Myelin stains reveal an anatomical framework for the representation of the digits in somatosensory area 3b of macaque monkeys.

Brain sections cut parallel to the cortical surface revealed myelin-light septa that isolated representations of the digits and parts of the face, teeth, and tongue in area 3b of adult and infant macaque monkeys. The widths of the bands of cortex representing individual digits, as measured by the distances between isolating septa, were proportionally similar in infant (2-4 week) and adult monkeys. However, the bands for digits 1-3 were somewhat narrower in infant than adult monkeys. There was little variation in absolute widths across individuals in the infant or adult groups, or between left and right hemispheres of the same group. Widths for digits 1-4 progressively decreased. The results suggest that these isomorphs of digits emerge in prenatal or early postnatal development and typical variations in postnatal hand use have little impact on subsequent development. As the hand representation in somatosensory cortex of monkeys may be significantly altered after the partial loss of peripheral nerve inputs, the physiological representation is not completely constrained by the isolating septa. Instead, the septa may serve as a persistent marker of normal organization in studies of cortical reorganization.