Typical fine tuning of brain functional organization is an activity-dependent process [Kandel et al.,2000]. According to Huttenlocher , plasticity changes over developmental time, with some mechanisms available throughout the lifetime (increase in synaptic strength, decrease in local inhibition, dendritric sprouting, formation of new synapses, formation of new neurons), whereas others only available to the early developing brain (use of unspecified labile synapses including silent synapses, competition for synaptic sites, persistence of normally transient connections, myelination). Changes in plasticity turn out to be region specific, suggesting [Thomas,2003] that there is no such general thing as “the brain's plasticity.” Many structures (e.g., dendrites, axons, and synapses) initially undergo exuberant growth followed by a period of pruning in which the processing of environmental input gradually sculpts the resulting brain structure.
Neural processing tends initially to be diffuse across several regions in both hemispheres, but with developmental time and the continuous processing of inputs, brain activity becomes increasingly restricted to more specific networks [Brown et al., 2008; Durston et al.,2006 and the discussion by Poldrack, this Special Issue, on the problem of the neurobiological plausibility of the gradual focalization hypothesis made on the basis of neuroimaging data]. If there is a gradual process of modularization over developmental time [Karmiloff-Smith,1992], as opposed to the notion of built-in modules, then this is likely to improve processing efficiency. A recent study by Minagawa-Kawai and colleagues  examined language-specific phonemic contrasts in infants from 3 months to 28 months and found differing age-specific onset of varying regions of the cortex. Another study suggests that comprehension of single words moves from bilateral processing between 13 and 17 months to left lateralized processing at 20 months [Mills et al.,1997]. Like vocabulary development, processing of human faces starts out with bilateral activity, with the brain displaying similar signatures for other stimuli like monkey faces [de Haan et al., 2002; Pascalis et al.,2001]. By the end of the first year, however, the brain becomes increasing fine-tuned for processing human faces, with other stimuli displaying different neural signatures, as well as increasing localization for human faces to specific networks in the right hemisphere [de Haan et al., 2002; Peelen et al.,2009]. We need to know more about how hemispheric differences influence developmental neural change. In adults, the RH seems to be involved in more parallel, coarse-grained, integrative processing, and the LH in more serial, fine-grained, predictive processing. How does this develop in children? Is information passage through corpus callosum always faster from RH to LH than from LH to RH, or does this alter over developmental time? Certainly, the thickness of the corpus callosum fibers changes developmentally over a lengthy period of time between infancy and adolescence [Keshavan et al., 2003]. Finally, short-range gray matter connectivity is greater in children, while white long-range connectivity develops considerably more slowly over time [Huttenlocher,2002]. All of these and other developmental changes must be taken into account when analyzing neuroimaging data over time. And, as shown in several articles in this Special Issue [Luna et al., Blakemore et al., Shaw et al.; see also, Crone et al.,2008], neural development doesn't end in childhood; puberty and adolescence witness many significant changes, which raises the important question of whether neural changes in learning and development are the same [Galván, this Special Issue]. Moreover, rather than focusing on chronological age in studies of adolescents, onset of puberty might be a more sensitive way of understanding interactions between hormonal and gray matter development, gender differences, as well as the general structural reorganization of the adolescent brain [Blakemore et al., this Special Issue].