Structural white matter deficits in high-functioning individuals with autistic spectrum disorder: a voxel-based investigation

Evidence for dysregulation of axonal growth and guidance in the etiology of ASD

Current theories concerning the cause of autism spectrum disorders (ASDs) have converged on the concept of abnormal development of brain connectivity. This concept is supported by accumulating evidence from functional imaging, diffusion tensor imaging, and high definition fiber tracking studies which suggest altered microstructure in the axonal tracts connecting cortical areas may underly many of the cognitive manifestations of ASD. Additionally, large-scale genomic studies implicate numerous gene candidates known or suspected to mediate neuritic outgrowth and axonal guidance in fetal and perinatal life. Neuropathological observations in postmortem ASD brain samples further support this model and include subtle disturbances of cortical lamination and subcortical axonal morphology. Of note is the relatively common finding of poor differentiation of the gray–white junction associated with an excess superficial white matter or “interstitial” neurons (INs). INs are thought to be remnants of the fetal subplate, a transient structure which plays a key role in the guidance and morphogenesis of thalamocortical and cortico-cortical connections and the organization of cortical columnar architecture. While not discounting the importance of synaptic dysfunction in the etiology of ASD, this paper will briefly review the cortical abnormalities and genetic evidence supporting a model of dysregulated axonal growth and guidance as key developmental processes underlying the clinical manifestations of ASD.

Brain anatomy of autism spectrum disorders I. Focus on corpus callosum

This brief review aims to examine the structural magnetic resonance imaging (sMRI) studies on corpus callosum in autism spectrum disorders (ASD) and discuss the clinical and demographic factors involved in the interpretation of results.

On the application of quantitative EEG for characterizing autistic brain: a systematic review

Autism-Spectrum Disorders (ASD) are thought to be associated with abnormalities in neural connectivity at both the global and local levels. Quantitative electroencephalography (QEEG) is a non-invasive technique that allows a highly precise measurement of brain function and connectivity. This review encompasses the key findings of QEEG application in subjects with ASD, in order to assess the relevance of this approach in characterizing brain function and clustering phenotypes. QEEG studies evaluating both the spontaneous brain activity and brain signals under controlled experimental stimuli were examined. Despite conflicting results, literature analysis suggests that QEEG features are sensitive to modification in neuronal regulation dysfunction which characterize autistic brain. QEEG may therefore help in detecting regions of altered brain function and connectivity abnormalities, in linking behavior with brain activity, and subgrouping affected individuals within the wide heterogeneity of ASD. The use of advanced techniques for the increase of the specificity and of spatial localization could allow finding distinctive patterns of QEEG abnormalities in ASD subjects, paving the way for the development of tailored intervention strategies.

Concordance of white matter and gray matter abnormalities in autism spectrum disorders: a voxel-based meta-analysis study

There are at least two fundamental unanswered questions in the literature on autism spectrum disorders (ASD): Are abnormalities in white (WM) and gray matter (GM) consistent with one another? Are WM morphometric alterations consistent with alterations in the GM of regions connected by these abnormal WM bundles and vice versa? The aim of this work is to bridge this gap. After selecting voxel-based morphometry and diffusion tensor imaging studies comparing autistic and normally developing groups of subjects, we conducted an activation likelihood estimation (ALE) meta-analysis to estimate consistent brain alterations in ASD. Multidimensional scaling was used to test the similarity of the results. The ALE results were then analyzed to identify the regions of concordance between GM and WM areas. We found statistically significant topological relationships between GM and WM abnormalities in ASD. The most numerous were negative concordances, found bilaterally but with a higher prevalence in the right hemisphere. Positive concordances were found in the left hemisphere. Discordances reflected the spatial distribution of negative concordances. Thus, a different hemispheric contribution emerged, possibly related to pathogenetic factors affecting the right hemisphere during early developmental stages. Besides, WM fiber tracts linking the brain structures involved in social cognition showed abnormalities, and most of them had a negative concordance with the connected GM regions. We interpreted the results in terms of altered brain networks and their role in the pervasive symptoms dramatically impairing communication and social skills in ASD patients.

Functional magnetic resonance imaging of autism spectrum disorders

This review presents an overview of functional magnetic resonance imaging findings in autism spectrum disorders (ASDS), although there is considerable heterogeneity with respect to results across studies, common themes have emerged, including: (i) hypoactivation in nodes of the "social brain" during social processing tasks, including regions within the prefrontal cortex, the posterior superior temporal sulcus, the amygdala, and the fusiform gyrus; (ii) aberrant frontostriatal activation during cognitive control tasks relevant to restricted and repetitive behaviors and interests, including regions within the dorsal prefrontal cortex and the basal ganglia; (iii) differential lateralization and activation of language processing and production regions during communication tasks; (iv) anomalous mesolimbic responses to social and nonsocial rewards; (v) task-based long-range functional hypoconnectivity and short-range hyper-connectivity; and (vi) decreased anterior-posterior functional connectivity during resting states. These findings provide mechanistic accounts of ASD pathophysiology and suggest directions for future research aimed at elucidating etiologic models and developing rationally derived and targeted treatments.

