Mapping Brain Maturation

When Huttenlocher began to demonstrate this succession of events in the human brain, magnetic resonance imaging (MRI) was still in its infancy. His histological work in the 1980s paved the way in showing that the time-course for synaptic blooming and pruning in the human brain varies enormously by brain region. For example, in the visual cortex, synaptic overproduction reaches a maximum at about the fourth postnatal month. Then synapse elimination starts, and this continues until preschool age, by which time synaptic density has reached the adult level. But in the medial prefrontal cortex, an area of the brain involved in executive, attentional and regulatory functions, the peak occurs at 3–4 years of age, and substantial decline does not occur until mid-to-late adolescence (1).

Early work by Yakovlev and Lecours documented the progression of myelination in the developing human brain (2). More recent work by Benes et al. with much larger samples showed similar results, with myelination continuing well into the third decade of life (3). Interestingly, the spatial and temporal pattern of these cellular changes seemed to parallel developmental changes in synaptic density. Simply put, myelination of the most dorsal regions of the brain responsible for higher cognitive functions seemed to continue well into adolescence, and more ventral and deep brain structures—some of which are responsible for relatively more primitive functions—were myelinated earlier.

However, the sequence of myelination might be more complex than this. A problem with many interpretations of the work by Yakovlev and Lecours (2), and of similar studies of myelogenesis, is that dorsal and ventral systems do not ultimately myelinate to the same degree: on average, dorsal cortex, at its most mature, is less myelinated than ventral cortex. Therefore, if myelination took the same time-course, then at any given time areas that myelinate more fully would appear more myelinated than those that myelinate less. Therefore, when the degree of myelination is observed on histologically stained sections or in MRI scans of the brain, one must also consider the ultimate extent of myelination for each system. Recent research studying patterning molecules, and rates of proliferation and neuronal migration, show a much more complex pattern of cortical maturation, occurring primarily from a rostral-lateral-ventral pole toward a dorsal-medial-caudal pole (reviewed in (4)).

This histological evidence suggested that brain development is a dynamic process of progressive and regressive changes. But histology gave only very fragmented evidence for late brain maturation, given the dearth of post-mortem data from childhood and adolescent years. By contrast, MRI can non-invasively document these large-scale processes of brain development, can provide insight into the sequence and timing of these developmental processes in longitudinal experiments, and can document how they occur in living subjects.

The first quantitative structural brain-imaging studies in normal children using MRI were conducted in the late 1980s and early 1990s. Terry Jernigan and colleagues showed that young adults actually had less cortical gray matter than children, despite the fact that adults had somewhat larger overall total brain volumes (5). They found that gray-matter volumes generally declined after age 7, perhaps because the advancement of white matter (i.e. myelination) throughout childhood began to overtake the overall rate of brain volume expansion, causing a net decrement in the amount of tissue appearing gray (or unmyelinated) on MRI. They then determined that the timing of gray-matter loss was different for different brain regions: loss was first observed in the deep motor nuclei in early childhood, then in the parietal and frontal lobes at around puberty (6). Although MRI did not assess synaptic density per se, this was the first in vivo evidence to support the post-mortem findings of Huttenlocher and of Yakovlev and Lecours.

The aforementioned studies used a method called volumetric parcellation. In this approach, the brain is subdivided into several, separate anatomical regions with different functions (e.g. the frontal lobe, the deep motor nuclei or the hippocampus), and their volumes are measured (Figure 1). Parcellation enables the compilation of growth curves that document how regional volumes vary with age. Results are typically illustrated using scatterplots but regional measurements are limited by anatomical structures (i.e. sulcal landmarks) and can also be reliably visualized and defined using MRI.

In the late 1990s, a few teams began to make composite 3D maps of developing brain structures (4, 7–12) showing, for example, the average pattern of age-related change in gray-matter thickness between childhood and young adulthood. More recently, these mapping techniques have become more popular because they provide more visual detail and group statistics regarding the rates of change and their regional variation. Time-lapse animations also can be computed to illustrate the changes.

Many of the basic functions of the brain, such as vision, hearing, speech, planning and emotional control are primarily handled in the cortex, so much of the work mapping brain development has focused on the cortex, although some groups have studied subcortical gray matter. The goal of cortical mapping is to create group average maps of cortical features of interest such as gray-matter thickness, gray-matter density, cortical shape, average sulcal patterning and hemispheric asymmetries, all of which change during development (for methods, see (13)). Next, statistics are defined that help localize age effects on these measurements, such as localized reductions or increases in gray-matter density or thickness, and brain growth.

