Planum temporale asymmetry in developmental dyslexia: Revisiting an old question

Abstract

Among the various asymmetrical structures of the human brain, the planum temporale, an anatomical region associated with a variety of auditory and language-related processes, has received particular attention. While its surface area has been shown to be greater in the left hemisphere compared to the right in about two-thirds of the general population, altered patterns of asymmetry were revealed by post mortem analyses in individuals with developmental dyslexia. These findings have been inconsistently replicated in magnetic resonance imaging studies of this disorder. In this report, we attempt to resolve past inconsistencies by analyzing the T1-weighted MR images of 81 children (mean age: 11 years, sd: 17 months), including 46 control (25 boys) and 35 dyslexic children (20 boys). We manually outlined Heschl's gyri, the planum temporale and the posterior rami of the Sylvian fissure on participants' brain images, using the same anatomical criteria as in post mortem studies. Results revealed an altered pattern of asymmetry of the planum temporale surface area in dyslexic boys only, with a greater proportion of rightward asymmetrical cases among dyslexic boys compared to control boys. Additionally, analyses of cortical thickness showed no asymmetry differences between groups for any of the regions of interest. Finally, a greater number of Heschl's gyrus full duplications emerged for the right hemisphere of dyslexic boys compared to controls. The present findings confirm and extend early post mortem observations. They also stress the importance of taking gender into account in studies of developmental dyslexia.

Keywords: developmental dyslexia; gender; planum temporale; reading; structural magnetic resonance imaging.

Figure 1

(a) Left cortical mesh model, seen from the lateral view. The black dotted line illustrates the cutting plane applied to visualize the superior temporal plane. At the caudal end of the Sylvian fissure, both the posterior ascending and descending rami are visible, overlaid in green. (b) The result of the aforementioned cutting operation is shown, seen from above, with Heschl's gyrus (red), the planum temporale (blue), and the posterior rami (green) labeled. The red dotted line shows our criterion for the anterior limit of Heschl's gyrus, and is the plane of the coronal section represented in c. (c) Coronal section through the same brain (left hemisphere depicted on the right side), the most anterior section in which the planum temporale (in blue) can be seen.

Figure 2

Example of (a) a common‐stem duplication and (b) a full posterior duplication. The head of the yellow arrow indicates Heschl's sulcus.

Figure 3

(a) Series of coronal sections with the PT marked in blue. The red circle shows the fusion between temporal and parietal lobes, which determines the posterior limit of the PT. (b) Coronal sections. Left, the PT is marked in blue. Right, a change in slope of the continuous plane characterizing the planum is seen, thus determining its posterior limit and the beginning of the posterior ramus.

Figure 4

(a) Mean surface areas of the left and right PT, PR, and H1 for control and dyslexic boys and girls. (b) Scatterplot illustrating individual variability for the left and right PT surface areas. Values are normalized for mean hemispheric surface area.

Figure 5

Histograms illustrating the frequency of planum temporale leftward, symmetrical and rightward asymmetrical cases in each group.