Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
550 APPENDIX C âoddballâ paradigm using event-related potentials) to study the develop- ment of explicit memory (see Bachevalier, 1992; Bachevalier et al., 1991, 1993; Nelson, C.A., 1994, 1995, 1996). Finally, Luciana and colleagues (e.g., Luciana and Nelson, 1998) have used an extensive battery of tasks to examine a range of cognitive behaviors. The use of neuropsychological tools have several advantages over the other approaches discussed below: (a) they are completely noninvasive, (b) they can be used across the lifespan, (c) parallel studies can be conducted across species, and (d) they can provide insight into specific behaviors. The neuropsychological approach also has shortcomings: (a) these tools only indirectly couple brain structure and function (i.e., because no direct mea- sures of the brain are taken) and thus may lack precision with regard to this relation; (b) when adopting such tools from the animal literature, it is important to consider whether both species are responding to the tasks the same way; (c) caution must be exercised when generalizing from clinical to normative samples; and (d) when used with the lesion method (i.e., the population under study has experienced a lesion to a particular part of the brain), it is important to be aware that the mapping of specific lesion to specific function may be less than one to one (i.e., a lesion in a particular area could affect the function of surrounding areas as well). METABOLIC PROCEDURES This class of tools depends on the ability to track various metabolic functions as they occur in real time. These include positron emission to- mography and functional magnetic resonance imaging, each of which is described below. Positron Emission Tomography Positron emission tomography (PET) scanning typically involves the injection of a natural substance such as oxygen or glucose that has been made radioactive. In so doing one is able to track the metabolism of this substance by those regions of the brain calling for its use. Positrons are emitted as the radioactive substance decays, and these positrons can be measured using a positron detector (i.e., PET scanner). The detector, in turn, computes the point of origin of these positrons, and thus localizes in the brain (within centimeters of resolution) the source of neural activity. A good example of this work comes from studies conducted by Chugani and his colleagues. Here a form of radioactive glucose (FDG) has been used in infants and children to infer the development of synapses (i.e., synapse formation requires energy and thus glucose can be used as an indirect marker for synaptogenesis; see Chugani, 1994; Chugani and Phelps, 1986;
STUDYING THE DEVELOPING HUMAN BRAIN 551 Chugani et al., 1987). The participants in these studies are typically studied under resting conditions (sometimes under sedation); that is, no task is being performed. A number of shortcomings with PET must be acknowledged. First, although the levels of radioactivity used in this work are relatively low, ethical constraints prevent samples of normally developing children from being evaluated; thus, currently all participants in this work require medical cause for doing the scan. Second, the spatial resolution of PET is typically confined to relatively large voxels (cubic centimeters of tissue), and thus it is difficult to pinpoint the locus of neural activity much beyond the centimeter range. Third, PET suffers from poor temporal resolution (i.e., on the order of minutes), and thus little useful information can be obtained about when brain activity is taking place. Finally, because a cyclotron is required to make the radioactive agents, PET studies are an expensive endeavor. Functional Magnetic Resonance Imaging Functional magnetic resonance imaging (fMRI) is a rapidly expanding technology that is increasingly finding a home in studies of development. The technique is based on the concept that deoxygenated hemoglobin is paramagnetic (paramagnetism refers to the ability of a normally nonmag- netic material to become magnetic) and thus can be detected using conven- tional magnetic resonance technology. When a particular part of the brain is called on to perform some task, that region receives increased blood flow and, as a by-product, increased oxygen. Increases and decreases in oxygen (generally on the order of 2 to 5 percent relative to background) are then monitored. By taking consecutive slices of the brain in various orientations, the MRI scanner is able to reconstruct where in the brain the greatest areas of activation occur. Over the past 10 years, there have been hundreds of studies using fMRI in the adult human. Increasingly, however, developmental investigators have begun to utilize this technique with children. For example, Casey and colleagues (e.g., Casey et al., 1995; Thomas et al., 1999), as well as Nelson and colleagues (e.g., Nelson et al., 2000) have used fMRI to study the development of working memory in normally developing children as young as 6 years. There are multiple advantages to fMRI. For example, it is completely noninvasive, does not require exposure to ionizing radiation, and can be performed in a relatively short period of time. Critically, the spatial resolu- tion of fMRI is comparable to conventional MRI and thus can provide detailed anatomic images along the lines of a few millimeters. There are also a number of limitations that must be acknowledged. For example, participants must sit very still so as to keep motion artifacts to a minimum.
552 APPENDIX C In addition, they must be able to tolerate a somewhat high (e.g., 90 dB) level of noise and a confining environment. In summary, both PET and, in particular, fMRI lend themselves to the study of developing brain function. Unfortunately, neither PET nor fMRI provides much useful information about the chronometry of mental events.
553 Biographical Sketches D Jack P. Shonkoff (Chair) is dean of the Heller Graduate School and Samuel F. and Rose B. Gingold professor of human development at Brandeis Uni- versity. He is an academic pediatrician whose work focuses on early child- hood health and development and the interactions among research, policy, and practice. For the National Academies, he has served as chair of the Board on Children, Youth, and Families and as a member of the Panel on Child Care Policy, the Steering Group for the National Forum on the Future of Children and Families, the Committee on the Assessment of Family Violence Interventions, and the Roundtable on Head Start Re- search. He serves as a member of the scientific core group of the John D. and Catherine T. MacArthur Foundation and the James S. McDonnell Foundation Research Network on Early Experience and Brain Develop- ment and serves on the board of ZERO TO THREE. He was elected to the Institute of Medicine in 1999 and is a member of the American Pediatric Society. Other honors include a Kellogg national fellowship, a fellowship from the National Center for Clinical Infant Programs, and the distin- guished contribution to child advocacy award from the Division of Child, Youth, and Family Services of the American Psychological Association. He has an M.D. from New York University School of Medicine. Deborah L. Coates is professor of psychology at the City University of New York. Prior to assuming this position, she was director of the Institute for Healthier Babies of the March of Dimes Birth Defects Foundation and associate professor of psychology at Catholic University. Her research