Cerebral lobes in autism: early hyperplasia and abnormal age effects

Executive processes in Asperger syndrome: patterns of performance in a multiple case series

Mixed evidence exists for executive dysfunction in autism spectrum disorders (ASD). This may be because of the nature of the tasks used, the heterogeneity of participants, and difficulties with recruiting appropriate control groups. A comprehensive battery of 'executive' tests was administered to 22 individuals with Asperger syndrome and 22 well-matched controls. Performance was analysed both between groups and on an individual basis to identify outliers in both the ASD and control groups. There were no differences between the groups on all 'classical' tests of executive function. However, differences were found on newer tests of executive function. Specifically, deficits in planning, abstract problem solving and especially multitasking. On the tests that discriminated the groups, all of the ASD individuals except one were identified as significantly impaired (i.e. below the 5th percentile of the control mean) on at least one executive measure. This study provides evidence for significant executive dysfunction in Asperger syndrome. Greatest dysfunction appeared in response initiation and intentionality at the highest level--the ability to engage and disengage actions in the service of overarching goals. These deficits are best observed through using more recent, ecologically valid tests of executive dysfunction. Moreover, performance on these measures correlated with autistic symptomatology.

An MRI study of increased cortical thickness in autism

OBJECTIVE

The purpose of this study was to examine cortical thickness in autism in light of the postmortem evidence of cortical abnormalities of the disorder.

METHOD

Magnetic resonance imaging (MRI) scans were acquired from 17 children with autism and 14 healthy comparison subjects, and sulcal and gyral thickness were measured for the total brain and for all lobes.

RESULTS

Increases in total cerebral sulcal and gyral thickness were observed in children with autism relative to comparison subjects. Similar findings were noted in the temporal and parietal lobes but not in the frontal and occipital lobes.

CONCLUSIONS

These preliminary findings indicate that increased cortical thickness may contribute to the increased gray matter volume and total brain size that have been observed in autism and may also be related to anomalies in cortical connectivity.

Sentence comprehension in autism: thinking in pictures with decreased functional connectivity

Comprehending high-imagery sentences like The number eight when rotated 90 degrees looks like a pair of eyeglasses involves the participation and integration of several cortical regions. The linguistic content must be processed to determine what is to be mentally imaged, and then the mental image must be evaluated and related to the sentence. A theory of cortical underconnectivity in autism predicts that the interregional collaboration required between linguistic and imaginal processing in this task would be underserved in autism. This functional MRI study examined brain activation in 12 participants with autism and 13 age- and IQ-matched control participants while they processed sentences with either high- or low-imagery content. The analysis of functional connectivity among cortical regions showed that the language and spatial centres in the participants with autism were not as well synchronized as in controls. In addition to the functional connectivity differences, there was also a group difference in activation. In the processing of low-imagery sentences (e.g. Addition, subtraction and multiplication are all math skills), the use of imagery is not essential to comprehension. Nevertheless, the autism group activated parietal and occipital brain regions associated with imagery for comprehending both the low and high-imagery sentences, suggesting that they were using mental imagery in both conditions. In contrast, the control group showed imagery-related activation primarily in the high-imagery condition. The findings provide further evidence of underintegration of language and imagery in autism (and hence expand the understanding of underconnectivity) but also show that people with autism are more reliant on visualization to support language comprehension.

Partially enhanced thalamocortical functional connectivity in autism

Based on evidence for thalamic abnormalities in autism, impairments of thalamocortical pathways have been suspected. We examined the functional connectivity between thalamus and cerebral cortex in terms of blood oxygen level dependent (BOLD) signal cross-correlation in 8 male participants with high-functioning autism and matched normal controls, using functional MRI during simple visuomotor coordination. Both groups exhibited widespread connectivity, consistent with known extensive thalamocortical connectivity. In a direct group comparison, overall more extensive connectivity was observed in the autism group, especially in the left insula and in right postcentral and middle frontal regions. Our findings are inconsistent with the hypothesis of general underconnectivity in autism and instead suggest that subcortico-cortical connectivity may be hyperfunctional, potentially compensating for reduced cortico-cortical connectivity.

An MRI study of increased cortical thickness in autism

OBJECTIVE

The purpose of this study was to examine cortical thickness in autism in light of the postmortem evidence of cortical abnormalities of the disorder.

METHOD

Magnetic resonance imaging (MRI) scans were acquired from 17 children with autism and 14 healthy comparison subjects, and sulcal and gyral thickness were measured for the total brain and for all lobes.

RESULTS

Increases in total cerebral sulcal and gyral thickness were observed in children with autism relative to comparison subjects. Similar findings were noted in the temporal and parietal lobes but not in the frontal and occipital lobes.

CONCLUSIONS

These preliminary findings indicate that increased cortical thickness may contribute to the increased gray matter volume and total brain size that have been observed in autism and may also be related to anomalies in cortical connectivity.

Functional and anatomical cortical underconnectivity in autism: evidence from an FMRI study of an executive function task and corpus callosum morphometry

The brain activation of a group of high-functioning autistic participants was measured using functional magnetic resonance imaging during the performance of a Tower of London task, in comparison with a control group matched with respect to intelligent quotient, age, and gender. The 2 groups generally activated the same cortical areas to similar degrees. However, there were 3 indications of underconnectivity in the group with autism. First, the degree of synchronization (i.e., the functional connectivity or the correlation of the time series of the activation) between the frontal and parietal areas of activation was lower for the autistic than the control participants. Second, relevant parts of the corpus callosum, through which many of the bilaterally activated cortical areas communicate, were smaller in cross-sectional area in the autistic participants. Third, within the autism group but not within the control group, the size of the genu of the corpus callosum was correlated with frontal-parietal functional connectivity. These findings suggest that the neural basis of altered cognition in autism entails a lower degree of integration of information across certain cortical areas resulting from reduced intracortical connectivity. The results add support to a new theory of cortical underconnectivity in autism, which posits a deficit in integration of information at the neural and cognitive levels.

