© 2001 Macmillan Magazines LtdFrom birth to teenage years,there is a fourfold increasein the volume of the human brain. During this period,there are also marked improvements in motor,cogni-tive and perceptual abilities.Although both of theseaspects of human development have been studied forseveral decades,it is only recently that investigators haveturned their attention to how they relate to one another.In other words, how does the physical growth ofthe brain relate to the emergence of new behaviouralabilities during infancy and childhood? Addressingthis question is not just of academic interest,but couldhave profound implications for clinical,educationaland social policies1.As adults, we have brains that arehighly structurally and functionally specialized; forexample,discrete regions of our cerebral cortex sup-port components of cognitive functions such as lan-guage and face processing.Although much of cogni-tive neuroscience and neuropsychology is concernedwith dissociating and identifying the functions ofthese regions in adults, the question of how such spe-cializations arise in the first place has received lessattention.One perspective is that the functional spe-cialization of regions of the cerebral cortex arisesthrough intrinsic genetic and molecular mechanisms,and that experience merely has a role in the final ‘finetuning’.An alternative view is that some aspects ofhuman functional brain development involve a pro-longed process of specialization that is shaped bypostnatal experience.A parallel debate to that among developmentalneuroscientists rages among developmental psycholo-gists. Some developmental psychologists argue thatthe human infant is born with ‘innate modules’and‘core knowledge’relevant to the physical and socialworld.Others propose that many of the changes inbehaviour observed during infancy are the result ofgeneral mechanisms of learning and plasticity.In this article, I summarize research on the postnatalneuroanatomical development of the human brain,andfindings on perceptual and cognitive development dur-ing infancy. I go on to outline three approaches tounderstanding the relationship between the two:func-tional brain development. The focus is on the first twoyears of life, as this is when the most pronouncedadvances in brain structure and behaviour occur.Human postnatal neuroanatomical developmentHuman brain development closely follows the sequenceofevents observed in other primates, albeit on a slowertimescale.A model of the evolution of the brain that suc-cessfully predicts the timing of different neural develop-mental events in various mammalian species2hasrecently been extended to human prenatal development3.The model predicts that the more delayed the generaltime course of development in a species, the larger therelative volume of the later developing structures (such asthe cerebral cortex, and particularly the frontal cortex).Inaccordance with this general prediction,the slowed rateFUNCTIONAL BRAIN DEVELOPMENTIN HUMANSMark H.JohnsonThere is a continuing debate in developmental neuroscience about the importance of activity-dependent processes. The relatively delayed rate of development of the human brain, comparedwith that of other mammals, might make it more susceptible to the influence of postnatalexperience. The human infant is well adapted to capitalize on this opportunity through primitivebiases to attend to relevant stimuli in its environment. The infant’s interaction with its environmenthelps to sculpt inter- and intraregional connections within the cortex, eventually resulting in thehighly specialized adult brain.Centre for Brain andCognitive Development,School of Psychology,Birkbeck College, Universityof London, Malet Street,London WC1E 7HX, UK.e-mail:[email protected] REVIEWS | NEUROSCIENCE VOLUME 2 | JULY 2001 | 475© 2001 Macmillan Magazines Ltd476 | JULY 2001 | VOLUME 2 www.nature.com/reviews/neuroREVIEWSThe differential time course of development of dif-ferent cortical regions can also be observed in the livinghuman brain by PET imaging12.In infants under 5 weeksof age,glucose uptake is highest in sensorimotor cortex,thalamus,brainstem and the cerebellar vermis,whereasby 3 months of age, there are considerable rises in activ-ity in the parietal, temporal and occipital cortices, basalganglia and cerebellar cortex. Maturational rises are notfound in the frontal and dorsolateral occipital cortexuntil approximately 6–8 months of age.An adult-likedistribution of resting activity within and across brainregions is observed by the end of the first year12.Thesemeasures,like the measures of synapse density, alsoshow an increase above adult levels. There is a continu-ing rise in overall resting brain metabolism (glucoseuptake) after the first year of life, with a peak — about150% of adult levels — at around 4–5 years of age forsome cortical areas.As with other species, regressive events are com-monly observed during human brain development.For example,in the primary visual cortex the meandensity of synapses per neuron starts to decrease at theend of the first year13.In humans,all cortical regionsstudied are subject to this rise and fall in synaptic den-sity, which declines to adult levels during later child-hood. The postnatal rise-and-fall developmentalsequence can also be seen in other measures of brainphysiology and anatomy. For example,PET studiesshow that although the overall level of glucose uptakereaches a peak during early childhood that is muchhigher than that observed in adults,the rates return toadult levels after about 9 years of age. The extent towhich these changes relate to those in synaptic densityis being investigated further.In addition to the formation of dendritic trees andtheir associated synapses,most fibres become myelin-ated during postnatal development. Owing to theincreased lipid content of the brain caused by this myeli-nation, structural MRI images can provide a cleargrey–white matter contrast,which allows quantitativevolume measurements to be made during development.Although there is some controversy about the interpre-tation of images from infants under 6 months (due tothe relatively high water content of both grey and whitematter at this age),the consensus is that brain structureshave the overall appearance of those in the adult by 2 years of age, and that all the main fibre tracts can beobserved by 3 years of age14,15.In some reports,it is sug-gested that after a rapid