Descriptive anatomy of Heschl's gyri in 430 healthy volunteers, including 198 left-handers

Abstract

This study describes the gyrification patterns and surface areas of Heschl's gyrus (HG) in 430 healthy volunteers mapped with magnetic resonance imaging. Among the 232 right-handers, we found a large occurrence of duplication (64 %), especially on the right (49 vs. 37 % on the left). Partial duplication was twice more frequent on the left than complete duplication. On the opposite, in the right hemisphere, complete duplication was 10 % more frequent than partial duplication. The most frequent inter-hemispheric gyrification patterns were bilateral single HG (36 %) and left single-right duplication (27 %). The least common patterns were left duplication-right single (22 %) and bilateral duplication (15 %). Duplication was associated with decreased anterior HG surface area on the corresponding side, independently of the type of duplication, and increased total HG surface area (including the second gyrus). Inter-hemispheric gyrification patterns strongly influenced both anterior and total HG surface area asymmetries, leftward asymmetry of the anterior HG surface was observed in all patterns except double left HG, and total HG surface asymmetry favored the side of duplication. Compared to right-handers, the 198 left-handers exhibited lower occurrence of duplication, and larger right anterior HG surface and total HG surface areas. Left-handers' HG surface asymmetries were thus significantly different from those of right-handers, with a loss of leftward asymmetry of their anterior HG surface, and with significant rightward asymmetry of their total HG surface. In summary, gyrification patterns have a strong impact on HG surface and asymmetry. The observed reduced lateralization of HG duplications and anterior HG asymmetry in left-handers highlights HG inter-hemispheric gyrification patterns as a potential candidate marker of speech lateralization.

Fig. 1

Heschl’s gyrus (HG) identification and delineation on individual and mean images. a The oblique section plane (orange line) used to reconstruct the slice passing through HG. b–d Examples of the three gyrification patterns of the left Heschl’s gyrus (yellow) in axial view: single HG (b), complete stem duplication (c), and complete posterior duplication (d). e Mean image of the 232 right-handers. f–h Mean anatomical images of subjects exhibiting the same HG configuration, 147 subjects had a single gyrus (f), 57 complete stem duplication (g) and 28 complete posterior duplication (h)

Fig. 2

Variations of the HG inter-hemispheric gyrification patterns between right-handers (solid lines) and left-handers (dotted lines). The better inter-hemispheric agreement in left-handers compared to right-handers is due to the more frequent occurrence of bilateral single HG. **p < 10⁻⁵. L1, single left; L2, left duplication; R1, single right; R2, right duplication

Fig. 3

Variations of divergent hemispheric gyrification patterns in right-handers (solid lines) and left-handers (dotted lines). Right duplications are more frequent than left duplications in both handedness groups. *p < 10⁻⁴, **p < 10⁻²

Fig. 4

Proportion of identical or asymmetrical hemispheric gyrification patterns in right-handers (solid lines) and left-handers (dotted lines). Bilateral single and bilateral duplication are more frequent than asymmetrical gyrification pattern in both groups. *p < 10⁻³, **p < 10⁻⁵

Fig. 5

Variation of mean aHG and totHG asymmetry depending on the inter-hemispheric gyrification pattern in 232 right-handers (solid lines) and 198 left-handers (dotted lines). Asymmetry was calculated using the raw mean data, by subtracting the right surface area from the left surface area, expressed in mm². *p < 7.10⁻³ compared to 0

Fig. 6

Effect of handedness on the right aHG surface area and aHG asymmetry. Data are adjusted for sex, age, educational level, and skull perimeter, as well as for right HG number for right aHG surface or inter-hemispheric gyrification pattern for aHG asymmetry. Surface area is in mm². Asymmetry is computed with the left minus right surface area subtraction. *p < 10⁻², **p < 10⁻³. RH right-handers, LH left-handers