A two-year longitudinal MRI study of the corpus callosum in autism

A growing body of literature has identified size reductions of the corpus callosum (CC) in autism. However, to our knowledge, no published studies have reported on the growth of CC volumes in youth with autism. Volumes of the total CC and its sub-divisions were obtained from 23 male children with autism and 23 age- and gender-matched controls at baseline and 2-year follow-up. Persistent reductions in total CC volume were observed in participants with autism relative to controls. Only the rostral body subdivision showed a normalization of size over time. Persistent reductions are consistent with the diagnostic stability and life-long impairment observed in many individuals with autism. Multi-modal imaging studies are needed to identify specific fiber tracks contributing to CC reductions.

Corpus Callosum Area in Children and Adults with Autism

Despite repeated findings of abnormal corpus callosum structure in autism, the developmental trajectories of corpus callosum growth in the disorder have not yet been reported. In this study, we examined corpus callosum size from a developmental perspective across a 30-year age range in a large cross-sectional sample of individuals with autism compared to a typically developing sample. Midsagittal corpus callosum area and the 7 Witelson subregions were examined in 68 males with autism (mean age 14.1 years; range 3-36 years) and 47 males with typical development (mean age 15.3 years; range 4-29 years). Controlling for total brain volume, increased variability in total corpus callosum area was found in autism. In autism, increased midsagittal areas were associated with reduced severity of autism behaviors, higher intelligence, and faster speed of processing (p=0.003, p=0.011, p=0.013, respectively). A trend toward group differences in isthmus development was found (p=0.029, uncorrected). These results suggest that individuals with autism benefit functionally from increased corpus callosum area. Our cross-sectional examination also shows potential maturational abnormalities in autism, a finding that should be examined further with longitudinal datasets.

Differences in white matter reflect atypical developmental trajectory in autism: A Tract-based Spatial Statistics study

Autism is a neurodevelopmental disorder in which white matter (WM) maturation is affected. We assessed WM integrity in 16 adolescents and 14 adults with high-functioning autism spectrum disorder (ASD) and in matched neurotypical controls (NT) using diffusion weighted imaging and Tract-based Spatial Statistics. Decreased fractional anisotropy (FA) was observed in adolescents with ASD in tracts involved in emotional face processing, language, and executive functioning, including the inferior fronto-occipital fasciculus and the inferior and superior longitudinal fasciculi. Remarkably, no differences in FA were observed between ASD and NT adults.

We evaluated the effect of age on WM development across the entire age range. Positive correlations between FA values and age were observed in the right inferior fronto-occipital fasciculus, the left superior longitudinal fasciculus, the corpus callosum, and the cortical spinal tract of ASD participants, but not in NT participants.

Our data underscore the dynamic nature of brain development in ASD, showing the presence of an atypical process of WM maturation, that appears to normalize over time and could be at the basis of behavioral improvements often observed in high-functioning autism.

Changes in the sulcal size associated with autism spectrum disorder revealed by sulcal morphometry

Autism spectrum disorder (ASD) is a complex, neurodevelopmental disorder with various structural abnormalities for different patient groups. Because of the heterogeneity of the disorder, several biomarkers have been suggested so far. Here, we explore the potential of sulcal surface and length as biomarkers. Three-dimensional T1-weighted images of 15 adolescents of normal intelligence with ASD and 15 age-, sex-, and intelligence quotient-matched control adolescents were analysed using Brainvisa 4.0 (http://www.brainvisa.info), which automatically extracts the cortical folds and labels them as 59 sulcal pieces. For each sulcus, the surface, length, and mean geodesic depth were computed using morphometry analysis within this software package. General linear model was conducted to compare the estimated values for the two groups, ASD and control. In the ASD group, the left insula and the right intraparietal sulcus (IPS) had significantly higher values for surface and length, respectively. Nonetheless for all sulcal pieces, the mean geodesic depth was not significantly different between the two groups. Our results suggest that sulcal surface and length can have correlation with morphological changes of cortex in ASD. Greater surface area and length in insula and IPS, respectively, may reflect greater folding. This could result in greater separation of functions with an impact upon the integrative functions of these regions.