Monozygotic twins with Asperger syndrome: Differences in behaviour reflect variations in brain structure and function

A pair of monozygotic twins discordant for symptoms of Asperger syndrome was evaluated at the age of 13.45 years using psychometric, morphometric, behavioural, and functional imaging methods. The lower-functioning twin had a smaller brain overall, a smaller right cerebellum, and a disproportionately large left frontal lobe, and manifested almost no differential activation between distractors of high and low-congruence with target visual stimuli. The higher-functioning twin manifested a typically autistic pattern of anterior deactivation and posterior hyperactivation in response to incongruent distractors, overlaid with a typically normal pattern of activation of superior frontal cortex. The morphometric results are consistent with known correlations between brain structure and behaviour in autism, and the physiological results suggest correspondences between structure and function.

Autism at the beginning: microstructural and growth abnormalities underlying the cognitive and behavioral phenotype of autism

Autistic symptoms begin in the first years of life, and recent magnetic resonance imaging studies have discovered brain growth abnormalities that precede and overlap with the onset of these symptoms. Recent postmortem studies of the autistic brain provide evidence of cellular abnormalities and processes that may underlie the recently discovered early brain overgrowth and arrest of growth that marks the first years of life in autism. Alternative origins and time tables for these cellular defects and processes are discussed. These cellular and growth abnormalities are most pronounced in frontal, cerebellar, and temporal structures that normally mediate the development of those same higher order social, emotional, speech, language, speech, attention, and cognitive functions that characterize autism. Cellular and growth pathologies are milder and perhaps nonexistent in other structures (e.g., occipital cortex), which are known to mediate functions that are often either mildly affected or entirely unaffected in autistic patients. It is argued that in autism, higher order functions largely fail to develop normally in the first place because frontal, cerebellar, and temporal cellular and growth pathologies occur prior to and during the critical period when these higher order neural systems first begin to form their circuitry. It is hypothesized that microstructural maldevelopment results in local and short distance overconnectivity in frontal cortex that is largely ineffective and in a failure of long-distance cortical-cortical coupling, and thus a reduction in frontal-posterior reciprocal connectivity. This altered circuitry impairs the essential role of frontal cortex in integrating information from diverse functional systems (emotional, sensory, autonomic, memory, etc.) and providing context-based and goal-directed feedback to lower level systems.

Corpus Callosum Morphometrics in Young Children with Autism Spectrum Disorder

This study assessed midsagittal corpus callosum cross sectional areas in 3-4 year olds with autism spectrum disorder (ASD) compared to typically developing (TD) and developmentally delayed (DD) children. Though not different in absolute size compared to TD, ASD callosums were disproportionately small adjusted for increased ASD cerebral volume. ASD clinical subgroup analysis revealed greater proportional callosum reduction in the more severely affected autistic disorder (AD) than in pervasive developmental disorder-not otherwise specified (PDD-NOS) children. DD children had smaller absolute callosums than ASD and TD. Subregion analysis revealed widely distributed callosum differences between ASD and TD children. Results could reflect decreased inter-hemispheric connectivity or cerebral enlargement due to increase in tissues less represented in the corpus callosum in ASD.

Advances in autism neuroimaging research for the clinician and geneticist

This review focuses on recent advances in the in vivo study of the whole brain in idiopathic autism. The brain is abnormally large in some but not all children with autism during post-natal development. Age-related changes in brain volume in autism are complex and appear to be abnormal from infancy into adulthood. Diffuse differences in total and regional gray and white matter volumes are found. The volumetric abnormalities appear to follow anomalous, complex, and non-uniform developmental curves. Diffuse abnormalities of brain chemical concentrations, neural network anatomy, brain lateralization, intra- and inter-hemispheric morphologic and functional connectivity, and serotonin synthesis capacity are also found. Abnormalities of head growth are first apparent during infancy. Abnormalities of total brain volume, gray and white matter volumes, brain chemistry, serotonin synthesis, and brain electrophysiology are evident by early childhood. Currently, no method of brain imaging helps with diagnosis or treatment of idiopathic autism, but ongoing research aims to unravel the heterogeneity of autism and may provide future diagnostic tools that inform treatment.

Cortical gray and white brain tissue volume in adolescents and adults with autism

BACKGROUND

A number of studies have found brain enlargement in autism, but there is disagreement as to whether this enlargement is limited to early development or continues into adulthood. In this study, cortical gray and white tissue volumes were examined in a sample of adolescents and adults with autism who had demonstrated total brain enlargement in a previous magnetic resonance imaging (MRI) study.

METHODS

An automated tissue segmentation program was applied to structural MRI scans to obtain volumes of gray, white, and cerebrospinal fluid (CSF) tissue on a sample of adolescent and adult males ages 13-29 with autism (n = 23) and controls (n = 15). Regional differences for brain lobes and brain hemispheres were also examined.

RESULTS

Significant enlargement in gray matter volume was found for the individuals with autism, with a disproportionate increase in left-sided gray matter volume. Lobe volume enlargements were detected for frontal and temporal, but not parietal or occipital lobes, in the subjects with autism. Age and nonverbal IQ effects on tissue volume were also observed.

CONCLUSIONS

These findings give evidence for left-lateralized gray tissue enlargement in adolescents and adults with autism, and demonstrate a regional pattern of cortical lobe volumes underlying this effect.

Large brains in autism: the challenge of pervasive abnormality

The most replicated finding in autism neuroanatomy-a tendency to unusually large brains-has seemed paradoxical in relation to the specificity of the abnormalities in three behavioral domains that define autism. We now know a range of things about this phenomenon, including that brains in autism have a growth spurt shortly after birth and then slow in growth a few short years afterward, that only younger but not older brains are larger in autism than in controls, that white matter contributes disproportionately to this volume increase and in a nonuniform pattern suggesting postnatal pathology, that functional connectivity among regions of autistic brains is diminished, and that neuroinflammation (including microgliosis and astrogliosis) appears to be present in autistic brain tissue from childhood through adulthood. Alongside these pervasive brain tissue and functional abnormalities, there have arisen theories of pervasive or widespread neural information processing or signal coordination abnormalities (such as weak central coherence, impaired complex processing, and underconnectivity), which are argued to underlie the specific observable behavioral features of autism. This convergence of findings and models suggests that a systems- and chronic disease-based reformulation of function and pathophysiology in autism needs to be considered, and it opens the possibility for new treatment targets.

