Meningitis and the evolution of dominance of righthandedness

Asymmetry of interhemispheric interaction in left-handed subjects

In right-handed subjects the execution of a simple finger-tapping task is associated with an asymmetry of interhemispheric interaction, probably suggesting that the dominant left hemisphere inhibits the right one. The present study investigated the left-handed subjects in order to elucidate whether this asymmetry is related to handedness. Nineteen healthy subjects performed unimanual left, right, and bimanual auditorily paced finger-tapping tasks while neuromagnetic activity was recorded with a 122-channel whole-head neuromagnetometer (MEG). Simultaneously, we recorded activity of the first dorsal interosseus (FDI) muscle of both hands. By using the analysis tool dynamic imaging of coherent sources (DICS), oscillatory activity at alpha as well as at beta frequency within the primary sensorimotor (S1/M1) and premotor (PMC) cortex was localized. As expected, we observed oscillatory coupling between S1/M1 and PMC contralateral to the moving hand. Furthermore, coupling between left PMC and bilateral S1/M1 occurred in each movement condition, suggesting that the left PMC modulates neural activity in bilateral primary sensorimotor cortices independent of the moving hand. Coupling between bilateral S1/M1 occurred more frequently and significantly stronger during the right hand condition. This result demonstrates the same interhemispheric coupling pattern as in right-handed subjects, suggesting that the asymmetry of this interaction is not due to hand dominance. A specialization of the left premotor cortex either for superior motor control per se or for the execution of sequential tasks might account for these results.

Increased occurrence of left-handedness after severe childhood bacterial meningitis: Support for the pathological left-handedness hypothesis

The hypothesis that brain damage during infancy causes pathological left-handedness was tested by assessing handedness in 182 survivors of childhood non-Hemophilus influenza type b bacterial meningitis in the Netherlands (mean age of 9.7 years). These children were selected randomly after clustering them into those with or without parental report on academic and behavioural problems. Medical records were obtained from the hospitals, while handedness and neurodevelopmental outcome were assessed at school age. Logistic regression analysis was used to study the relationship between a severity score of bacterial meningitis and handedness. Fifteen percent were left-handed. Severity of childhood bacterial meningitis was related to left-handedness (Odds ratio (OR) 6.2, 95% confidence interval (CI) 2.0-18.6 for those with a total severity score above the median as compared to those below). Compared to non-left-handed children, left-handed children had lower IQ (mean difference -6.6, 95% CI -12 to -1.2), tended to have lower vocabulary scores on WISC-r (-1.0, -2.1 to 0), and lower Beery scores on visual-motor integration (-4.9, -10.1 to 0.4). Left-handed children also tended to have more combined academic and behavioural limitations (OR 2.7, 95% CI 0.9-8.6), lower manual speed of the dominant hand (mean difference -9 taps, p < 0.05) and better manual steadiness in the non-dominant hand (mean difference of contact's time -2.7 s, p < 0.05). Left-handed post-meningitic children generally have worse neurodevelopmental outcome than non-left-handed survivors. Our results support the role of early life brain damage in left-handedness.