Age- and disease-related influences on structural and functional hemispheric differences

The influence of normal aging and a disease on the brain’s hemispheric specialization has been investigated in behavioral, structural and functional studies. However, results have been controversial, suggesting a more pronounced hemispheric asymmetry in response to age- and disease-related influences on the one hand (right hemi-aging model) and a reduction of functional lateralization on the other hand (HAROLD model). The intention of this work was to investigate possible differences in the susceptibility to age- and disease-related influences between the left and the right hemisphere. The effect of age on the functional lateralization of figural and verbal working memory was investigated in two age groups by means of functional magnetic resonance imaging (fMRI). Both working memory functions revealed a stronger left- compared to right-hemispheric recruitment in the old age group. Moreover, the left-lateralization of verbal working memory was found to be associated with a stronger left-hemispheric recruitment in high performing older age. In a second study, the influence of the unilaterally beginning neurodegenerative disease Corticobasal Syndrome (CBS) on brain volumetric and behavioral characteristics was explored by means of deformation field morphometry (DFM) in two groups of patients that differed in the initially affected side of body. Patients with initial right-sided impairment (r-CBS) revealed atrophy predominantly in frontoparietal areas and showed, except from apraxia, no other cognitive deficits. In contrast, patients with impairment of the left body side (l-CBS) revealed more widespread atrophy, extending from frontoparietal to orbitofrontal and temporal regions; and apraxia, perceptional and memory deficits could be found.Taken together, these findings about age- and disease-related influences on the brain’s structural organization support the idea of a difference in susceptibility between the left and the right hemisphere. They are in consistence with the assumption of the right hemi-aging model and challenge the often reported age-related reduction in hemispheric asymmetry