Autistic spectrum disorders

Autistic spectrum disorders (ASD), are a group of disorders characterised by qualitative abnormalities in social and emotional behaviour and are associated with restricted, stereotyped and repetitive interests and activities. There has been considerable understanding of ASD in recent years. This educational review paper focuses on four areas of interest and relevance to trainees preparing for the membership examination of the Royal College of Psychiatrists (MRCPsych): (a) diagnosing ASD; (b) epidemiology of ASD; (c) aetiology, including genetic, cognitive and neurochemical/neuropathological theories in ASD; and (d) treatment of ASD. Relevant papers are discussed and recommendations for further reading are provided.

Macrocephaly and the control of brain growth in autistic disorders

Autism is a childhood-onset neuropsychiatric disorder characterized by marked impairments in social interactions and communication, with restricted stereotypic and repetitive patterns of behavior, interests, and activities. Genetic epidemiology studies indicate that a strong genetic component exists to this disease, but these same studies also implicate significant environmental influence. The disorder also displays symptomatologic heterogeneity, with broad individual differences and severity on a graded continuum. In the search for phenotypes to resolve heterogeneity and better grasp autism's underlying biology, investigators have noted a statistical overrepresentation of macrocephaly, an indicator of enlarged brain volume. This feature is one of the most widely replicated biological findings in autism. What then does brain enlargement signify? One hypothesis invoked for the origin of macrocephaly is a reduction in neuronal pruning and consolidation of synapses during development resulting in an overabundance of neurites. An increase in generation of cells is an additional mechanism for macrocephaly, though it is less frequently discussed in the literature. Here, we review neurodevelopmental mechanisms regulating brain growth and highlight one underconsidered potential causal mechanism for autism and macrocephaly--an increase in neurogenesis and/or gliogenesis. We review factors known to control these processes with an emphasis on nuclear receptor activation as one signaling control that may be abnormal and contribute to increased brain volume in autistic disorders.

Neural connectivity and cortical substrates of cognition in hominoids

Cognitive functions and information processing recruit discrete neural systems in the cortex and white matter. We tested the idea that specific regions in the cerebrum are differentially enlarged in humans and that some of the neural reorganizational events that took place during hominoid evolution were species-specific and independent of changes in absolute brain size. We used magnetic resonance images of the living brains of 10 human and 17 ape subjects to obtain volumetric estimates of regions of interest. We parcellated the white matter in the frontal and temporal lobes into two sectors, including the white matter immediately underlying the cortex (gyral white matter) and the rest of white matter (core). We outlined the dorsal, mesial, and orbital subdivisions of the frontal lobe and analyzed the relationship between cortex and gyral white matter within each subdivision. For all regions analyzed, the observed human values are as large as expected, with the exception of the gyral white matter, which is larger than expected in humans. We found that orangutans had a relatively smaller orbital sector than any other great ape species, with no overlap in individual values. We found that the relative size of the dorsal subdivision is larger in chimpanzees than in bonobos, and that the ratio of gyral white matter to cortex stands out in Pan in comparison to Gorilla and Pongo. Individual variability, possible sex differences, and hemispheric asymmetries were present not only in humans, but in apes as well. Differences in the distribution of neural connectivity and cortical sectors were identified among great ape species that share similar absolute brain sizes. Given that these regions are part of neural systems with distinct functional attributes, we suggest that the observed differences may reflect different evolutionary pressures on regulatory mechanisms of complex cognitive functions, including social cognition.

White matter abnormalities in autism detected through transverse relaxation time imaging

While neuroimaging studies have reported neurobiological abnormalities in autism, the underlying tissue abnormalities remain unclear. Quantitative transverse relaxation time (T2) imaging permits the examination of tissue abnormalities in vivo, with increased T2 largely reflecting increased tissue water. Blood flow and the presence of tissue iron may also affect T2. In this study, we used voxel-based relaxometry of the cerebrum and global averages to examine T2 abnormalities in autism. Nineteen males with autism (age: 9.2 +/- 3.0 years) and 20 male controls (age: 10.7 +/- 2.9 years) underwent magnetic resonance imaging at 3.0 T. Quantitative T2 maps, generated through gradient echo sampling of the free induction decay and echo, were segmented into gray matter, white matter, and cerebrospinal fluid. Average cerebral gray and white matter T2 were determined and compared between groups. To assess localized T2 differences, the quantitative T2 maps were warped to a template created for this study, smoothed, and compared using statistical parametric mapping. Patients with autism had an increase in average cerebral white matter T2, although no group differences were seen in average cerebral gray matter T2. Patients with autism also had bilateral regional T2 increases in the gray matter and associated white matter of the parietal lobes (primary sensory association areas) and occipital lobes (visual association areas) and in the white matter within the supplementary motor areas in the frontal lobes. The regional and global elevations in white matter T2 suggest abnormalities of white matter tissue water content in autism, which may represent a neurobiological basis for the aberrant cortical connectivity hypothesized to underlie the disorder.

Prefrontal white matter in pathological liars

BACKGROUND

Studies have shown increased bilateral activation in the prefrontal cortex when normal individuals lie, but there have been no structural imaging studies of deceitful individuals.

AIMS

To assess whether deceitful individuals show structural abnormalities in prefrontal grey and white matter volume.

METHOD

Prefrontal grey and white matter volumes were assessed using structural magnetic resonance imaging in 12 individuals who pathologically lie, cheat and deceive ('liars'),16 antisocial controls and 21 normal controls.

RESULTS

Liars showed a 22-26% increase in prefrontal white matter and a 36-42% reduction in prefrontal grey/white ratios compared with both antisocial controls and normal controls.

CONCLUSIONS

These findings provide the first evidence of a structural brain deficitinliars, they implicate the prefrontal cortex as an important (but not sole) component in the neural circuitry underlying lying and provide an initial neurobiological correlate of a deceitful personality.

Autism: a window onto the development of the social and the analytic brain

Although the neurobiological understanding of autism has been increasing exponentially, the diagnosis of autism spectrum conditions still rests entirely on behavioral criteria. Autism is therefore most productively approached using a combination of biological and psychological theory. The triad of behavioral abnormalities in social function, communication, and restricted and repetitive behaviors and interests can be explained psychologically by an impaired capacity for empathizing, or modeling the mental states governing the behavior of people, along with a superior capacity for systemizing, or inferring the rules governing the behavior of objects. This empathizing-systemizing theory explains other psychological models such as impairments of executive function or central coherence, and may have a neurobiological basis in abnormally low activity of brain regions subserving social cognition, along with abnormally high activity of regions subserving lower-level, perceptual processing--a pattern that may result from a skewed balance of local versus long-range functional connectivity.

