Symmetries in human brain language pathways correlate with verbal recall

Abstract

Lateralization of language to the left hemisphere is considered a key aspect of human brain organization. We used diffusion tensor MRI to perform in vivo virtual dissection of language pathways to assess the relationship between brain asymmetry and cognitive performance in the normal population. Our findings suggest interhemispheric differences in direct connections between Broca's and Wernicke's territories, with extreme leftward lateralization in more than half of the subjects and bilateral symmetrical distribution in only 17.5% of the subjects. Importantly, individuals with more symmetric patterns of connections are better overall at remembering words using semantic association. Moreover, preliminary analysis suggests females are more likely to have a symmetrical pattern of connections. These findings suggest that the degree of lateralization of perisylvian pathways is heterogeneous in the normal population and, paradoxically, bilateral representation, not extreme lateralization, might ultimately be advantageous for specific cognitive functions.

Fig. 1.

Left–right hemispheric differences in perisylvian language pathways. (A) Tractography reconstruction of language pathways in the average DT-MRI data set. In both hemispheres, the anterior indirect segment (green) connects posterior inferior frontal cortex (B, Broca's territory) and inferior parietal cortex (G, Geschwind's territory), whereas the posterior indirect segment (yellow) connects superior posterior temporal cortex (W, Wernicke's territory) and Geschwind's territory. In the left hemisphere, but not in the right, the direct segment (red) connects Wernicke's and Broca's territories. (B–D) Segment overlap maps derived from dissection of single brains show L-R asymmetry only in the direct segment (B) and L-R symmetry in the anterior indirect (C) and posterior indirect (D) segments. “Hotter” (yellow) colors correspond to a higher degree of overlap, whereas “cooler” (red) colors reflect less overlap.

Fig. 2.

Lateralization of language pathways and behavioral correlates. (A) Distribution of the lateralization pattern of the direct long segment (red). (B) Significant correlation between the lateralization index (streamlines) of the direct segment and performances on the CVLT (number of words correctly recalled of 80). (C) Performances in the CVLT according to the three lateralization groups (∗, P < 0.01 vs. Group 1; †, P < 0.001 vs. Group 1). (D) Distribution of the lateralization groups between genders. (E) Performances in the CVLT according to the lateralization pattern and gender (striped colors are females) (∗, P < 0.05 vs. Group 1; †, P < 0.01 vs. Group 1; ‡, P < 0.05 vs. Group 1).