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Voxel-based morphometry has been used to investigate the associations between regional gray and white matter volume and cognitive function. The manuscript introduces these studies' methods, cautions, results, and problems related to the reports of the results. In many studies of ours that investigating these associations using the huge sample size and robust statistics, the associations were weak and wide spread in the brain. These patterns of results suggest the difficulty in finding the specific neural bases that underlie individual cognitive functions through VBM.

Accumulated evidence shows that neural information processing takes place in superficial layers of the brain called the gray matter. Synapses, which connect different neurons reside in the gray matter and are considered the major components of information processing and plasticity. On the other hand, myelinated axons lie beneath the gray matter. These bundles of cables connect neurons in the different brain regions to form functional neural circuits. Myelinated axons were of little of interest to neuroscientists and have long been ignored in the formation of functional neuronal circuits. Recent evidence shows that myelin formed by oligodendrocytes shows plastic changes depending on neuronal activity. In this issue, we discuss the plastic changes of myelin and its functional role in learning and training.