Autism as a neural systems disorder: a theory of frontal-posterior underconnectivity

The underconnectivity theory of autism attributes the disorder to lower anatomical and functional systems connectivity between frontal and more posterior cortical processing. Here we review evidence for the theory and present a computational model of an executive functioning task (Tower of London) implementing the assumptions of underconnectivity. We make two modifications to a previous computational account of performance and brain activity in typical individuals in the Tower of London task (Newman et al., 2003): (1) the communication bandwidth between frontal and parietal areas was decreased and (2) the posterior centers were endowed with more executive capability (i.e., more autonomy, an adaptation is proposed to arise in response to the lowered frontal-posterior bandwidth). The autism model succeeds in matching the lower frontal-posterior functional connectivity (lower synchronization of activation) seen in fMRI data, as well as providing insight into behavioral response time results. The theory provides a unified account of how a neural dysfunction can produce a neural systems disorder and a psychological disorder with the widespread and diverse symptoms of autism.

Abnormal white matter integrity in young children with autism

This study investigated white matter integrity in young children with autism using diffusion tensor imaging (DTI). Twenty-two children with autism, mean age 3:2 years, and 32 controls, mean age 3:4 years, participated in the study. Tract-based spatial statistics (TBSS) revealed white matter abnormalities in several distinct clusters within the genu and body of the corpus callosum (CC), left superior longitudinal fasciculus (SLF) and right and left cingulum (Cg). TBSS-VOIs analysis was performed in the clusters where differences in fractional anisotropy (FA) were detected to investigate the relationship between changes in FA and diffusivity indices. In all VOIs, increase in FA was caused by a decrease in radial diffusivity (Dr), while no changes in axial diffusivity (Da) or mean diffusivity (MD) were observed. Tractography analysis was applied to further study the CC, SLF, and Cg. Witelson parcellation scheme was used for the CC. Significant increase in FA was seen in children with autism in the mid-body of the CC as well as in the left Cg. It is suggested that such abnormal white matter integrity in young children with autism may adversely affect connectivity between different brain regions and may be linked to some of the behavioral impairments apparent in autism.

Detecting abnormalities of corpus callosum connectivity in autism using magnetic resonance imaging and diffusion tensor tractography

The corpus callosum (CC) has emerged as one of the primary targets of autism research. To detect aberrant CC interhemispheric connectivity in autism, we performed T1-weighted magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI)-based tractography in 18 children with high functioning autism (HFA) and 16 well-matched typically developing (TD) children. We compared global and regional T1 measures (CC volume, and CC density), and the DTI measures [fractional anisotropy (FA), apparent diffusion coefficient (ADC), average fiber length (AFL), and fiber number (FN)] of transcallosal fibers, between the two groups. We also evaluated the relationships between scores on the Childhood Autism Rating Scale (CARS) and CC T1 or DTI measurements. Significantly less white matter density in the anterior third of the CC, and higher ADC and lower FN values of the anterior third transcallosal fiber tracts were found in HFA patients compared to TD children. These results suggested that the anterior third CC density and transcallosal fiber connectivity were affected in HFA children.

Regional differences in grey and white matter in children and adults with autism spectrum disorders: an activation likelihood estimate (ALE) meta-analysis

Structural alterations in brain morphology have been inconsistently reported in children compared to adults with autism spectrum disorder (ASD). We assessed these differences by performing meta-analysis on the data from 19 voxel-based morphometry studies. Common findings across the age groups were grey matter reduction in left putamen and medial prefrontal cortex (mPFC) and grey matter increases in the lateral PFC, while white matter decreases were seen mainly in the children in frontostriatal pathways. In the ASD sample, children/adolescents were more likely than adults to have increased grey matter in bilateral fusiform gyrus, right cingulate and insula. Results show that clear maturational differences exist in social cognition and limbic processing regions only in children/adolescents and not in adults with ASD, and may underlie the emotional regulation that improves with age in this population.

Disrupted cortical connectivity theory as an explanatory model for autism spectrum disorders

Recent findings of neurological functioning in autism spectrum disorder (ASD) point to altered brain connectivity as a key feature of its pathophysiology. The cortical underconnectivity theory of ASD (Just et al., 2004) provides an integrated framework for addressing these new findings. This theory suggests that weaker functional connections among brain areas in those with ASD hamper their ability to accomplish complex cognitive and social tasks successfully. We will discuss this theory, but will modify the term underconnectivity to 'disrupted cortical connectivity' to capture patterns of both under- and over-connectivity in the brain. In this paper, we will review the existing literature on ASD to marshal supporting evidence for hypotheses formulated on the disrupted cortical connectivity theory. These hypotheses are: 1) underconnectivity in ASD is manifested mainly in long-distance cortical as well as subcortical connections rather than in short-distance cortical connections; 2) underconnectivity in ASD is manifested only in complex cognitive and social functions and not in low-level sensory and perceptual tasks; 3) functional underconnectivity in ASD may be the result of underlying anatomical abnormalities, such as problems in the integrity of white matter; 4) the ASD brain adapts to underconnectivity through compensatory strategies such as overconnectivity mainly in frontal and in posterior brain areas. This may be manifested as deficits in tasks that require frontal-parietal integration. While overconnectivity can be tested by examining the cortical minicolumn organization, long-distance underconnectivity can be tested by cognitively demanding tasks; and 5) functional underconnectivity in brain areas in ASD will be seen not only during complex tasks but also during task-free resting states. We will also discuss some empirical predictions that can be tested in future studies, such as: 1) how disrupted connectivity relates to cognitive impairments in skills such as Theory-of-Mind, cognitive flexibility, and information processing; and 2) how connection abnormalities relate to, and may determine, behavioral symptoms hallmarked by the triad of Impairments in ASD. Furthermore, we will relate the disrupted cortical connectivity model to existing cognitive and neural models of ASD.

