106 Review TRENDS in Cognitive Science Vol9 No.3 March 2005 and latest in the dorsolateral prefrontal cortex 6.11. information on directionality and regularity of mvelinated These findings are consistent wi non-h and humt e pre imotor cortex in Cr 88-5 indirect meas ures of white matter suggest res norm un ma 10 00 cortex and pres n oncluding that gray matter loss during this per t of working men y was correlated po a sculpt ng process of th bra ely with prefrontal-parieta 124 ent 39,421 t DTI this period of developmentas well 1,13,51.One frontostriatal the report sn subcortical and accrate) performance on a GoNo Th ctional 19 and longitudinal studies 6]sho nt st identified in an fMRI study usin the same task elopment n port ons of the ganglia to 42. Similarly, combined DTI and alytica dual ce ofs this in white of brain correlated with development,but specificity of the invo hite me ement of part r roung adulthood [6,13,14,17,20 evide ed by nitive ability 241Th e dissociations are important ncre density hes cha when distinguishing bet odend l ,both regre Fun tional organization of the develop an brain pr th previo e e nges in bral beingne-tuned with overabun tal volume ing of ele ant conne nt corte How do the ese structural changes rel to cognitive nition is characte hanges: nges cortica developmen ze by the ability to filter and suppress irrelevan prefro lobe stru on and ity to filte and suppres 2Rrprae MRI-b of appropriat es co prefrontal volume asures of cogniti interference competing sources lessening with Many parad ms use dy 21.22).Together th studies su t that,perhaps not and Go/No o-go tasks 33351.Collectively.im n brain dev lopment are task as probe chil tasks than do pment of h man brain connectivity ults sed on ectional [36]and long tudina ctionsa ng revie ned with the 371 reg elimination of an overabundan stre ngthening of of syne es and the focal or fine-tuned,whereas regior elated rele evelopm ase MR s diffusion tensor im TD,P vide a potential tool activity ohser a variety of or examin ing the role of co connectivity in the pment within ides DTI Anding nted previously and latest in the dorsolateral prefrontal cortex [6,10]. These findings are consistent with non-human and human primate postmortem studies showing that the prefrontal cortex matures at a more protracted rate than sensor￾imotor cortex in synaptic density [11,12]. Cross-sectional studies of normative brain maturation during childhood and adolescence have shown somewhat similar patterns concluding that gray matter loss during this period reflects a sculpting process of the immature brain into the fully functioning mature one [10,13–16]. As such, the pattern of development observed is suggested to reflect the ongoing neuronal regressive events, such as pruning and the elimination of connections. Developmental changes in subcortical regions occur over this period of development as well [10,13,15–19]. One of the more reliable patterns reported is in subcortical regions to which association cortex projects. For example, both cross-sectional [19] and longitudinal studies [6] show patterns of development in portions of the basal ganglia to which the prefrontal cortex directly projects. Again, this pattern could reflect the gradual elimination of connec￾tions, with strengthening of others. What is the corresponding pattern of change in white matter volume over this period? In contrast to an inverted U-shaped pattern of development in gray matter with age, white matter volume increases in a roughly linear pattern, increasing throughout development until approximately young adulthood [6,13,14,17,20], as evidenced by increases in volume and density [17]. These changes presumably reflect ongoing myelination of axons by oligodendrocytes enhancing neuronal conduction and communication. Therefore, both regressive and progress￾ive processes are occurring in parallel, which could enhance neural and cognitive processes. Connections are being fine-tuned with the elimination of an overabun￾dance of synapses and strengthening of relevant connec￾tions with development and experience. How do these structural changes relate to cognitive changes? Developmental changes in cortical development have been found to correlate with behavioral performance measures. Sowell and colleagues [4] showed an association between prefrontal lobe structural maturation and mem￾ory function using neuropsychological measures. Similar associations have been reported between MRI-based prefrontal volume and specific measures of cognitive control (i.e. the ability to override an inappropriate response in favor of another) in cross-sectional studies [21,22]. Together these studies suggest that, perhaps not surprisingly, functional changes in brain development are reflected in structural changes. Development of human brain connectivity The MRI-based morphometry studies reviewed suggest that cortical connections are being fine-tuned with the elimination of an overabundance of synapses and the strengthening of relevant connections with development and experience. Recent advances in MR technology, such as diffusion tensor imaging (DTI), provide a potential tool for examining the role of cortical connectivity in the development of cognitive and brain development in greater detail. This method (see Box 1) provides information on directionality and regularity of myelinated fiber tracts (e.g. [23]). Until recently, few studies have linked brain connec￾tivity measures with cognitive development, although indirect measures of white matter suggest regional development in prefrontal cortex and presumably in function [23]. Of these studies, one showed that develop￾ment of working memory capacity was correlated posi￾tively with prefrontal–parietal connectivity [24], consistent with imaging studies showing differential recruitment of these regions in children relative to adults [39,42]. Using a similar approach, Liston and colleagues [25] showed that DTI-based connectivity in both frontos￾triatal and posterior fiber tracts correlated with age, but only frontostriatal connectivity correlated with efficient (fast and accurate) performance on a Go/No-go task. The prefrontal fiber tracts examined were defined by regions of interest identified in an fMRI study using the same task [42]. Similarly, combined DTI and fMRI analytical approaches have shown the importance of the maturation of prefrontal–parietal connectivity in the performance of a working memory task [26]. As such, DTI and fMRI-based measures with these regions correlated across age. In these studies, measures of brain connectivity were correlated with development, but specificity of the invol￾vement of particular fiber tracts in cognitive performance was shown by dissociating the particular tract [25] or cognitive ability [24]. These dissociations are important when distinguishing between performance and age￾related differences in imaging measures. Functional organization of the developing human brain What do the previously described changes in brain structure, such as prolonged development of the prefron￾tal volume and connectivity, mean in terms of function? The development of the prefrontal cortex is thought to play an important role in the maturation of higher cognitive abilities [27,28]. Mature cognition is character￾ized by the ability to filter and suppress irrelevant information and actions (sensorimotor processes), in favor of relevant ones (i.e. cognitive control) [27]. A child’s capacity to filter information and suppress inappropriate actions in favor of appropriate ones continues to develop across the first two decades of life, with susceptibility to interference from competing sources lessening with maturity [29–32]. Many paradigms used to study cogni￾tive development require cognitive control such as the Stroop and Go/No-go tasks [33–35]. Collectively, imaging studies using these tasks as probes show that children recruit distinct but often more prefrontal regions, both ventral and dorsal, when performing these tasks than do adults. Based on cross-sectional [36] and longitudinal studies [37], regions whose brain activity correlates with task performance (reaction time and/or accuracy) become more focal or fine-tuned, whereas regions not correlated with task performance decrease in activity with age (see Figure 1). It has been suggested that this pattern of activity, observed across a variety of paradigms, reflects development within, and refinement of, projections to and from these regions [28,30,34,36,38], consistent with the DTI findings presented previously. 106 Review TRENDS in Cognitive Sciences Vol.9 No.3 March 2005 www.sciencedirect.com