In another longitudinal study, changes in cortical thickness were measured (in millimeters) in a group of 45 normally developing children studied between 5 and 11 years of age (12). Each child was studied twice with a two-year scanning interval. Maps of cortical thickness were created for each child, and the average cortical-thickness maps were remarkably similar to maps created from postmortem data by von Economo (15). The average cortical thickness for the normally developing children and from the post-mortem data are shown in Figure 2. As can be seen on the medial brain surface, the cortex is thickest in the most dorsal aspects of the frontal and parietal lobes (~4–5 mm), and thinnest in the visual cortices of the occipital lobes surrounding the calcarine sulcus (2.0–2.5 mm). These data revealed cortical thinning of ~0.15–0.30 mm per year in normally developing children, most prominently in right dorsal frontal and bilateral parietal regions. Cortical thickening was also observed with increases of ~0.10–0.15 mm per year in the classical language regions of the temporal and frontal lobes (Figure 3). The brain grew at a rate of ~0.4–1.5 mm per year in many regions, most prominently in the frontal and occipital regions. This pattern of results is similar to those observed in cross-sectional and longitudinal data of normal brain maturation (4, 16), but the new methods enabled changes in cortical thickness to be assessed in millimeters for the first time. Additionally, this study confirmed that dynamic but distinct changes occur in posterior temporal and frontal language regions, where gray matter continues to increase in thickness. We speculate that cortical changes in these regions could be related to changing language abilities that continue after 5 years of age (e.g. changes in reading ability) (12); functional and structural imaging studies are underway to test these hypotheses.

Changes in cortical thickness during normal development relate to cognitive changes as children and adolescents mature. In the same 45 normally developing children already described, cortical thinning in the left dorsal frontal and parietal lobes correlated with improved performance on a test of general verbal intellectual functioning (12, 17). Greater left hemisphere gray-matter thinning was associated with improved performance on the vocabulary subtest of a standardized IQ measure (17) (Figure 4). When permutation analyses were conducted to correct for multiple comparisons in the brain-behavior analyses, change in cortical thickness was correlated with change in the behavioral measure only in dorsal frontal and parietal cortex of the left hemisphere. Cortex in these regions tends to thin with increased age. Thus, the results of negative relationships between cortical thickness and vocabulary improvements are consistent with what would be expected. Considering these data, it seems reasonable to speculate that the brain changes are related to cognitive changes during development, although few studies have addressed this issue.

Most brain-mapping studies of development have focused on the cortex, but developmental changes in subcortical structures have also been characterized. For example, one study mapped a front-to-back wave of growth through the corpus callosum at 3–15 years of age (22) (Figure 6). The continuous myelination of interhemispheric fibers results in increased axonal conduction speed and transmission of information between the brain hemispheres. Perhaps the main surprise from the subcortical studies is the progressive volume loss in the deep gray-matter nuclei—especially the caudate—at around puberty. In contrast to the basal ganglia, temporal lobe gray-matter structures (the amygdala and hippocampus) seem to increase in volume during childhood and adolescence.

We cannot conclude directly what cellular changes are involved in the dynamic maturational processes described here. MRI measures changes in the volume and density of brain structures, but lacks the resolution to characterize the cellular mechanism (e.g. dendritic remodeling, cell death, synaptic pruning or myelination) of such changes. Moreover, gray-matter volume and thickness reflect not only the dendritic and synaptic processes that occur in neurons but also the complex architecture of neurons, glia (i.e. myelin) and vasculature. However, we can say that the gray-matter changes described in this review correlate well with the post-mortem sequence of regionally variable increased synaptic pruning and myelination during adolescence and early adulthood.

