Imaging the developing brain: what have we learned about cognitive development?

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The human brain undergoes significant changes in both its structural architecture and functional organization across the life span. Advances in neuroimaging techniques over the past decade have allowed us to track these changes safely in the human in vivo. We review the imaging literature on the neurobiology of cognitive development, focusing specifically on cognitive task-dependent changes observed in brain physiology and anatomy across childhood and adolescence. The findings suggest that cortical function becomes fine-tuned with development. Brain regions associated with more basic functions such as sensory and motor processes mature first, followed by association areas involved in top-down control of behavior.

Introduction

The year 2005 marks a decade since the first application of fMRI to developmental questions [1]. It is well established that brain development and cognitive maturation occur concurrently during childhood and adolescence 2, 3, 4, but much less is known about the direct relationship between neural and cognitive development. This review highlights what we have learned about the biological substrates of cognitive development over the past decade, advances and limitations of the methods, and the relevance of these studies to understanding and developing interventions for individuals with atypical development. Emphasis is placed on studies that inform how cognitive development and learning map onto changes in brain anatomy, connectivity and physiology across childhood and adolescence.

Contemporary non-invasive neuroimaging methods have provided developmental scientists with the opportunity to track safely cognitive and neural processes underlying human development (see Box 1). These methods have advanced the field of developmental neuroscience by providing evidence of changes in structural architecture and functional organization in the developing brain in vivo. However, these measures provide only an indirect measure of brain structure and function. Changes in the volume of a structure or amount of activity as measured by MR methods lack the resolution to characterize the mechanism of change definitively. Histological evidence suggests that brain development is a dynamic process of regressive and progressive changes (see Box 2). As such, MRI-based cortical changes observed with development may be a combination of myelination, dendritic pruning and changes in the vascular, neuronal and glial density. Nonetheless, the methodologies provide information about regional development that, in conjunction with histological studies, could tease these processes apart. Furthermore, because these tools permit not only scanning of children but also repeated scanning of the same individual over time, they can provide measurement of general neuroanatomical changes with both learning and development. This review highlights MR-based measures of change in neuroanatomy, cortical connectivity and cortical function with learning and development.

Section snippets

Neuroanatomical development of the human brain

Several structural imaging studies have mapped the neuroanatomical course of human brain development [5]. The findings parallel many of the post-mortem histological findings, providing validity for in vivo imaging measures, and also parallel behavioral and cognitive development. Claims of causality between coincidental changes in brain and behavioral development is a common trap into which one could fall by simply assuming linear changes across systems and direct associations between these

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

Functional organization of the developing human brain

What do the previously described changes in brain structure, such as prolonged development of the prefrontal 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 characterized 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

Cortical organization with learning

The importance of tracking cortical changes in individuals over time is perhaps most evident in the area of learning. Using repeated scanning of the same individuals, Karni and others (e.g. 47, 48) have shown rapid learning effects in primary motor cortex of adults during motor sequence learning that were apparent within a single imaging session, but that increased over weeks of training. This use of fMRI to trace learning-related changes in cortical areas is currently being used by others in

Future directions

Future research will no doubt take advantage of the ability to image children multiple times with fMRI over the course of learning to delineate developmental and experience-based processes in cortical activity (see Box 3). For example, to determine whether the immature brain after extended practice engages in the same neural processes as the mature brain, we could compare brain activity in the mature system with brain activity in the immature system both before and after extended experience.

Conclusions

Findings from both cross-sectional and longitudinal imaging studies of late childhood and adolescence show that brain regions associated with more basic functions such as motor and sensory processes mature first, followed by association areas involved in top-down control of thoughts and action. This pattern of development is paralleled by a shift from diffuse to more focal recruitment of cortical regions with learning and cognitive development. Fine-tuning of cortical systems occurs with

Acknowledgements

This work was supported in part by P01 MH62196, R01 MH63255 and R21 DA15882 to B.J.C.

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