Poster Slam Session C, Wednesday, August 21, 2019, 10:30 - 10:45 am, Finlandia Hall, James Magnuson
The left Frontal Aslant Tract supports sentence planning: Evidence from direct electrical stimulation and longitudinal diffusion MRI in brain tumor patients
Benjamin Chernoff1, Webster Pilcher2, Bradford Mahon1,2;1Carnegie Mellon University, 2University of Rochester Medical Center
Sentence planning unfolds at multiple levels of processing, including planning of message, syntactic, and morphophonological elements. Patients with damage to a recently discovered white matter pathway in the brain connecting the inferior frontal gyrus (IFG) to the pre-supplementary motor area (SMA), the left Frontal Aslant Tract (FAT), exhibit impaired sentence production and dysfluent speech in the absence of impairments to semantic processing, lexical access, articulation, or non-speech motor function (e.g., limb or orofacial apraxia). However, the specific role of the FAT has not been situated within existing neurocognitive models of language processing. Here, we test mechanistic hypotheses about the language computations supported by the FAT, using two different approaches. The first approach examined how sentence production is impaired by FAT damage compared to damage to other white matter pathways connecting the IFG, through longitudinal case series of patients with low-grade tumors affecting the frontal aslant tract, the arcuate fasciculus, and the uncinate fasciculus. We studied these patients pre- and post-operatively using structural and functional MRI, as well as standardized neuropsychological tests of speech production. We found that impairments for verbal fluency, picture naming, and sentence repetition are respectively associated with damage to the left FAT, uncinate fasciculus, and arcuate fasciculus. Damage to these pathways was quantified using both Diffusion Tensor Imaging and functional connectivity. In the second approach, we designed a novel sentence production task to test our hypothesis that the left FAT is a key pathway for integrating syntagmatic and positional-level planning during sentence production. We refer to this as the ‘Syntagmatic Constraints On Positional Elements’ (SCOPE) hypothesis. A core prediction made by the SCOPE hypothesis is that disruption of the FAT should specifically disrupt sentence production at phrasal boundaries, with no impairment for articulation. We test this prediction by measuring sentence production latencies in a patient undergoing direct electrical stimulation (DES) mapping of the frontal aslant tract during an awake craniotomy to remove a left hemisphere brain tumor. The patient produced cued sentences such as ‘The red square is above the yellow circle’, and we measured the intra-word and inter-word durations as a function of stimulation (on, off and location relative to the tract). We found that stimulation prolonged inter-word pauses before the start of the noun phrases and at the verb, while inter-word durations internal to noun phrases were, if anything shorter in the context of stimulation compared to without stimulation. Stimulation of the frontal aslant tract had no effect on articulation time. These results provide initial support for the SCOPE hypothesis, and motivate novel directions for future research to explore the functions of this recently discovered component of the language system.
Themes: Language Production, Syntax
Method: White Matter Imaging (dMRI, DSI, DKI)
Poster C50