Mapping fetal and neonatal brains presents another set of practical and technical challenges. Aside from the obvious difficulty in obtaining motion-artifact-free images, at these ages the degree of myelination produces considerably less contrast in images obtained using the most commonly applied T1 and T2 weighted pulse sequences. Nevertheless, considerable insight into the earliest myelination and differentiation of the telencephalon and diencephalon can be seen by imaging from fetal stages in utero through the first six months after birth (23). These imaging results are consistent with post-mortem measures of increased myelination before birth (24–26). A general caudal-to-rostral progression can be appreciated in this age group. Furthermore, morphological differentiation of several structures such as the internal capsule (a fan-like structure of white matter separating the lentiform nucleus from the caudate and dorsal thalamus) precedes changes in white-matter, often by many months. Utilizing specific magnetic resonance pulse sequences to identify white matter, in an approach called diffusion tensor imaging (DTI), can also help characterize the changes associated with myelination. Neil et al. (27) found dramatic and generalized changes in water apparent diffusion coefficients and diffusion anisotropy. These diffusion parameters, measurable using DTI, are locally sensitive to the degree of myelination (which provides resistance to water diffusion) and to fiber orientation (which constrains the principal directions of water diffusion to be aligned with axons). Gilmore et al. (28) found significant increases in fractional anisotropy in the anterior and posterior regions (genu and splenium) of the corpus callosum during development that reflect underlying changes to water content and cytoarchitecture during maturation.

Beginning in 1992, scientists led by Judith Rapoport at the National Institute of Mental Health employed MRI technology to scan a group of ~50 teenagers repeatedly as their schizophrenia developed and a group of >500 healthy children as controls. The collected data were analyzed using brain-mapping methods developed to detect subtle changes in the cortex (30). Healthy subjects lost gray matter at a subtle rate of 1–2% per year in the parietal cortices, with very little detectable change in the other lobes of the brain. By contrast, the childhood-onset schizophrenia patients showed a rapid progressive loss of gray matter in superior frontal and temporal cortices, reaching 3–4% per year in some regions. This pattern was seen in both boys and girls. Early deficits in parietal brain regions that support language and associative thinking spread forwards into the temporal lobes, supplementary motor cortices and frontal eye fields. The deficits spread anatomically over a period of 5 years, consistent with the characteristic neuromotor, sensory and visual search impairments in the disease. In temporal cortices, including primary auditory regions, severe gray-matter loss was absent at disease onset but later became pervasive. Patients with the worst brain tissue loss also had the worst symptoms, which included hallucinations, delusions, bizarre and psychotic thoughts, hearing voices and depression.

The notion of schizophrenia as a dynamically emerging disease of neurodevelopment is still controversial. The spreading patterns of deficits corroborate the idea that the normal developmental process of dendritic and synaptic pruning might be abnormally accelerated or derailed in schizophrenia (31). Even so, they might represent a separate process entirely that begins in adolescence—twins studies have hinted that the earliest deficits, occurring in the parietal cortex, might have a non-genetic trigger (32). Some histological studies suggest that there is decreased neuropil in schizophrenia, but autopsy material is scarce and findings are inconsistent. As in normal development, the observed cortical changes might also be glial or vascular in origin, rather than purely neuronal (33). Intracortical contrast might also be affected by changes in myelination (34), and by changes in lipid metabolism during the use of atypical antipsychotic agents. It is possible that in schizophrenia, changes in gray-matter density also reflect, at least in part, increasing intracortical myelination. Still, four dimensional maps such as these offer a biological marker of disease progression for drug trials, where brain mapping can assess how well antipsychotic medications oppose advancement of the disease (35).

Brain imaging has also helped to identify circumscribed alterations in brain structure that are associated with developmental disorders that have genetic causes. Williams syndrome (WS), for example, is a developmental disorder associated with a deletion of ~20 genes in the 7q11.23 region of chromosome 7. WS subjects have mild to moderate mental retardation, but have remarkable proficiencies in language skills, social drive and musical ability. Figure 7 shows a range of brain regions affected by this genetic deletion: in general, the cortex is thinner, except in perisylvian language regions, where it is thicker by 5–10%; cortical complexity is also significantly increased (36). In post-mortem data from WS subjects, Holinger et al. (37) observed larger cells in Heschl’s gyrus in the primary auditory area—a finding congruent with the cortical thickening seen here. WS subjects also have disproportionately reduced white matter, so cortical cells fitting over a smaller white-matter area can ‘pile up’, and the cortex can thicken owing to a crowding effect. Some Wnt signaling genes, which direct early cell differentiation and segmentation, are in the deleted 7q11.23 chromosomal region, suggesting a genetic basis for the cortical dysmorphology observed on MRI. Other genes deleted in WS might normally contribute to fissure formation in the human cortex, and their absence might account for the abnormal cortical thickening. Ongoing work is examining the changes in folding complexity in the same regions that show thickened cortex, to ensure that this thickening is not simply a consequence of increased gyrus formation (e.g. microgyria often appears as thickened cortex, when in fact it is thinner, four-layered cortex). If cortical thickness and complexity were positively correlated in general, the frontal and parietal regions of greater gyrification found in studies of WS (e.g. (38)) would be expected to show greater cortical thickness, but this was not found. There is, therefore, no simple relationship between thickness and complexity, and careful study of these measures in larger samples is warranted. Alternatively, the cortical thickening might represent an adaptive response to the genetic deletion, perhaps even from increased use or overuse of specific cortical networks. Identification of such neuroanatomical characteristics of WS is important for better understanding of the scope and timing of the cortical anomaly in WS.