Statistical distribution of blood serotonin as a predictor of early autistic brain abnormalities

BACKGROUND

A wide range of abnormalities has been reported in autistic brains, but these abnormalities may be the result of an earlier underlying developmental alteration that may no longer be evident by the time autism is diagnosed. The most consistent biological finding in autistic individuals has been their statistically elevated levels of 5-hydroxytryptamine (5-HT, serotonin) in blood platelets (platelet hyperserotonemia). The early developmental alteration of the autistic brain and the autistic platelet hyperserotonemia may be caused by the same biological factor expressed in the brain and outside the brain, respectively. Unlike the brain, blood platelets are short-lived and continue to be produced throughout the life span, suggesting that this factor may continue to operate outside the brain years after the brain is formed. The statistical distributions of the platelet 5-HT levels in normal and autistic groups have characteristic features and may contain information about the nature of this yet unidentified factor.

RESULTS

The identity of this factor was studied by using a novel, quantitative approach that was applied to published distributions of the platelet 5-HT levels in normal and autistic groups. It was shown that the published data are consistent with the hypothesis that a factor that interferes with brain development in autism may also regulate the release of 5-HT from gut enterochromaffin cells. Numerical analysis revealed that this factor may be non-functional in autistic individuals.

CONCLUSION

At least some biological factors, the abnormal function of which leads to the development of the autistic brain, may regulate the release of 5-HT from the gut years after birth. If the present model is correct, it will allow future efforts to be focused on a limited number of gene candidates, some of which have not been suspected to be involved in autism (such as the 5-HT4 receptor gene) based on currently available clinical and experimental studies.

The functional neuroanatomy of spatial attention in autism spectrum disorder

This study investigated the functional neuroanatomical correlates of spatial attention impairments in autism spectrum disorders (ASD) using an event-related functional magnetic resonance imaging (FMRI) design. Eight ASD participants and 8 normal comparison (NC) participants were tested with a task that required stimulus discrimination following a spatial cue that preceded target presentation by 100 msec (short interstimulus interval [ISI]) or 800 msec (long ISI). The ASD group showed significant behavioral spatial attention impairment in the short ISI condition. The FMRI results showed a reduction in activity within frontal, parietal, and occipital regions in ASD relative to the NC group, most notably within the inferior parietal lobule. ASD behavioral performance improved in the long ISI condition but was still impaired relative to the NC group. ASD FMRI activity in the long ISI condition suggested that the rudimentary framework of normal attention networks were engaged in ASD including bilateral activation within the frontal, parietal, and occipital lobes. Notable activation increases were observed in the superior parietal lobule and extrastriate cortex. No reliable activation was observed in the posterior cerebellar vermis in ASD participants during either long or short ISI conditions. In addition, no frontal activation during short ISI and severely reduced frontal activation during long ISI was observed in the ASD group. Taken together, these findings suggest a dysfunctional cerebello-frontal spatial attention system in ASD. The pattern of findings suggests that ASD is associated with a profound deficit in automatic spatial attention abilities and abnormal voluntary spatial attention abilities. This article also describes a method for reducing the contribution of physical eye movements to the blood-oxygenation level dependent activity in studies of ASD.

When is the brain enlarged in autism? A meta-analysis of all brain size reports

BACKGROUND

Multiple studies have reported increased brain size in autism, while others have found no difference from normal. These conflicting results may be due to a lack of accounting for age-related changes in brain enlargement, use of small sample sizes, or differences in data acquisition methods.

METHODS

Reports of autism head circumference (HC), magnetic resonance imaging (MRI), and post-mortem brain weight (BW) that met specific criteria were identified and analyzed. Percent difference from normal values (%Diff) and standardized mean differences (SMD) were calculated to compare brain size across studies and measurement methods. Curve fitting, analysis of variance, and heterogeneity analyses were applied to assay the effects of age and measurement type on reported brain size in autism.

RESULTS

A fitted curve of HC and MRI %Diff values from 15 studies revealed a largely consistent pattern of brain size changes. Specifically, brain size in autism was slightly reduced at birth, dramatically increased within the first year of life, but then plateaued so that by adulthood the majority of cases were within normal range. Analysis of variance of MRI and post-mortem %Diff values by age group (young child, older child, adult) and measurement type (MRI, BW) revealed a significant main effect of both age and measurement type, with the youngest ages (2-5) showing the greatest deviation from normal. Random effects heterogeneity analysis revealed a significant effect of age on HC and MRI SMD.

CONCLUSIONS

These findings reveal a period of pathological brain growth and arrest in autism that is largely restricted to the first years of life, before the typical age of clinical identification. Study of the older autistic brain, thus, reflects the outcome, rather than the process, of pathology. Future research focusing on this early process of brain pathology will likely be critical to elucidate the etiology of autism.

Variability in spatial normalization of pediatric and adult brain images

OBJECTIVE

Normalization of brain images is a necessity for group comparisons of source analyses based on realistic head models. In this paper we compared the outcome of a linear registration method for brain images of psychiatric and control groups of different ages in order to assess the relative adequacy of normalization in such diverse groups.

METHODS

Magnetic Resonance images (MRI) of the brains of pediatric and adolescent subjects (mean ages 19 and 10.5 years) with a pervasive developmental disorder (PDD) and their healthy controls were included. A simple voxel-wise test of the group variances in image intensities was performed to evaluate regional differences in registration quality. Dipole analysis of visual P1 was performed to establish whether source locations were comparable across groups.

RESULTS

Significant differences between pediatric groups were found in white matter and thalamic regions of the brain. For all other group-wise comparisons, differences were confined to skull and neck regions. Dipole locations were found to be more anteriorly located in the adolescent groups.

CONCLUSIONS

The normalization procedure used in this paper is based on a brain template of normal adult brains from a restricted age group, and the results show that the use of this method in pediatric groups is less adequate. The method seems suitable for use in psychiatric groups. Also, the generators of visual P1 in PDD patients were found to be comparable to controls.

SIGNIFICANCE

The results suggest that this existing normalization method can be used in diverse populations, but is less suitable for pediatric images.

