Poster E34, Thursday, August 22, 2019, 3:45 – 5:30 pm, Restaurant Hall
Paragrammatism, agrammatism and the cortical organization of syntax: a lesion-symptom mapping study
William Matchin1, Alexandra Basilakos1, Dirk den Ouden1, Brielle Stark2, Julius Fridriksson1, Gregory Hickok3;1University of South Carolina, 2Indiana University, 3University of California, Irvine
INTRODUCTION: The cortical organization of syntax has been difficult to determine, in part because similar syntactic effects in functional neuroimaging studies are elicited in the inferior frontal gyrus (IFG) and the posterior middle temporal gyrus (pMTG). Agrammatic speech is associated with damage to frontal structures including the pars triangularis of the IFG (Wilson et al., 2010; den Ouden et al., 2019). However, unlike the pMTG, damage to the IFG does not reliably impair basic sentence and syntactic comprehension abilities (Dronkers et al., 2004; Wilson & Saygin, 2004; Pillay et al., 2017; Rogalsky et al., 2018). Kleist (1914) originally proposed two kinds of syntactic disturbances in the speech of people with aphasia: agrammatism (simplification of grammatical structure, omission of function words/morphemes), and paragrammatism (error-filled misuse of grammatical elements and structures, leading to “sentence monsters”, e.g. “two women is ugly”, “…wanted to make a trick her”). Matchin & Hickok (forthcoming) suggest that agrammatism results from damage to a morpho-syntactic sequencing system in the pars triangularis and paragrammatism results from damage to a hierarchical syntactic system in the pMTG. However, as the cortical locus of paragrammatism is largely unknown, we performed a voxel-based lesion-symptom mapping (VLSM) study in 53 patients with chronic aphasia secondary to a single-event left hemisphere stroke. METHODS: Four expert raters classified subjects’ spoken narrative discourse samples as agrammatic, paragrammatic, or no grammatical deficit, with consensus obtained through discussion. Subjects’ lesion maps were manually drawn and warped to MNI space, and VLSM analyses were performed. Lesion volume was always included as a covariate. RESULTS: Region of interest analyses identified a clear double dissociation: damage to the left pars triangularis of IFG was significantly associated with agrammatism, t(51) = 2.959, p = 0.002, but not paragrammatism, t(51) = -1.542, p = 0.935, while damage to the left pMTG was significantly associated with paragrammatism, t(51) = 3.087, p = 0.002, but not agrammatism, t(51) = -1.429, p = 0.920. Neither syndrome was significantly associated with damage to the anterior MTG. Whole brain analyses revealed non-overlapping effects of agrammatism in inferior and middle frontal brain regions and paragrammatism in posterior temporal and parietal brain regions. Secondary analyses adding speech fluency (words per minute) as an additional covariate revealed the same significant double dissociation, albeit weaker, with the same spatial distribution. DISCUSSION: We showed that these qualitatively distinct grammatical deficits, agrammatism and paragrammatism, correspond to inferior frontal and posterior temporal damage (respectively) as proposed by Matchin & Hickok (forthcoming). While both brain regions appear to support syntactic processing, broadly construed, their patterns strongly diverge with respect to grammatical deficits in aphasia, consistent with distinct roles in linear morpho-syntactic processing in IFG and hierarchical lexical-syntactic processing in pMTG.
Themes: Syntax, Disorders: Acquired
Method: Other