ADHD is a much more common, and less debilitating, disorder of childhood than childhood-onset schizophrenia and WS. Nonetheless, volumetric structural brain-imaging studies of ADHD patients have demonstrated subtle reductions in total brain volume, and in volumes of the right frontal lobe and caudate nucleus (39, 40). To further understanding of brain abnormalities in ADHD, brain mapping studies were conducted in 27 children and adolescents with ADHD and in 46 age-matched and gender-matched control subjects (41). As predicted, abnormal morphology was observed in the frontal cortices of ADHD subjects, with reduced regional brain size localized primarily to inferior portions of dorsal prefrontal cortices bilaterally. Brain size was also reduced in anterior temporal cortices bilaterally. Gray-matter density was prominently increased in large portions of the posterior temporal and inferior parietal cortices bilaterally (Figure 7). The etiology and cognitive and behavioral sequelae of this increased gray-matter density and regionally reduced brain size are not yet understood, but these regions of the brain are known to be part of an action-attention network probably involved in the symptomatology of ADHD (discussed in (41)).

Brain mapping has also identified anomalies in developmental disorders with known environmental causes. Fetal alcohol syndrome (FAS) is a severe neurodevelopmental disorder observed in children whose mothers abused alcohol during pregnancy (42, 43). Not all women who abuse alcohol during pregnancy have children with FAS, but it is quickly becoming more evident from animal and both postmortem and in vivo human studies that the brain is especially vulnerable to the teratogenic effects of alcohol. Children with FAS display prenatal growth deficiency, developmental delay, craniofacial anomalies (i.e. microcephaly, epicanthal folds and short palpebral fissures) and limb defects. Cognitive deficits in these children are significant and mental retardation is frequently observed, but a clear neuropsychological profile has yet to be discovered (44).

In vivo quantitative MRI studies have confirmed brain morphological abnormalities in children prenatally exposed to alcohol. In addition to the microcephaly frequently observed in these children, volumetric MRI studies have shown that within the cortex, only the parietal lobes were significantly reduced in volume above and beyond the generalized brain size reduction. White-matter hypoplasia (i.e. incomplete development) was more significant than gray-matter hypoplasia, and hippocampal volume was relatively intact (45). These volumetric findings prompted use of cortical mapping techniques to assess brain abnormalities on a more local level. Localized abnormalities in brain size and gray-matter density were identified in a group of 21 children, adolescents and young adults with severe prenatal alcohol exposure compared with 21 age-matched and gender-matched non-exposed subjects (46). The resulting brain maps con-firmed parietal lobe size reductions observed in the volumetric studies, and revealed frontal lobe size reductions that had not been previously observed. In fact, frontal lobe abnormalities were predicted given these children’s deficits on neuropsychological tests of executive functioning (47). Furthermore, gray-matter density increases were observed bilaterally in the inferior parietal and posterior temporal lobes (Figure 7). These mapping studies suggest that brain growth continues to be adversely affected long after the pre-natal insult of alcohol exposure to the developing brain, and the regions most prominently affected are consistent with the behavioral and neuropsychological deficits in these children.

It is interesting to note the regional correspondence of gray-matter density abnormalities observed in three of the four neurodevelopmental disorders reviewed here. As shown in Figure 7, gray-matter density increases were observed in perisylvian (language) cortices in ADHD (41), WS (36) and FAS (46). The magnitude of increase in gray matter relative to their respective control groups varies across the disorders, with maxima of ~30% in ADHD, 10% in WS and 50% in FAS. The spatial extent of these gray-matter density increases also varies across the disorders, and is bilateral in ADHD and FAS, but more prominent in the right hemisphere in WS. These disorders are distinct in terms of their etiologies and neurocognitive deficit profiles, although various levels of language impairment are observed in all three disorders. Facial dysmorphology is present in both FAS and WS. Of particular relevance might be that ADHD diagnoses are common among children with FAS (48) and WS (49). Perhaps the similar profile of gray-matter density abnormalities observed in these three groups are related to the common symptoms of ADHD. Functional and structural imaging studies evaluating children that have ADHD, WS with ADHD, WS without ADHD, FAS with ADHD, and FAS without ADHD might yield interesting results, although they would be difficult given the relative rarity of WS in particular. These examples are illustrative only and summarize visualizations derived from different studies (e.g. case-control studies of FAS, WS and ADHD). Further studies are required in which the magnitudes and spatial patterns of the effects in the different patient groups are directly compared. Such direct comparisons would have to take into account possible confounding factors, because the children were assessed on different scanners at different imaging centers.