Significance of abnormalities in developmental trajectory and asymmetry of cortical serotonin synthesis in autism

The role of serotonin in prenatal and postnatal brain development is well documented in the animal literature. In earlier studies using positron emission tomography (PET) with the tracer alpha[(11)C]methyl-l-tryptophan (AMT), we reported global and focal abnormalities of serotonin synthesis in children with autism. In the present study, we measured brain serotonin synthesis in a large group of autistic children (n = 117) with AMT PET and related these neuroimaging data to handedness and language function. Cortical AMT uptake abnormalities were objectively derived from small homotopic cortical regions using a predefined cutoff asymmetry threshold (>2 S.D. of normal asymmetry). Autistic children demonstrated several patterns of abnormal cortical involvement, including right cortical, left cortical, and absence of abnormal asymmetry. Global brain values for serotonin synthesis capacity (unidirectional uptake rate constant, K-complex) values were plotted as a function of age. K-complex values of autistic children with asymmetry or no asymmetry in cortical AMT uptake followed different developmental patterns, compared to that of a control group of non-autistic children. The autism groups, defined by presence or absence and side of cortical asymmetry, differed on a measure of language as well as handedness. Autistic children with left cortical AMT decreases showed a higher prevalence of severe language impairment, whereas those with right cortical decreases showed a higher prevalence of left and mixed handedness. Global as well as focal abnormally asymmetric development in the serotonergic system could lead to miswiring of the neural circuits specifying hemispheric specialization.

Reduced functional connectivity between V1 and inferior frontal cortex associated with visuomotor performance in autism

Some recent evidence has suggested abnormalities of the dorsal stream and possibly the mirror neuron system in autism, which may be responsible for impairments of joint attention, imitation, and secondarily for language delays. The current study investigates functional connectivity along the dorsal stream in autism, examining interregional blood oxygenation level dependent (BOLD) signal cross-correlation during visuomotor coordination. Eight high-functioning autistic men and eight handedness and age-matched controls were included. Visually prompted button presses were performed with the preferred hand. For each subject, functional connectivity was computed in terms of BOLD signal correlation with the mean time series in bilateral visual area 17. Our hypothesis of reduced dorsal stream connectivity in autism was only in part confirmed. Functional connectivity with superior parietal areas was not significantly reduced. However, the autism group showed significantly reduced connectivity with bilateral inferior frontal area 44, which is compatible with the hypothesis of mirror neuron defects in autism. More generally, our findings suggest that dorsal stream connectivity in autism may not be fully functional.

Why the frontal cortex in autism might be talking only to itself: local over-connectivity but long-distance disconnection

Although it has long been thought that frontal lobe abnormality must play an important part in generating the severe impairment in higher-order social, emotional and cognitive functions in autism, only recently have studies identified developmentally early frontal lobe defects. At the microscopic level, neuroinflammatory reactions involving glial activation, migration defects and excess cerebral neurogenesis and/or defective apoptosis might generate frontal neural pathology early in development. It is hypothesized that these abnormal processes cause malformation and thus malfunction of frontal minicolumn microcircuitry. It is suggested that connectivity within frontal lobe is excessive, disorganized and inadequately selective, whereas connectivity between frontal cortex and other systems is poorly synchronized, weakly responsive and information impoverished. Increased local but reduced long-distance cortical-cortical reciprocal activity and coupling would impair the fundamental frontal function of integrating information from widespread and diverse systems and providing complex context-rich feedback, guidance and control to lower-level systems.

Brain overgrowth in autism during a critical time in development: implications for frontal pyramidal neuron and interneuron development and connectivity

While abnormalities in head circumference in autism have been observed for decades, it is only recently that scientists have begun to focus in on the developmental origins of such a phenomenon. In this article we review past and present literature on abnormalities in head circumference, as well as recent developmental MRI studies of brain growth in this disorder. We hypothesize that brain growth abnormalities are greatest in frontal lobes, particularly affecting large neurons such as pyramidal cells, and speculate how this abnormality might affect neurofunctional circuitry in autism. The relationship to clinical characteristics and other disorders of macrencephaly are discussed.

Brain development in autism: early overgrowth followed by premature arrest of growth

Due to the relatively late age of clinical diagnosis of autism, the early brain pathology of children with autism has remained largely unstudied. The increased use of retrospective measures such as head circumference, along with a surge of MRI studies of toddlers with autism, have opened a whole new area of research and discovery. Recent studies have now shown that abnormal brain overgrowth occurs during the first 2 years of life in children with autism. By 2-4 years of age, the most deviant overgrowth is in cerebral, cerebellar, and limbic structures that underlie higher-order cognitive, social, emotional, and language functions. Excessive growth is followed by abnormally slow or arrested growth. Deviant brain growth in autism occurs at the very time when the formation of cerebral circuitry is at its most exuberant and vulnerable stage, and it may signal disruption of this process of circuit formation. The resulting aberrant connectivity and dysfunction may lead to the development of autistic behaviors. To discover the causes, neural substrates, early-warning signs and effective treatments of autism, future research should focus on elucidating the neurobiological defects that underlie brain growth abnormalities in autism that appear during these critical first years of life.

Plasticity of nonneuronal brain tissue: roles in developmental disorders

Neuronal and nonneuronal plasticity are both affected by environmental and experiential factors. Remodeling of existing neurons induced by such factors has been observed throughout the brain, and includes alterations in dendritic field dimensions, synaptogenesis, and synaptic morphology. The brain loci affected by these plastic neuronal changes are dependent on the type of experience and learning. Increased neurogenesis in the hippocampal dentate gyrus is a well-documented response to environmental complexity ("enrichment") and learning. Exposure to challenging experiences and learning opportunities also alters existing glial cells (i.e., astrocytes and oligodendrocytes), and up-regulates gliogenesis, in the cerebral cortex and cerebellum. Such glial plasticity often parallels neuronal remodeling in both time and place, and this enhanced morphological synergism may be important for optimizing the functional interaction between glial cells and neurons. Aberrant structural plasticity of nonneuronal elements is a contributing factor, as is aberrant neuron plasticity, to neurological and developmental disorders such as epilepsy, autism, and mental retardation (i.e., fragile X syndrome). Some of these nonneuronal pathologies include abnormal cerebral and cerebellar white matter and myelin-related proteins in autism; abnormal myelin basic protein in fragile X syndrome (FXS); and abnormal astrocytes in autism, FXS, and epilepsy. A number of recent studies demonstrate the possibility of using environmental and experiential intervention to reduce or ameliorate some of the neuronal and nonneuronal abnormalities, as well as behavioral deficits, present in these neurological and developmental disorders.

