Planum temporale asymmetry in people who stutter

Abstract

Purpose: Previous studies have reported that the planum temporale - a language-related structure that normally shows a leftward asymmetry - had reduced asymmetry in people who stutter (PWS) and reversed asymmetry in those with severe stuttering. These findings are consistent with the theory that altered language lateralization may be a cause or consequence of stuttering. Here, we re-examined these findings in a larger sample of PWS.

Methods: We evaluated planum temporale asymmetry in structural MRI scans obtained from 67 PWS and 63 age-matched controls using: 1) manual measurements of the surface area; 2) voxel-based morphometry to automatically calculate grey matter density. We examined the influences of gender, age, and stuttering severity on planum temporale asymmetry.

Results: The size of the planum temporale and its asymmetry were not different in PWS compared with Controls using either the manual or the automated method. Both groups showed a significant leftwards asymmetry on average (about one-third of PWS and Controls showed rightward asymmetry). Importantly, and contrary to previous reports, the degree of asymmetry was not related to stuttering severity. In the manual measurements, women who stutter had a tendency towards rightwards asymmetry but men who stutter showed the same degree of leftwards asymmetry as male Controls. In the automated measurements, Controls showed a significant increase in leftwards asymmetry with age but this relationship was not observed in PWS.

Conclusions: We conclude that reduced planum temporale asymmetry is not a prominent feature of the brain in PWS and that the asymmetry is unrelated to stuttering severity.

Fig. 1

Location and shape of the Planum Temporale (PT). (A) Lateral surface view (left) of the left hemisphere indicating the Sylvian fissure (green arrows) and the bifurcation at its posterior limit into ascending and descending rami (yellow arrow). Sagittal slice (right) through the left hemisphere at 48 mm from the midline indicating Heschl’s gyrus (blue arrow), Heschl’s sulcus (red arrow), and the PT (purple arrow). (B) Axial slice from a structural scan for one participant in this study. A – anterior, P – posterior, R – right, L – left. (C) Simplified line drawing based on Geschwind & Levitsky’s (1968) figure showing the location of Heschl’s gyrus and the PT. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Determining the posterior and anterior boundaries of the PT. (A). A case of a posterior duplication of Heschl’s gyrus. Heschl’s sulcus is indicated by the red arrow on the sagittal slice 39 mm from the midline. (B). Axial slice showing the duplication of Heschl’s gyrus in the same person. (C). A case of a heart-shaped Heschl’s gyrus with an incomplete duplication showing the “common stem” formation. Heschl’s sulcus is indicated by the red arrow on a sagittal slice 40 mm from the midline. (D). Knife-cut method of determining the posterior boundary in the absence of a descending ramus. A sagittal slice 51 mm from the midline is shown. The dotted yellow line indicates the trajectory of a plane through the Sylvian fissure and the yellow arrow indicates the point where this line intersects the parietal wall and the posterior boundary. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Surface area measurements of the PT. (A). Left and right hemisphere measurements shown for individual PWS and Controls. (B). Asymmetry Quotients for individual participants. Red symbols – PWS, blue symbols – Controls, filled circles – right handers, open circles – left handers, solid black lines – group means, L – left, R – right. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Gender differences in PT surface area asymmetry. Red symbols – PWS, blue symbols – Controls, filled symbols – male, open symbols – female, solid lines – group means. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

The (lack of) relationship between stuttering severity and asymmetry of the PT. Open symbols – females who stutter, filled symbols – males who stutter, R – Right, L – Left.

Fig. 6

Left-greater-than-right asymmetry of grey matter density in the PT. The brain image is the study-specific template of grey matter density averaged from 63 PWS and 63 Controls. Coloured areas indicate a significant asymmetry in the amount of grey matter (voxel-wise statistics, p < 0.05 corrected using permutation testing) in the PT of the left hemisphere. Red – PWS, Blue – Controls. Sagittal slices are shown through the left hemisphere at 40, 44 and 48 mm from the mid-sagittal plane. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7

Grey matter density estimates for the PT obtained using voxel-based morphometry (standard space location ±40, −36, 18). See legend of Fig. 3 for details.

Fig. 8

The relationship between age and asymmetry of grey matter density in the PT. This was significant in B. Controls (blue) but not in A. PWS (red). R – Right, L – Left. Solid lines represent the regression line with 95% confidence intervals shown by the dashed lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 9

The relationship between age and grey matter density in the left and right PT. This was significant for the left PT in B. Controls (blue) but not in A. PWS (red).Grey matter density in the right PT showed no significant relationship with age in either group (PWS – orange; Controls – purple). Solid lines represent the regression line with 95% confidence intervals shown by the dashed lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)