Poster A28, Thursday, August 16, 10:15 am – 12:00 pm, Room 2000AB

Morpheme-based word production is associated with ventral-stream white matter pathways

Maya Yablonski1, Benjamin Menashe1, Michal Ben-Shachar1;1Bar-Ilan University

Language users possess extensive knowledge about the internal structure of words and their constituent morphemes. Although ample behavioral evidence has shown that morphology is an important factor in skilled reading, the white matter underpinnings of morphological skills remain largely unknown. In a recent study, we found that implicit morphological knowledge is associated with microstructural properties in bilateral ventral-stream pathways of adult English readers. Here, we examine white matter associations with morpheme-based word production in adult Hebrew readers. This extends our measurements to a non-linear, morphologically rich language, using a task that requires morpheme abstraction, lexical search and access, in response to an orally presented word. To assess root-based fluency, participants were requested to generate within 30s as many Hebrew words as possible derived from a common root (e.g., upon hearing the target miSGeRet, frame, participants orally generated words like SaGuR, is closed, maSGeR, a welder, and so on, all derived from the same triconsonantal root, SGR). Similarly, in a pattern-based fluency task, participants had to generate words derived from a common morphological pattern (e.g., upon hearing the word maGReSa, a shredder, participants generated maMTeRa, a sprinkler, maVReGa, a screwdriver, etc.). In accordance with the dual-stream model of speech processing (Hickok & Poeppel, 2007), we hypothesized that root-based fluency would be associated with the bilateral ventral-stream tracts, which support semantic processing and lexical access. We further hypothesized that pattern-based fluency would be associated with the left dorsal stream, due to the phonological regularities introduced by the morphological pattern in Hebrew words. Participants were 45 adult native Hebrew-speakers (29 females). They all completed both fluency tasks (as well as other cognitive tests) and underwent an MRI scan (3T Siemens scanner, 64 diffusion directions at b=1000 s/mm2, 3 volumes at b=0; isotropic voxel size: 1.7*1.7*1.7mm3). Dorsal and ventral tracts of interest were identified bilaterally in each participant’s native space, using deterministic tractography and automatic tract segmentation (Yeatman et al., 2012). Spearman’s correlations were calculated between each fluency measure and two diffusivity parameters, fractional anisotropy (FA) and mean diffusivity (MD), averaged across the length of the tract. The results show that root-based fluency is positively correlated with the mean FA of the inferior fronto-occipital fasciculus (IFOF) bilaterally, and with the left arcuate fasciculus (AF). In addition, negative correlations were found between root-based fluency and mean MD of the bilateral IFOF, bilateral inferior longitudinal fasciculus, left uncinate fasciculus and left AF. Pattern-based fluency was not significantly correlated with any of the tracts tested. In sum, root-based fluency was correlated with ventral-stream tracts, bilaterally, as well as with the left AF, a dorsal-stream tract. Importantly, partial correlations controlling for phonemic or semantic fluency eliminated the effects in the left AF, but maintained significance in the bilateral ventral pathways. These results suggest that the left AF contributes to verbal fluency tasks broadly, and not specifically to morpheme-based fluency. Taken together, our findings point to the involvement of bilateral ventral-stream pathways in morphological processing, across stimulus and response modality, cross-linguistically, and across morphological systems.

Topic Area: Grammar: Morphology