Localized enlargement of the frontal cortex in early autism

BACKGROUND

Evidence from behavioral, imaging, and postmortem studies indicates that the frontal lobe, as well as other brain regions such as the cerebellum and limbic system, develops abnormally in children with autism. It is not yet clear to what extent the frontal lobe is affected; that is, whether all regions of frontal cortex show the same signs of structural maldevelopment.

METHODS

In the present study, we measured cortical volume in four subregions of the frontal cortex in 2-year-old to 9-year-old boys with autism and normal control boys.

RESULTS

The dorsolateral region showed a reduced age effect in patients when compared with control subjects, with a predicted 10% increase in volume from 2 years of age to 9 years of age compared with a predicted 48% increase for control subjects. In a separate analysis, dorsolateral and medial frontal regions were significantly enlarged in patients aged 2 to 5 years compared with control subjects of the same age, but the precentral gyrus and orbital cortex were not.

CONCLUSIONS

These data indicate regional variation in the degree of frontocortical overgrowth with a possible bias toward later developing or association areas. Possible mechanisms for these regional differences are discussed.

Auditory spatial localization and attention deficits in autistic adults

The objective of this study was to compare autistic adults and matched control subjects in their ability to focus attention selectively on a sound source in a noisy environment. Event-related brain potentials (ERPs) were recorded while subjects attended to a fast paced sequence of brief noise bursts presented in free-field at a central or peripheral location. Competing sequences of noise bursts at adjacent locations were to be ignored. Both behavioral measures of target detection and auditory ERP amplitudes indicated that control subjects were able to focus their attention more sharply on the relevant sound source than autistic subjects. These findings point to a fundamental deficit in the spatial focusing of auditory attention in autism, which may be a factor that impedes social interactions and sensory-guided behavior, particularly in noisy environments.

Larger Brains in Medication Naive High-Functioning Subjects with Pervasive Developmental Disorder

BACKGROUND

Are brain volumes of individuals with Pervasive Developmental Disorder (PDD) still enlarged in adolescence and adulthood, and if so, is this enlargement confined to the gray and/or the white matter and is it global or more prominent in specific brain regions.

METHODS

Brain MRI scans were made of 21 adolescents with PDD and 21 closely matched controls.

RESULTS

All brain volumes, except the white matter, were significantly larger in patients. After correction for brain volume, ventricular volumes remained significantly larger in patients.

CONCLUSIONS

Patients showed a proportional, global increase in gray matter and cerebellum volume, and a disproportional increase in ventricular volumes. Thus, at least in high-functioning patients with PDD, brain enlargement may still be present in adult life.

Autism and head circumference in the first year of life

BACKGROUND

It has been reported that children with autism and pervasive developmental disorder have a significantly smaller head circumference at birth and that their head circumference then increases disproportionately rapidly in the first year of life.

METHODS

We attempted to replicate these findings using 15 narrowly defined autistic children from the National Collaborative Perinatal Project and approximately 40,000 nonautistic control subjects.

RESULTS

The autistic group had a slightly but not significantly larger head circumference at birth. At 4 months, the head circumference in the autistic group was not significantly larger than that of control subjects, but body weight and length were significantly larger in the autistic group.

CONCLUSIONS

We believe this is the first report of significant general body growth in autistic children in infancy; the larger head circumference may be part of this excessive general growth.

Less white matter concentration in autism: 2D voxel-based morphometry

Autism is a neurodevelopmental disorder affecting behavioral and social cognition, but there is little understanding about the link between the functional deficit and its underlying neuroanatomy. We applied a 2D version of voxel-based morphometry (VBM) in differentiating the white matter concentration of the corpus callosum for the group of 16 high functioning autistic and 12 normal subjects. Using the white matter density as an index for neural connectivity, autism is shown to exhibit less white matter concentration in the region of the genu, rostrum, and splenium removing the effect of age based on the general linear model (GLM) framework. Further, it is shown that the less white matter concentration in the corpus callosum in autism is due to hypoplasia rather than atrophy.

Behavioral and cellular consequences of increasing serotonergic activity during brain development: a role in autism?

The hypothesis explored in this review is that the high levels of serotonin in the blood seen in some autistic children (the so-called hyperserotonemia of autism) may lead to some of the behavioral and cellular changes also observed in the disorder. At early stages of development, when the blood-brain Barrier is not yet fully formed, the high levels of serotonin in the blood can enter the brain of a developing fetus and cause loss of serotonin terminals through a known negative feedback function of serotonin during development. The loss of serotonin innervation persists throughout subsequent development and the symptoms of autism appear. A review of the basic scientific literature on prenatal treatments affecting serotonin is given, in support of this hypothesis, with an emphasis on studies using the serotonin agonist, 5-methoxytryptamine (5-MT). In work using 5-MT to mimic hyperserotonemia, Sprague-Dawley rats are treated from gestational day 12 until postnatal 20. In published reports, these animals have been found to have a significant loss of serotonin terminals, decreased metabolic activity in cortex, changes in columnar development in cortex, changes in serotonin receptors, and "autistic-like" behaviors. In preliminary cellular findings given in this review, the animals have also been found to have cellular changes in two relevant brain regions: 1. Central nucleus of the amygdala, a brain region involved in fear-responding, where an increase in calcitonin gene related peptide (CGRP) was found 2. Paraventricular nucleus of the hypothalamus, a brain region involved in social memory and bonding, where a decrease in oxytocin was found. Both of these cellular changes could result from loss of serotonin innervation, possibly due to loss of terminal outgrowth from the same cells of the raphe nuclei. Thus, increased serotonergic activity during development could damage neurocircuitry involved in emotional responding to social stressors and may have relevance to the symptoms of autism.

Neuroimaging in disorders of social and emotional functioning: what is the question?