Probably the best known twin database is the Finnish twin registry, which covers information on twin births in Finland from 1940 and has been a resource to many epidemiological studies recognized worldwide. These data enabled investigation of which brain regions develop under tight genetic control and which are more influenced by the environment. Before that, twins had been studied using MRI and some similarities in the volumes of brain structures had been noted in identical twins, with weaker resemblance in fraternal twins. In 2001, utilizing this database, the first study of twins using brain mapping revealed aspects of brain structure that are largely inherited (50).

Forty healthy adult subjects, consisting of ten monozygotic and ten dizygotic twin pairs (five male pairs and five female pairs in each group) were drawn from a twin cohort consisting of all the same-sex twins born in Finland between 1940 and 1957 for which both members of each pair were alive and still residing in Finland. All the subjects were scanned and 3D maps of gray matter and models of cortical surface anatomy were derived. Monozygotic pairs were then matched with the dizygotic pairs. Gray-matter volumes in the frontal parts of the brain were more closely matched in the identical twins than in twins who were less similar genetically. One interpretation of the findings is that areas of the brain involved in process and rule learning (e.g. frontal lobe structures) are under more tight genetic control, at least in terms of gross structural development, than areas of the brain that map life experience and content (the temporal and parietal lobes).

The finding that the gross anatomy of the frontal cortex develops under such strong genetic control is also of practical use for genetic studies of disease. These genetically mediated structural differences have even been linked, using genetic brain-mapping methods, with known genetic markers that are overrepresented in patients with frontal lobe deficits (51).

As we have shown, MRI used in both cross-sectional and longitudinal studies gives precious data on normal and pathological brain development. Key information is now coming from the large-scale analysis of data from children scanned longitudinally using MRI. Moreover, advances in MRI techniques other than anatomical MRI are now offering additional perspectives on the functional development of the brain. Functional MRI studies are now emerging as a tool to study brain development, as are fiber-mapping studies using DTI to track connections and how they develop during maturation.

However, several limitations need to be acknowledged. Some are technical. For example, because it requires considerable statistical power to detect systematic differences in brain structure, sample size is a major issue, especially when rare disorders are considered. MRI data can be heterogeneous. Scan databases are only now becoming large enough to stratify images of brain development by symptom profiles, therapeutic response and currently identified risk factors.

MRI has some limits in resolving distinctions between gray and white matter that are relevant when assessing maturational changes. Recent algorithms for measuring cortical thickness (13) rely on definition of a boundary between gray and white matter, or at least on a proportion of tissue identified as gray matter (in the case of gray-matter density). As gray matter becomes more myelinated, the border between gray and white changes, and can be displaced toward the pia. One factor in the cortical thinning seen in adolescence is not absolute diminution of the cortical gray matter, but rather a change in the gray–white segmentation boundary because of increasing intracortical myelin with age, which might shift the gray–white junction further into the cortical mantle. Other anatomical features might also affect the accurate segmentation of gray and white matter on magnetic resonance images. The inner band of Baillarger, for instance, is so richly myelinated that in brain sections it is visible with the naked eye, close to the gray–white interface in layer 5b; this might affect the ability to define an accurate cortical boundary in conventional magnetic resonance images. Further studies with improved tissue classification algorithms, tissue relaxometry and high-field imaging (providing increased resolution and contrast-to-noise ratio) are likely to resolve some of these limitations.

Other issues concern the interpretation given to some data. The key question is: what does a change in the volume of a brain region mean, especially when one considers pathology (Figure 8)? Even if the volume and shape of brain structures are determined, to identify the underlying cellular cause of these differences one must rely on using additional sources of information, such as post-mortem tissue. Nevertheless, these MRI studies are likely to help us better understand the links between changes in the brain and cognitive development, in addition to the clinical course of development and treatment of illnesses.