INTRODUCTION

Between birth and 2 years of age, the human cortex undergoes tremendous development, with region-specific and layer-specific patterns of synaptic maturation, overgrowth, and pruning that are undoubtedly influenced by environmental input and complex patterns of neurotransmitter expression (e.g., Huttenlocher & Dabholkar, 1997; Moore & Guan, 2001). During this period, the newborn, who is totally dependent on caregivers for survival, turns into a walking, talking, thinking, self-aware being. These anatomical and functional changes across development should be reflected in vivo in the electrical brain activity that can be measured at the scalp.

In practice, collecting data from infants can be rather difficult. While studies that condition a behavioral response, such as sucking or looking, are probably the most advanced of the techniques available, there remain considerable problems in the type and amount of data that can be collected from preverbal infants with short attention spans and immature motor response systems, especially in the first months after birth. Postmortem studies of brain development can also be problematic because death in infancy is usually associated with abnormalities that may invalidate generalizations to normal development. Many of the imaging techniques available for the study of adult brain responses are difficult to apply to human infants. For example, fMRI and MEG require that the subject remain very still throughout the testing period. It is thus possible to test sleeping infants, but rather difficult to test awake infants (Anderson et al., 2001; Hattori et al., 2001; Souweidane et al., 1999).