How and when can environmental influences change cerebral cortex? An experimental training study of twins with birth weight differences

Human cortical morphology is genetically programmed but also influenced by environment both in development and adulthood. Determining the timing of these influences across the lifespan is a key challenge. Here we test what makes genetically identical brains differ and converge. Mono-(MZ) and dizygotic (DZ) twins (n = 206, age 16-79 yrs) with known extent of birth weight (BW) discordance, had MRIs pre-and post-10 weeks immersive virtual reality navigation training in a train-rest-rest/rest-train-rest-design, or as passive controls. As a measure of between-twin similarity, we calculated “brainprints” from 272 structural cortical features, to assess effects of genetic (MZ/DZ) and environmental variation at early (BW discordance) and later life stages (training status). Baseline brainprint similarity was higher in MZ than DZ twins, but greater BW discordance yielded less similarity in MZ (r =-.54, p <.0001), dominated by cortical area effects (t =-6.748, p <.0001). In contrast, training increased brainprint similarity of MZ relative to DZ twins (zygosity x training; t =-2.864, p =.0046), mostly by cortical curvature (t =-4.401, p<.0001). Follow-up analyses indicated training increased white matter curvature and surface area. The findings demonstrate that in adulthood, early life environmental difference persistently contributes to make the brains of genetically identical twins deviate, while concurrent environmental influence in the form of training still can cause their brainprints to converge at the grey-white-matter boundary. This indicates how these early and later environmental influences on the cortex can be distinguished, and how cortical characteristics can be modified.