Developmental neuroimaging: a developmental psychologist looks ahead

The study of the mature human brain has moved forward at a rapid pace over the last 10 years, thanks in large part to the invention and use of a variety of neuroimaging methods. As the papers in this volume attest, many of these methods are now available for the study of the developing human brain, both in normal children and in children with a variety of developmental disabilities. These methods will allow developmental neuroanatomists and neurophysiologists to chart changes in brain morphology, long-range connectivity and activity as children grow and learn. Combinations of these methods, moreover, will allow developmental neuroscientists to probe specific brain processes and growth patterns. By combining functional magnetic resonance imaging (fMRI) or near infrared spectroscopy (NIRS) with event-related potentials (ERP) or magnetoencephalography (MEG), for example, investigators will be able to chart with considerable precision both where and when particular patterns of neural activity occur as children perform particular tasks. By combining diffusion tensor imaging (DTI) and fMRI, moreover, investigators will be able to relate developmental changes in connectivity across brain areas to changes in patterned activity within individual areas. Knowledge of human brain development promises to grow at an unprecedented rate through the use of these methods and approaches. Here, however, I consider a different question: How will these methods for studying development of the brain contribute to knowledge of development of the human mind? Will developmental neuroimaging bring new insights to developmental psychology? I suggest an optimistic answer to this question: Developmental neuroimaging is likely to offer new insights into questions that have been central to developmental psychology for centuries. Developmental neuroimaging also may shed light on aspects of the mature human mind that have long eluded those who study adults. Before turning to this suggestion, however, I must digress and consider how neuroimaging experiments have affected the study of mature psychological processes. Neuroimaging – particularly the functional brain imaging methods of positron emission tomography (PET), fMRI, ERP and MEG – has swept the field of human cognitive psychology over the last decade. Its contribution to understanding human cognitive processes, however, has been uneven. When functional neuroimaging methods have probed psychological functions that already were well understood on a behavioral level, they have provided rich and useful descriptions of the neural activity that accompanies these functions. These descriptions serve as neural signatures for the psychological mechanisms that accomplish the functions. In contrast, where functional neuroimaging methods have been used to study functions whose nature had eluded behavioral scientists, their findings have been as variable and inconclusive as those of the canon of behavioral research that preceded them. To illustrate the first of these generalizations, consider functional neuroimaging studies of number processing in adults. Prior to the advent of fMRI, a wealth of behavioral evidence pointed to the existence of a cognitive system for representing and reasoning about number: what Dehaene (1997) calls ‘number sense’. Evidence for number sense came from experiments on animals who were trained to discriminate particular numbers of objects or events (e.g. Meck & Church, 1983), on normal adults given number discrimination tasks under conditions that prevented verbal counting or other symbolic strategies (e.g. van Oeffelen & Vos, 1982), and on neurological patients who showed striking dissociations between number sense and other linguistic and calculation skills (e.g. Warrington, 1982). In all these populations, number representations were found to be abstract and amodal, to encompass increasing numerosities with no clear upper bound, and to be subject to Weber’s Law: The discriminability of two numerosities depended on their ratio. In the context of these behavioral findings, Dehaene and others have explored the neural mechanisms of number sense through coordinated studies using fMRI