A Role for the Frontal Aslant Tract in Speech Planning: A Neurosurgical Case Study

Abstract

Frontal and temporal white matter pathways play key roles in language processing, but the specific computations supported by different tracts remain a matter of study. A role in speech planning has been proposed for a recently described pathway, the frontal aslant tract (FAT), which connects the posterior inferior frontal gyrus to the pre-SMA. Here, we use longitudinal functional and structural MRI and behavioral testing to evaluate the behavioral consequences of a lesion to the left FAT that was incurred during surgical resection of a frontal glioma in a 60-year-old woman, Patient AF. The pattern of performance in AF is compared, using the same measures, with that in a 37-year-old individual who underwent a left anterior temporal resection and hippocampectomy (Patient AG). AF and AG were both cognitively intact preoperatively but exhibited specific and doubly dissociable behavioral deficits postoperatively: AF had dysfluent speech but no word finding difficulty, whereas AG had word finding difficulty but otherwise fluent speech. Probabilistic tractography showed that the left FAT was lesioned postoperatively in AF (but not AG) whereas the inferior longitudinal fasciculus was lesioned in AG (but not AF). Those structural changes were supported by corresponding changes in functional connectivity to the posterior inferior frontal gyrus: decreased functional connectivity postoperatively between the posterior inferior frontal gyrus and pre-SMA in AF (but not AG) and decreased functional connectivity between the posterior inferior frontal gyrus and the middle temporal gyrus in AG (but not AF). We suggest from these findings that the left FAT serves as a key communicative link between sentence planning and lexical access processes.

A general role for ventral white matter pathways in morphological processing: going beyond reading

“…Our regression analyses confirmed that the correlations between FAT microstructure and morpheme-based fluency were not explained by standard verbal fluency tests. The apparent specificity of the correlations with the morpheme-based task are in line with several case studies where surgical resection damaging the left FAT induced speech disruptions that were sensitive to grammatical constraints (Chernoff et al, 2019(Chernoff et al, , 2018Sierpowska et al, 2015). Of particular relevance is the case reported by Sierpowska and colleagues (2015), where damage to the left FAT elicited production errors characterized by inappropriate application of morphological rules.…”

A common neural signature of brain injury in concussion and subconcussion

“…Smoothing and regularization were performed using FSL's fugue tool (www.fmrib.ox.ac.uk/fsl), and three-dimensional (3D) Gaussian smoothing was applied using sigma = 4 mm (25). The magnitude image was warped on the basis of this smoothing, with y as the warp direction, following a previous study (26). Eddy current correction was performed using FSL's eddy_correct tool, which takes each volume of the diffusion-weighted image and registers it to the B0 image to correct for both eddy currents and motion.…”

The usefulness and limitations of diffusion tensor imaging – a review study

“…Nevertheless, functional MRI also can unambiguously estimate the exact location of speech areas. Researchers should put more effort into studying DTI in terms of speech preservation as awake craniotomy resection is still more reliable in this case [37].…”

“…In clinical practice, FAT could be determined to be a means of DTI in patients for whom an awake craniotomy is planned [43]. Baker et al suggested that FAT and "crossed FAT" are of great importance for tumours infiltrating SMA and pre-SMA [37,44].…”