Social and emotional processing uses neural systems involving structures ranging from the brain stem to the associational cortex. Neuroimaging research has attempted to identify abnormalities in components of these systems that would underlie the behavioral abnormalities seen in disorders of social and emotional processing, notably autism spectrum disorders, the focus of this review. However, the findings have been variable. The most replicated anatomic finding (a tendency toward large brains) is not modular, and metabolic imaging and functional imaging (although showing substantial atypicality in activation) are not consistent regarding specific anatomic sites. Moreover, autism spectrum disorder demonstrates substantial heterogeneity on multiple levels. Here evidence is marshaled from a review of neuroimaging data to support the claim that abnormalities in social and emotional processing on the autism spectrum are a consequence of systems disruptions in which the behaviors are a final common pathway and the focal findings can be variable, downstream of other pathogenetic mechanisms, and downstream of more pervasive abnormalities.

Abnormal activity patterns in premotor cortex during sequence learning in autistic patients

BACKGROUND

Evidence for frontal abnormality in autism has accumulated in recent years. Our own studies have shown abnormal activation in prefrontal cortex during finger tapping and visuomotor coordination. Studies in healthy adults suggest reduced premotor and increased prefrontal activity during advanced learning stages. We examined hemodynamic changes during visuomotor learning in autistic patients.

METHODS

We studied eight high-functioning autistic patients and eight control subjects during learning of an 8-digit sequence over a period of 8 min, using functional magnetic resonance imaging.

RESULTS

Autistic patients showed overall less prefrontal activation during late visuomotor learning; however, the main finding was a complementary one of enhanced activation in right pericentral and premotor cortex. In the autism group, Brodmann areas 3, 4, and 6 of the right hemisphere became more involved during late learning stages (trials 25-48), compared with early stages (trials 1-24). This effect was not seen in the control group.

CONCLUSIONS

Our findings suggests that in autistic patients 1) primary sensorimotor and premotor cortex, which is normally predominant in early stages of visuomotor learning, plays an atypical role in later stages, even when learning is evident; and 2) handedness and side of execution interact with asymmetry of visuomotor learning activations, contrary to what is seen in normal adults.

Imaging data in autism: from structure to malfunction

During the last two decades, neuroimaging studies have improved our knowledge of brain development and contributed to our understanding of disorders involving the developing brain. Differences in cerebral anatomy have been determined in autism spectrum disorder (ASD). Morphological studies by magnetic resonance imaging have provided evidence of structural differences in ASD compared with the normal population. This has enhanced our view of autism as a neurobiological disorder corresponding with different stages and events in brain development. Alterations in volume of the total brain and specifically the cerebellum, frontal lobe, and limbic system have been identified. There appears to be a pattern of increased and then decreased rate of brain growth over time. We integrate these observations with neurobehavioral findings to provide a developmental hypothesis of the pathophysiology of autism.

The autistic brain: birth through adulthood

PURPOSE OF REVIEW

We discuss evidence of brain maldevelopment in the first years of life in autism and new neuroanatomical and functional evidence from later ages of development.

RECENT FINDINGS

Head circumference, an accurate indicator of brain size in children, was reported to jump from normal or below normal size in the first postnatal months in autistic infants to the 84 th percentile by about 1 year of age; this abnormally accelerated growth was concluded by 2 years of age. Infants with extreme head (and therefore brain) growth fell into the severe end of the clinical spectrum and had more extreme neuroanatomical abnormalities. In the frontal and temporal lobes in autism, there have been reports of abnormal increases in gray and white matter at 2 to 4 years; reduced metabolic measures; deviant diffusion tensor imaging results in white matter; underdeveloped cortical minicolumns; and reduced functional activation during socio-emotional, cognitive and attention tasks. Cerebellar abnormalities included abnormal volumes, reduced number and size of Purkinje neurons in the vermis and hemispheres, molecular defects, and reduced functional activation in posterior regions.

SUMMARY

A new neurobiological phenomenon in autism has been described that precedes the onset of clinical behavioral symptoms, and is brief and age-delimited to the first two years of life. The neurobiological defects that precede, trigger, and underlie it may form part of the developmental precursors of some of the anatomical, functional, and behavioral manifestations of autism. Future studies of the first years of life may help elucidate the factors and processes that bring about the unfolding of autistic behavior.

Depletion of MAP2 expression and laminar cytoarchitectonic changes in dorsolateral prefrontal cortex in adult autistic individuals

The neuropathological substrates underlying the characteristic clinical phenotype of autism are unknown. Neuroimaging studies have identified a decrease in task-related activation in the dorsolateral prefrontal cortex in autism. In the current study, we have analysed the dorsolateral prefrontal cortex in two adult individuals with a clinical diagnosis of autism, using Nissl staining and MAP2 immunohistochemistry. There was unchanged density of both neuronal and glial cell pools, although the autistic individuals had ill-defined neocortical cellular layers, substantially depleted MAP2 neuronal expression, and reduced dendrite numbers. Further studies on a larger number of individuals with autism are needed to establish the clinical relevance of the described changes, especially to determine whether the loss of dendritic markers is age associated or disease specific.

Autism as a disorder of neural information processing: directions for research and targets for therapy

The broad variation in phenotypes and severities within autism spectrum disorders suggests the involvement of multiple predisposing factors, interacting in complex ways with normal developmental courses and gradients. Identification of these factors, and the common developmental path into which they feed, is hampered by the large degrees of convergence from causal factors to altered brain development, and divergence from abnormal brain development into altered cognition and behaviour. Genetic, neurochemical, neuroimaging, and behavioural findings on autism, as well as studies of normal development and of genetic syndromes that share symptoms with autism, offer hypotheses as to the nature of causal factors and their possible effects on the structure and dynamics of neural systems. Such alterations in neural properties may in turn perturb activity-dependent development, giving rise to a complex behavioural syndrome many steps removed from the root causes. Animal models based on genetic, neurochemical, neurophysiological, and behavioural manipulations offer the possibility of exploring these developmental processes in detail, as do human studies addressing endophenotypes beyond the diagnosis itself.

Dynamic mapping of human cortical development during childhood through early adulthood

We report the dynamic anatomical sequence of human cortical gray matter development between the age of 4-21 years using quantitative four-dimensional maps and time-lapse sequences. Thirteen healthy children for whom anatomic brain MRI scans were obtained every 2 years, for 8-10 years, were studied. By using models of the cortical surface and sulcal landmarks and a statistical model for gray matter density, human cortical development could be visualized across the age range in a spatiotemporally detailed time-lapse sequence. The resulting time-lapse "movies" reveal that (i) higher-order association cortices mature only after lower-order somatosensory and visual cortices, the functions of which they integrate, are developed, and (ii) phylogenetically older brain areas mature earlier than newer ones. Direct comparison with normal cortical development may help understanding of some neurodevelopmental disorders such as childhood-onset schizophrenia or autism.

