A growing number of structural neuroimaging studies have reported significant changes in gray matter density or volume and white matter microstructure in the adult human brain following training. Such reports appear consistent with animal studies of training-dependent structural plasticity showing changes in, for example, dendritic spines. However, given the microscopic nature of these changes in animals and the relatively low spatial resolution of MRI, it is unclear that such changes can be reliably detected in humans. Here, we critically evaluate the robustness of the current evidence in humans, focusing on the specificity, replicability, and the relationship of the reported changes with behavior. We find that limitations of experimental design, statistical methods, and methodological artifacts may underlie many of the reported effects, seriously undermining the evidence for training-dependent structural changes in adult humans. The most robust evidence, showing specificity of structural changes to training, task and brain region, shows changes in anterior hippocampal volume with exercise in elderly participants. We conclude that more compelling evidence and converging data from animal studies is required to substantiate structural changes in the adult human brain with training, especially in the neocortex.
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