The Neural Basis of Successful Word Reading in Aphasia

Abstract

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.

Figures

Figure 1.

Example trials from the behavioral tasks given for participant inclusion outside the MRI environment. The left image shows an example of the pseudoword rhyme matching task. The right image shows an example of the semantic picture matching task.

Figure 2.

Lesion overlap map across 21 patients.

Figure 3.

Group z score contrasts. (A) Correct–error. The area more active for correctly read words (warm colors) was in the left AG. Areas more active for incorrect words (cool colors) were located bilaterally in the posterior inferior frontal cortex (mainly precentral gyrus and pars opercularis) and posterior medial frontal cortex (mainly SMA). (B) Areas modulated by RT including the bilateral SMA, posterior precuneus and lingual gyrus, and right IFG and adjacent precentral gyrus/sulcus.

Figure 4.

Overlap between performance on word reading task and semantic regions (red) in healthy controls. Activation associated with correctly read (correct) trials is shown in green, with semantic overlap shown in yellow. Activation associated with incorrectly read (error) trials is shown in blue, with semantic overlap shown in cyan.