Functional anatomy of impaired selective attention and compensatory processing in autism

In autism, physiological indices of selective attention have been shown to be abnormal even in situations where behaviour is intact. This divergence between behaviour and physiology suggests the action of some compensatory process of attention, one which may hold clues to the aetiology of autism's characteristic cognitive phenotype. Six subjects with autism spectrum disorders and six normal control subjects were studied with functional magnetic resonance imaging while performing a bilateral visual spatial attention task. In normal subjects, the task evoked activation in a network of cortical regions including the superior parietal lobe (P<0.001), left middle temporal gyrus (P=0.002), left inferior (P<0.001) and middle (P<0.02) frontal gyri, and medial frontal gyrus (P<0.02). Autistic subjects, in contrast, showed activation in the bilateral ventral occipital cortex (P<0.03) and striate cortex (P<0.05). Within the task condition, a region-of-interest comparison of attend-left versus attend-right conditions indicated that modulation of activation in the autistic brain as a function of the lateral focus of spatial attention was abnormally decreased in the left ventral occipital cortex (P<0.03), abnormally increased in the left intraparietal sulcus (P<0.01), and abnormally variable in the superior parietal lobe (P<0.03). These results are discussed in terms of a model of autism in which a pervasive defect of neural and synaptic development produces over-connected neural systems prone to noise and crosstalk, resulting in hyper-arousal and reduced selectivity. These low-level attentional traits may be the developmental basis for higher-order cognitive styles such as weak central coherence.

Activation of methionine synthase by insulin-like growth factor-1 and dopamine: a target for neurodevelopmental toxins and thimerosal

Methylation events play a critical role in the ability of growth factors to promote normal development. Neurodevelopmental toxins, such as ethanol and heavy metals, interrupt growth factor signaling, raising the possibility that they might exert adverse effects on methylation. We found that insulin-like growth factor-1 (IGF-1)- and dopamine-stimulated methionine synthase (MS) activity and folate-dependent methylation of phospholipids in SH-SY5Y human neuroblastoma cells, via a PI3-kinase- and MAP-kinase-dependent mechanism. The stimulation of this pathway increased DNA methylation, while its inhibition increased methylation-sensitive gene expression. Ethanol potently interfered with IGF-1 activation of MS and blocked its effect on DNA methylation, whereas it did not inhibit the effects of dopamine. Metal ions potently affected IGF-1 and dopamine-stimulated MS activity, as well as folate-dependent phospholipid methylation: Cu(2+) promoted enzyme activity and methylation, while Cu(+), Pb(2+), Hg(2+) and Al(3+) were inhibitory. The ethylmercury-containing preservative thimerosal inhibited both IGF-1- and dopamine-stimulated methylation with an IC(50) of 1 nM and eliminated MS activity. Our findings outline a novel growth factor signaling pathway that regulates MS activity and thereby modulates methylation reactions, including DNA methylation. The potent inhibition of this pathway by ethanol, lead, mercury, aluminum and thimerosal suggests that it may be an important target of neurodevelopmental toxins.

Outcome classification of preschool children with autism spectrum disorders using MRI brain measures

OBJECTIVE

To test the hypothesis that a combination of magnetic resonance imaging (MRI) brain measures obtained during early childhood distinguish children with autism spectrum disorders (ASD) from typically developing children and is associated with functional outcome.

METHOD

Quantitative MRI technology was used to measure gray and white matter volumes (cerebrum and cerebellum), total brain volume, and the area of the cerebellar vermis in 52 boys with a provisional diagnosis of autism (aged 1.9-5.2 years) and 15 typically developing young children (aged 1.7-5.2 years). Diagnostic confirmation and cognitive outcome data were obtained after the children reached 5 years of age.

RESULTS

A discriminant function analysis of the MRI brain measures correctly classified 95.8% of the ASD cases and 92.3% of the control cases. This set of variables also correctly classified 85% of the ASD cases as lower functioning and 68% of the ASD cases as higher functioning.

CONCLUSIONS

These results indicate that variability in cerebellar and cerebral size is correlated with diagnostic and functional outcome in very young children with ASD.

A systems neuroscience approach to autism: biological, cognitive, and clinical perspectives

Autism is a behaviorally defined disorder characterized by a broad constellation of symptoms. Numerous studies directed to the biological substrate demonstrate clear effects of neurodevelopmental differences that will likely point to the etiology, course, and long-term outcomes of the disorder. Consistently replicated research on the neural underpinnings of autism is reviewed. In general, results suggest several main conclusions: First, autism is a heterogeneous disorder and is likely to have multiple possible etiologies; second, structural brain studies have indicated a variety of diffuse anatomical differences, reflective of an early developmental change in the growth or pruning of neural tissue, rather than localized lesions; similarly, neurochemical studies suggest early, neuromodulatory discrepancies rather than gross or localized abnormalities; and finally, there are a number of limitations on studies of brain activity that to date preclude definitive answers to questions of how the brain functions differently in autism. The large number of active research programs investigating the cognitive neuroscience of autism spectrum disorders, in combination with the exciting development of new methodologies and tools in this area, indicates the drama and excitement of work in this area.

Autism clinical trials: biological and medical issues in patient selection and treatment response

Biomedical measures are critical in the initial patient-screening and -selection phases of a clinical trial in autism and related disorders. These measures can also play an important role in the assessment and characterization of response and can provide an opportunity to study underlying etiologic and pathophysiologic processes. Thus, biomedical measures, including clinical laboratory analyses, metabolic screening, and chromosomal analysis, are used to screen for potential safety-related problems, to decrease biological and genetic heterogeneity, and to define subgroups. Neurobiological measures can be examined as possible predictors, modifiers or surrogates of therapeutic response, and adverse effects. Neurobiological research measures can also be used to study mechanisms and extent of drug action and to perform baseline and longitudinal investigations of possible pathophysiologic alterations. The potential utility and desirability of specific measures are considered and the general approach to choosing measures for incorporation is discussed.