Semantic network activation facilitates oral word reading in chronic aphasia.

People with aphasia often show partial impairments on a given task. This trial-to-trial variability offers a potential window into understanding how damaged language networks function. We test the hypothesis that successful word reading in participants with phonological system damage reflects semantic system recruitment. Residual semantic and phonological networks were defined with fMRI in 21 stroke participants with phonological damage using semantic- and rhyme-matching tasks. Participants performed an oral word reading task, and activation was compared between correct and incorrect trials within the semantic and phonological networks. The results showed a significant interaction between hemisphere, network activation, and reading success. Activation in the left hemisphere semantic network was higher when participants successfully read words. Residual phonological regions showed no difference in activation between correct and incorrect trials on the word reading task. The results provide evidence that semantic processing supports successful phonological retrieval in participants with phonological impairment.

The Neural Basis of Successful Word Reading in Aphasia.

Understanding the neural basis of recovery from stroke is a major research goal. Many functional neuroimaging studies have identified changes in brain activity in people with aphasia, but it is unclear whether these changes truly support successful performance or merely reflect increased task difficulty. We addressed this problem by examining differences in brain activity associated with correct and incorrect responses on an overt reading task. On the basis of previous proposals that semantic retrieval can assist pronunciation of written words, we hypothesized that recruitment of semantic areas would be greater on successful trials. Participants were 21 patients with left-hemisphere stroke with phonologic retrieval deficits. They read words aloud during an event-related fMRI paradigm. BOLD signals obtained during correct and incorrect trials were contrasted to highlight brain activity specific to successful trials. Successful word reading was associated with higher BOLD signal in the left angular gyrus. In contrast, BOLD signal in bilateral posterior inferior frontal cortex, SMA, and anterior cingulate cortex was greater on incorrect trials. These data show for the first time the brain regions where neural activity is correlated specifically with successful performance in people with aphasia. The angular gyrus is a key node in the semantic network, consistent with the hypothesis that additional recruitment of the semantic system contributes to successful word production when phonologic retrieval is impaired. Higher activity in other brain regions during incorrect trials likely reflects secondary engagement of attention, working memory, and error monitoring processes when phonologic retrieval is unsuccessful.

Lesion localization of speech comprehension deficits in chronic aphasia.

OBJECTIVE: Voxel-based lesion-symptom mapping (VLSM) was used to localize impairments specific to multiword (phrase and sentence) spoken language comprehension. METHODS: Participants were 51 right-handed patients with chronic left hemisphere stroke. They performed an auditory description naming (ADN) task requiring comprehension of a verbal description, an auditory sentence comprehension (ASC) task, and a picture naming (PN) task. Lesions were mapped using high-resolution MRI. VLSM analyses identified the lesion correlates of ADN and ASC impairment, first with no control measures, then adding PN impairment as a covariate to control for cognitive and language processes not specific to spoken language. RESULTS: ADN and ASC deficits were associated with lesions in a distributed frontal-temporal parietal language network. When PN impairment was included as a covariate, both ADN and ASC deficits were specifically correlated with damage localized to the mid-to-posterior portion of the middle temporal gyrus (MTG). CONCLUSIONS: Damage to the mid-to-posterior MTG is associated with an inability to integrate multiword utterances during comprehension of spoken language. Impairment of this integration process likely underlies the speech comprehension deficits characteristic of Wernicke aphasia.

Surface errors without semantic impairment in acquired dyslexia: a voxel-based lesion-symptom mapping study.

Patients with surface dyslexia have disproportionate difficulty pronouncing irregularly spelled words (e.g. pint), suggesting impaired use of lexical-semantic information to mediate phonological retrieval. Patients with this deficit also make characteristic 'regularization' errors, in which an irregularly spelled word is mispronounced by incorrect application of regular spelling-sound correspondences (e.g. reading plaid as 'played'), indicating over-reliance on sublexical grapheme-phoneme correspondences. We examined the neuroanatomical correlates of this specific error type in 45 patients with left hemisphere chronic stroke. Voxel-based lesion-symptom mapping showed a strong positive relationship between the rate of regularization errors and damage to the posterior half of the left middle temporal gyrus. Semantic deficits on tests of single-word comprehension were generally mild, and these deficits were not correlated with the rate of regularization errors. Furthermore, the deep occipital-temporal white matter locus associated with these mild semantic deficits was distinct from the lesion site associated with regularization errors. Thus, in contrast to patients with surface dyslexia and semantic impairment from anterior temporal lobe degeneration, surface errors in our patients were not related to a semantic deficit. We propose that these patients have an inability to link intact semantic representations with phonological representations. The data provide novel evidence for a post-semantic mechanism mediating the production of surface errors, and suggest that the posterior middle temporal gyrus may compute an intermediate representation linking semantics with phonology.

Cerebral localization of impaired phonological retrieval during rhyme judgment.

OBJECTIVE: Computation of a prearticulatory phonological representation (phonological access, or phonological retrieval) is an essential process in speech production whose neural localization is not clear. This study combined a specific behavioral measure of phonological access and multivariate voxel-based lesion-symptom mapping (VLSM) in a series of left hemisphere stroke patients to identify brain regions critical for this process. METHODS: Phonological access was assessed in 40 chronic ischemic stroke patients using a silent rhyming task to avoid confounds with motor planning and articulation deficits. Additional covariates were incorporated in the VLSM analysis to control for orthographic and working memory demands of the rhyming task, and for age, education, and total lesion volume. The resulting t statistic maps were thresholded at voxelwise p < 0.001 and cluster-corrected at a familywise error of p < 0.05. RESULTS: Phonological access impairment was correlated with damage to a focal region of cortex and white matter caudal to the posterior sylvian fissure, which included the posterior supramarginal gyrus and adjacent anterior angular gyrus, planum temporale, and posterior superior temporal gyrus. No correlation was observed with Broca's area, insula, or sensorimotor cortex. An additional VLSM showed no correlation between damage in this posterior perisylvian region and spoken word comprehension. INTERPRETATION: This is the first demonstration of a specific lesion correlate for phonological access impairment. Although this posterior perisylvian region overlaps with some versions of the classical Wernicke area, the present results demonstrate its involvement in prearticulatory phonological production rather than speech perception or lexical-semantic processes.