The frontal aslant tract underlies speech fluency in persistent developmental stuttering

Abstract

The frontal aslant tract (FAT) is a pathway that connects the inferior frontal gyrus with the supplementary motor area (SMA) and pre-SMA. The FAT was recently identified and introduced as part of a "motor stream" that plays an important role in speech production. In this study, we use diffusion imaging to examine the hypothesis that the FAT underlies speech fluency, by studying its properties in individuals with persistent developmental stuttering, a speech disorder that disrupts the production of fluent speech. We use tractography to quantify the volume and diffusion properties of the FAT in a group of adults who stutter (AWS) and fluent controls. Additionally, we use tractography to extract these measures from the corticospinal tract (CST), a well-known component of the motor system. We compute diffusion measures in multiple points along the tracts, and examine the correlation between these diffusion measures and behavioral measures of speech fluency. Our data show increased mean diffusivity in bilateral FAT of AWS compared with controls. In addition, the results show regions within the left FAT and the left CST where diffusivity values are increased in AWS compared with controls. Last, we report that in AWS, diffusivity values measured within sub-regions of the left FAT negatively correlate with speech fluency. Our findings are the first to relate the FAT with fluent speech production in stuttering, thus adding to the current knowledge of the functional role that this tract plays in speech production and to the literature of the etiology of persistent developmental stuttering.

Frontal aslant tract projections to the inferior frontal gyrus

“…Although little is known about the functional properties of the FAT, there is a growing interest in that novel tract [3,9,12,15,16,19,20]. The functional roles of FAT can be scrutinised to motor functions and language processing.…”

“…The symptoms include the inability to produce grammatically correct and fluent speech with a preserved comprehension in most cases. On the other hand, damage to the pre-SMA causes a mild to severe speech impairment ranging from affected fluency to complete speech arrest (mutism) [10][11][12]20]. A recent study of patients suffering from primary progressive aphasia showed that pathological changes within the FAT affect verbal fluency with no influence on the overall language impairment, grammar deficits, repetition and single word comprehension [5].…”

“…Moreover, most FAT fibres originate at pars opercularis (IFG-Op) of the IFG which can explain its functional role in the language model, supported by mapping techniques [4,5]. In dominant hemisphere FAT is associated with the initiation of speech and the fluency [9,12]. Speech arrest was observed in frontal lobe tumours infiltrating FAT as well as during its intraoperative electrical stimulation in awake surgeries [11,16].…”

Prolongamentos na fala de adultos com e sem gagueira

“…Prolongation is one of the stuttering-like disfluencies described as the main clinical manifestation of the disorder [3][4][5]9,[17][18][19]29,30 . However, fluent people can also show prolongations in the speech flow, especially at the end of words 20,21 .…”

“…Persistent Developmental Stuttering is a fluency disorder that affects about 1% of the adult population 1 characterized by disruptions in the speech flow that can be manifested through prolongations, blocks and repetitions [2][3][4][5] . These disfluencies are involuntary, occur at a higher frequency in relation to general population 6 and are considered the main characteristic of the disorder 7 .…”