The primary function of the frontal motor strip (area 4) is motor control. Evidence for greater lipid content consistent with lateralized, heavy myelination of fibers in area 4 comports with this function. These relations, tested in the NCANDA adolescents, were supported with Pearson correlations and the multiple mediation model indicating a relation between greater normalized myelin content in the right hemisphere and faster performance on the Grooved Pegboard Test, especially in the older than younger adolescents when performing in the challenging condition using the non-dominant (typically the left) hand. In apparent contrast to area 4, lower normalized myelin content (i.e., relative to the global mean) in the mid or posterior cingulate cortices (areas p24' and 23c) predicted faster grooved pegboard performance especially by the non-dominant hand. This relation can be explained by recognizing that a function of the cingulate cortex is to support voluntary saccades (Vogt 2016), control over which is critical for eye-hand coordinated movement needed for successful grooved pegboard performance. The complementary relation of unimodal (area 4) and multimodal (areas p24' and 23c) cortices between normalized myelin content
