correlation Study

Jeffrey Binder1, Sara B. Pillay1, Colin J. Humphries1, Peter Kraegel1, Diane S. Book1;1Medical College of Wisconsin, Milwaukee, WI, USA

Introduction: Severe phoneme perception impairment ("pure word deafness") from stroke is rare and typically follows bilateral cortical auditory system damage. Cases with unilateral left hemisphere lesions have occasionally been reported, but the precise lesion correlate is unclear, as are the incidence and severity of phoneme perception deficits arising from unilateral lesions. Methods: 57 patients with chronic unilateral left hemisphere stroke performed an AB discrimination task in which they judged whether two consecutively presented nonwords contained the same consonants and vowels. Stimuli were natural recordings of CV, VC, CCV, and VCC syllables. The 160 trials included 80 minimal pair contrasts of vowel color, manner, place, or voicing. To focus attention on phoneme identity, the 80 "same" trials were always acoustically different recordings of the same syllable. As a control for non-specific executive deficits and phonological short-term memory processes, patients performed a visual nonword rhyme matching task in which they judged which of two printed nonwords (e.g., bix and fise) rhymes with a third (e.g., fricks). Voxel-based lesion-symptom mapping (VLSM) was performed after labeling lesions on high-resolution T1-weighted MRI scans. Results: Accuracy on the phoneme discrimination task averaged 91.7% (SD 7.9, range 66.9 to 99.1). Thirteen patients (23%) performed more than 2 SD below the mean of an age-matched control cohort (n = 24, mean 96.3%, SD 4.8). Relative to controls, performance was worst on place contrasts (mean Z score = -4.4; 32 patients impaired), followed by voicing contrasts (mean Z score = -1.6; 18 patients impaired), manner contrasts (mean Z score = -1.2; 14 patients impaired), and vowel contrasts (mean Z score = -0.2; 8 patients impaired). Lesioned voxels associated specifically with impaired phoneme discrimination (after removing variance associated with the nonword rhyming task) were in the mid-portion of the left middle temporal gyrus and adjacent superior temporal sulcus (centered around -28 on the stereotaxic Y axis), extending medially into deep temporal lobe white matter and the retrolenticular internal capsule. Conclusions: Unilateral left hemisphere lesions can impair phoneme perception. The incidence in this unselected sample was 23%, although the severity of the deficit was generally mild. We included performance on a visual phonological task as a covariate in the VLSM analysis because the phoneme discrimination task inevitably depends on executive and short-term memory processes in addition to the phoneme perceptual process of interest. Damage correlated specifically with phoneme perception impairment occurred in high-level association cortex of the auditory ventral stream and deep temporal white matter pathways. The white matter lesions likely affect auditory radiations projecting from the ipsilateral medial geniculate body as well as auditory transcallosal tracts carrying information from the right hemisphere, effectively disconnecting the left-lateralized, mid-temporal phoneme recognition system from auditory input, analogous to the visual disconnection that occurs in some cases of pure alexia.

Topic Area: Perception: Speech Perception and Audiovisual Integration

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