Different patterns of language activation in post-stroke aphasia are detected by overt and covert versions of the verb generation fMRI task

Abstract

Background: Post-stroke language functions depend on the relative contributions of the dominant and non-dominant hemispheres. Thus, we aimed to identify the neural correlates of overt and covert verb generation in adult post-stroke aphasia.

Material and methods: Sixteen aphasic LMCA stroke patients (SPs) and 32 healthy controls (HCs) underwent language testing followed by fMRI while performing an overt event-related verb generation task (ER-VGT) isolating activations related to noun-verb semantic processing or to articulation and auditory processing, and a covert block design verb generation task (BD-VGT).

Results: BD-VGT activation patterns were consistent with previous studies, while ER-VGT showed different patterns in SPs relative to HCs including less left-hemispheric involvement during semantic processing and predominantly right-sided activation related to articulation and auditory processing. ER-VGT intra-scanner performance was positively associated with activation during semantic associations in the left middle temporal gyrus for HCs (p=0.031) and left middle frontal gyrus for SPs (p=0.042). Increased activation in superior frontal/cingulate gyri was associated with better intra-scanner performance (p=0.020). Lesion size negatively impacted verbal fluency tested with Controlled Oral Word Association Test (p=0.0092) and the Semantic Fluency Test (p=0.033) and trended towards a negative association with verb generation performance on the event-related verb generation task (p=0.081).

Conclusion: Greater retention of pre-stroke language skills is associated with greater involvement of the left hemisphere with different cortical recruitment patterns observed in SPs versus HCs. Post-stroke verbal fluency may depend more upon the structural and functional integrity of the dominant left hemisphere language network rather than the shift to contralateral homologues.

Figure 1

T1-weighted structural images of left middle cerebral artery lesions in 16 patients with ischemic stroke. Slice location selected to show the extent of the aphasia-producing stroke. Each axial slice is presented in radiological convention.

Figure 2

Group z-score maps for the block-design version of the verb generation task (BD-VGT). (A) Brain regions show increased activation related to covert verb generation in healthy subjects, with clusters of activation significant at p<0.05, corrected for multiple voxel comparisons and z-scores of activated voxels ranging from 9.8 (blue) to 20 (red) and a cluster threshold of at least 30 contiguous voxels. LMCA stroke group did not show activated clusters at this threshold. (B) At a lower threshold of p<0.001 uncorrected, both healthy (top panel) and LMCA stroke (bottom panel) subjects exhibit increased activation related to covert verb generation. Z-scores of activated voxels range from 3.3 (blue) to 20 (red). Each z-score map is presented in radiological convention, with left on the picture corresponding to the right hemisphere, and are superimposed on an average T1-weighted image generated from all subjects of each group. The 7 axial slices selected for each display panel range in Talairach coordinates from z=−25 mm (left) to z=+47 mm (right).

Figure 3

Group z-score maps for the event-related version of the verb generation task (ER-VGT). Brain regions show increased activation related to (A) processing noun-verb semantic associations and (B) articulation and auditory processing in healthy (top panel) and LMCA stroke (bottom panel) subjects. Clusters of activation are significant at p<0.05, corrected for multiple voxel comparisons, with z-scores of activated voxels ranging from 9 (blue) to 20 (red) and a cluster threshold of at least 30 contiguous voxels. Each z-score map is presented in radiological convention, with left on the picture corresponding to the right hemisphere, and are superimposed on an average T1-weighted image generated from all subjects of each group. The 7 axial slices selected for each display panel range in Talairach coordinates from z=−25 mm (left) to z=+47 mm (right).