Reduced interhemispheric Stroop-like interference in childhood and ageing

One of the most important structural changes that occur in the brain of both children and older people relates to the corpus callosum (CC), the largest neural pathway that connects the two cerebral hemispheres. The CC consists of around 200 million fibres that continue to myelinate during childhood until puberty and that endures a process of demyelination in aging. Hence, a primary question of interest is whether these changes in callosal myelination have functional consequences on interhemispheric communication. To investigate interhemispheric function in children and older people, 10 children (aged 7-8 years), 12 older adults (aged 61-82 years) and 20 control young adults (aged 18-23 years) were tested with a divided visual field Stoop-like task. In this modified version of the Stroop paradigm, a number of dots (2 or 3) were paired with either an incongruent or congruent Arabic digit (“2” or “3”). The stimuli were presented tachistoscopically with the two components either distributed across the two hemifields (bilateral presentation) or presented to a single visual half-field (unilateral presentation). The Stroop effect, the difference in reaction times between the incongruent and congruent trials, was not affected by stimulus presentation in the control group. However, for both children and older adults, the Stroop effect was significantly smaller for bilaterally-presented stimuli than for unilaterally-presented stimuli, indicating reduced interhemispheric interference in these groups. These findings suggest that age-related callosal thinning may have specific functional consequences likely due to a decrease in interhemispheric connectivity. In particular, the condition appears to affect the ability of one hemisphere to interfere with the operation of the other hemisphere