As the psychiatrist Barry Reisberg first observed twenty years ago, the decline of an Alzheimer's patient mirrors in reverse the neurological development of a child. The earliest capacities a child develops--raising the head (at one to three months), smiling (two to four months), sitting up unassisted (six to ten months)--are the last capacities an Alzheimer's patient loses. Brain development in a growingchild is consolidated through a process called myelinization, wherein the axonal connections among neurons are gradually strengthened by sheathings of the fatty substance myelin. Apparently, since the last regions of the child's brain to