Agrammatic comprehension caused by a glioma in the left frontal cortex

It has been known that lesions in the left inferior frontal gyrus (L. IFG) do not always cause Broca's aphasia, casting doubt upon the specificity of this region. We have previously devised a picture-sentence matching task for a functional magnetic resonance imaging (fMRI) study, and observed that both pars triangularis (L. F3t) of L. IFG (extending to pars opercularis (L. F3op)) and the left lateral premotor cortex (L. LPMC) are selectively involved in syntactic processing. The present study with lesion-symptoms mapping was conducted to examine whether the function of these regions is indeed critical for syntactic comprehension. Using the same picture-sentence matching task, we examined 21 patients with a glioma in the left frontal cortex but with no apparent disability in verbal/written communication or intelligence quotient. This task included three main conditions of sentence types: canonical/subject-initial active sentences, non-canonical/subject-initial passive sentences, and non-canonical/object-initial scrambled sentences. The patients preoperatively underwent a high-resolution 3D-MRI, and voxel-based lesion-symptom mapping was employed for the error rates data. We found that the patients with a lesion in L. F3op/F3t or L. LPMC showed differential patterns of condition-selective deficits in the comprehension of sentences. More specifically, the L. F3op/F3t-damaged patients had more profound deficits in the comprehension of non-canonical sentences, whereas the L. LPMC-damaged patients had more profound deficits in the comprehension of object-initial scrambled sentences. These results establish that a lesion in L. F3op/F3t or L. LPMC is sufficient to cause agrammatic comprehension.