Asymmetry and dyslexia

Abstract

Developmental language disorders are characterized by a maturational trajectory that deviates or lags that of normal children. Given the wide variation in the rate of normal language development, diagnosis and classification of these disorders poses severe problems for the clinician. Our laboratory has been searching for anatomical signatures that could aid the development of a neurobiologically based classification. Quantitative analysis of the magnetic resonance imaging (MRI) brain scans of a series of samples of children and adults with reading and language disorders has identified two clusters with contrasting anatomical and reading profiles. Individuals with small symmetrical brain structures tend to have deficits in multiple domains of written and oral language whereas those with larger asymmetrical structures are more likely to have the isolated phonological deficits seen in adults with compensated dyslexia. Surprisingly, the anatomical risk factors that define these clusters do not form a continuum of increasing severity but deviate in opposite directions from normal. Individuals with moderate brain size and asymmetry typically demonstrate the best overall performance. Further research should determine if phonological impairments in the two clusters are associated with differing genetic and environmental risk factors requiring different types of intervention.

Figure 1

Information processing maps in the temporal lobes (red oval) are differentiated from perisylvian auditory maps that are biased toward fast automatic digital processing in the time (temporal) domain, while parietal information processing maps (turquoise oval) differentiate from somatosensory and visual maps that are biased towards slower spatial reconstruction. Because of the anatomical asymmetry of the Sylvian fissure, temporal maps dominate left hemisphere processing and parietal maps dominate right hemisphere processing. Central sulcus, outlined in red, separates frontal and parietal lobes.

Figure 2

Location of the planum temporale in the posterior Sylvian fissure (arrows) and the planum parietale (depicted in blue). In the most common variant, the central sulcus separates the precentral and postcentral gyri and the planum parietale rises in the supramarginal gyrus, posterior to the postcentral sulcus.

Figure 3

Sagittal MRI images of the left and right hemispheres of a severely dyslexic man illustrating anomalous Sylvian fissure variants in both the left and right hemisphere. The X’s depict anomalous extra gyri interspersed between the central sulcus and the parietal planum. On the right, the planum parietale terminating the Sylvian fissure rises directly posterior to the central sulcus rather than the postcentral sulcus (compare the number of sulci descending from the superior surface anterior to the planum parietale on the left and right). These anomalies increase the asymmetries between the temporal and parietale lobes in the two hemispheres, leading, it is argued, to inefficient bottom up processing.

Figure 4

Contrasting views of the relation between reading and oral language deficits. A: Tallal proposes that auditory processing problems lead to phonological processing problems which lead to reading and oral language deficits. The severity of the auditory processing problem is expected to correlate with the severity of the reading and language deficits. B: The multidimensional view of Bishop & Snowling (2005). In this view, comprehension and phonological deficits have independent causes. Children with SLI have deficits in both phonological and comprehension, while some children with reading deficits actually have better phonological processing than comprehension.

Figure 5

Contrary to the prediction that anomalous asymmetry of the planum would be associated with worse auditory processing, auditory function is actually better in the students wih anomalous (negative) asymmetry. Auditory procedures are described in King, Lombardino, Crandell & Leonard (2003).

Figure 6

The reading profiles of the individuals in the samples discussed in this article. WA: Woodcock Johnson (WJ) Word Attack; PC: WJ Passage Comprehension; RAN: rapid naming of colors, objects, letters and/or numbers; VIQ: estimated verbal intelligence quotient; RD: reading disability; Samples are described in Table 1.

Figure 7

Results from anatomical risk index studies. A: College students ranked in order of their discrepancy between PC and WA. B. Graphic depiction of how the seven anatomical measurements are added together to produce positive and negative indices. AntVerm: anterior lobe of the cerebellar vermis; PT/PP: summed asymmetries of the planum temporale and planum parietale; CV: cerebral volume (adjusted for sex); LH2: surface area of second Heschl’s gyrus (left); LH: surface area of first Heschl’s gyrus (left). C. Relationship between anatomical risk index and PC-WA discrepancy. Students ordered as in 7a. Students with negative or small discrepancies have negative anatomical risk indices. Students with large positive discrepancies have positive anatomical risk indices. D. GT sample: children with poor comprehension tend to have negative anatomical risk indices while children with normal comprehension tend to have positive anatomical risk indices.

Figure 8

This horizontal MRI image shows the overlain fiber tracings from diffusion tensor imaging of two men with different structural anatomy. The fibers depicted in yellow come from a man with normal leftward planar asymmetry. The fibers traced in blue come from a man with symmetrical plana temporale due to a large planum temporale in the right hemisphere (bottom of figure). Note the absence of connections between the temporal and frontal lobes in the right hemisphere in the man with normal asymmetry. We hypothesize that the parietal lobe dominates frontal circuits on the right when temporal lobe connections are missing. Image modified from (Leonard, Eckert, & Kuldau, 2006), used with permission of Blackwell Publishing.