Frontiers in Psychology (Apr 2014)
Does Handedness Affect the Cerebral Organization of Speech and Language in Individuals with Aphasia?
Abstract
Introduction The relationship between handedness and cerebral organization has been a longstanding area of inquiry in the aphasia literature and has been studied in both neurologic patients and healthy individuals (Binder et al., 1996; Goodglass & Quadfasel, 1954; Knecht et al., 2000; Pujol et al., 1999). In the current study, we had the opportunity to use voxel-based lesion symptom mapping (VLSM) to identify the neural correlates of specific aspects of speech and language, comparing right-handed and non-right-handed individuals (i.e., left and ambidextrous individuals) in a large sample of well-characterized left hemisphere patients with aphasia. Our primary question was whether both groups would show similar VLSM patterns for distinct language functions, including fluency and auditory comprehension, or whether non-right-handed individuals would show a distinct pattern of language correlates in the left hemisphere. Methods The current study included behavioral and imaging data from 176 patients who were identified from our research database and met the following criteria: a single, left hemisphere stroke; native English speaker; at least 12 months post-onset; no prior neurologic or psychiatric history; and available brain imaging. Of these patients, 23 individuals reported being left-handed or ambidextrous, based on the Edinburgh Handedness Inventory. The right-handed and non-right-handed groups did not differ significantly with respect to age (mean = 61.1 and 57.0 years, respectively), education (mean = 14.1 and 14.6 years), lesion volume (111.6 and 134.9cc), or overall aphasia severity (WAB AQ score mean = 69.7 and 69.8). Patients’ lesion data were reconstructed from scans collected in the chronic phase of stroke. To determine the neural correlates of fluency and auditory comprehension scores from the WAB, we applied voxel-based lesion symptom mapping (VLSM) to the lesion and behavioral data (see Baldo et al., 2012; Bates et al., 2003). For VLSM, t-tests are run at every voxel, comparing performance on a particular behavioral measure in patients with a lesion versus without a lesion involving that voxel. Thus, for any particular voxel, a t-test is run with lesion status as the independent variable (lesioned or not) and behavioral performance (here, fluency and comprehension scores) as the dependent variable. Results Overall, the VLSM maps for right-handed and non-right-handed individuals with aphasia showed a large degree of overlap. For fluency, both groups showed significant voxels in left insular cortex, portions of Broca’s area, and underlying white matter, with the coordinates of the highest t-value in the insula. For auditory comprehension, both groups had significant regions predominantly in the left middle temporal gyrus, with some extension into the left inferior temporal gyrus and left inferior frontal cortex. Both groups’ comprehension maps had the highest t-value at coordinates corresponding to the left posterior middle temporal gyrus. Conclusions Although some earlier studies suggested distinct cerebral organizations for right- versus non-right-handed individuals, the neural correlates of fluency and comprehension were greatly overlapping between these groups in our sample of left hemisphere patients with aphasia.
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