Cerebral Asymmetries: Complementary and Independent Processes

Most people are right-handed and left-cerebrally dominant for speech, leading historically to the general notion of left-hemispheric dominance, and more recently to genetic models proposing a single lateralizing gene. This hypothetical gene can account for higher incidence of right-handers in those with left cerebral dominance for speech. It remains unclear how this dominance relates to the right-cerebral dominance for some nonverbal functions such as spatial or emotional processing. Here we use functional magnetic resonance imaging with a sample of 155 subjects to measure asymmetrical activation induced by speech production in the frontal lobes, by face processing in the temporal lobes, and by spatial processing in the parietal lobes. Left-frontal, right-temporal, and right-parietal dominance were all intercorrelated, suggesting that right-cerebral biases may be at least in part complementary to the left-hemispheric dominance for language. However, handedness and parietal asymmetry for spatial processing were uncorrelated, implying independent lateralizing processes, one producing a leftward bias most closely associated with handedness, and the other a rightward bias most closely associated with spatial attention

Asymmetries of the arcuate fasciculus in monozygotic twins: genetic and nongenetic influences.

We assessed cerebral asymmetry for language in 35 monozygotic twin pairs. Using DTI, we reconstructed the arcuate fasciculus in each twin. Among the male twins, right-handed pairs showed greater left-sided asymmetry of connectivity in the arcuate fasciculus than did those with discordant handedness, and within the discordant group the right-handers had greater left-sided volume asymmetry of the arcuate fasciculus than did their left-handed co-twins. There were no such effects in the female twins. Cerebral asymmetry for language showed more consistent results, with the more left-cerebrally dominant twins also showing more leftward asymmetry of high anisotropic fibers in the arcuate fasciculus, a result applying equally to female as to male twins. Reversals of arcuate fasciculus asymmetry were restricted to pairs discordant for language dominance, with the left-cerebrally dominant twins showing leftward and the right-cerebrally dominant twins rightward asymmetry of anisotropic diffusion in the arcuate fasciculus. Because monozygotic twin pairs share the same genotype, our results indicate a strong nongenetic component in arcuate fasciculus asymmetry, particularly in those discordant for cerebral asymmetry

Concordance for handedness and language dominance in male and female twin pairs.

<p>Twin pairs who were excluded from the analysis due to the intractability of the arcuate fasciculus are shown in brackets.</p

Cerebral language dominance.

<p>Asymmetry indices (AI) for the FA values of the arcuate fasciculus according to cerebral language dominance are shown. Twin pairs with discordant language dominance showed reversed AI_FA asymmetry.</p

Brain activity and the arcuate fasciculus.

<p>Averaged brain activity during word generation task (<b>A</b>) and reconstruction of the arcuate fasciculus on the <b>B</b>) 1% threshold and <b>C</b>) 2% threshold, respectively. Colour bars represent the number of subjects showing arcuate fasciculus fibers in that voxel.</p