Involvement of the anterior thalamic radiation in boys with high functioning autism spectrum disorders: a Diffusion Tensor Imaging study

BACKGROUND

Autism has been hypothesized to reflect neuronal disconnection. Several recent reports implicate the key thalamic relay nuclei and cortico-thalamic connectivity in the pathophysiology of autism. Accordingly, we aimed to focus on evaluating the integrity of the thalamic radiation and sought to replicate prior white matter findings in Korean boys with high-functioning autism spectrum disorders (ASD) using Diffusion Tensor Imaging (DTI).

METHODS

We compared fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD) and radial diffusivity (RD) in 17 boys with ASD and 17 typically developing controls in the anterior thalamic radiation (ATR), superior thalamic radiation (STR), posterior thalamic radiation (PTR), corpus callosum (CC), uncinate fasciculus (UF) and inferior longitudinal fasciculus (ILF).

RESULTS

The two groups were group-matched on age, IQ, handedness and head circumference. In whole-brain voxel-wise analyses, FA was significantly reduced and MD was significantly increased in the right ATR, CC, and left UF in subjects with ASD (p<0.05, corrected). We found significantly lower FA in right and left ATR, CC, left UF and right and left ILF and significantly higher MD values of the CC in the ASD group in region of interest-based analyses. We also observed significantly higher RD values of right and left ATR, CC, left UF, left ILF in subjects with ASD compared to typically developing boys and significantly lower AD values of both ILF. Right ATR and right UF FA was significantly negatively correlated with total SRS score within the ASD group (r=-.56, p=.02).

CONCLUSIONS

Our preliminary findings support evidence implicating disturbances in the thalamo-frontal connections in autism. These findings highlight the role of hypoconnectivity between the frontal cortex and thalamus in ASD.

Biological motion processing as a hallmark of social cognition

Visual processing of biological motion (BM) produced by living organisms is of immense value for successful daily-life activities and, in particular, for adaptive social behavior and nonverbal communication. Investigation of BM perception in neurodevelopmental disorders related to autism, preterm birth, and genetic conditions substantially contributes to our understanding of the neural mechanisms underpinning the extraordinary tuning to BM. The most prominent research outcome is that patients with daily-life deficits in social cognition are also compromised on visual body motion processing. This raises the question of whether performance on body motion perception tasks may serve a hallmark of social cognition. Overall, the findings highlight the role of structural and functional brain connectivity for proper functioning of the neural circuitry involved in BM processing and visual social cognition that share topographically and dynamically overlapping neural networks.

Voxel-based meta-analysis of regional white-matter volume differences in autism spectrum disorder versus healthy controls

BACKGROUND

We conducted a meta-analysis of voxel-based morphometry (VBM) studies in autism spectrum disorder (ASD) to clarify the changes in regional white-matter volume underpinning this condition, and generated an online database to facilitate replication and further analyses by other researchers.

METHOD

PubMed, ScienceDirect, Web of Knowledge and Scopus databases were searched between 2002 (the date of the first white-matter VBM study in ASD) and 2010. Manual searches were also conducted. Authors were contacted to obtain additional data. Coordinates were extracted from clusters of significant white-matter difference between patients and controls. A new template for white matter was created for the signed differential mapping (SDM) meta-analytic method. A diffusion tensor imaging (DTI)-derived atlas was used to optimally localize the changes in white-matter volume.

RESULTS

Thirteen datasets comprising 246 patients with ASD and 237 healthy controls met inclusion criteria. No between-group differences were found in global white-matter volumes. ASD patients showed increases of white-matter volume in the right arcuate fasciculus and also in the left inferior fronto-occipital and uncinate fasciculi. These findings remained unchanged in quartile and jackknife sensitivity analyses and also in subgroup analyses (pediatric versus adult samples).

CONCLUSIONS

Patients with ASD display increases of white-matter volume in tracts known to be important for language and social cognition. Whether the results apply to individuals with lower IQ or younger age and whether there are meaningful neurobiological differences between the subtypes of ASD remain to be investigated.

Strength of lateralisation for processing facial emotion in relation to autistic traits in individuals without autism

A great number of studies have shown that non-clinical individuals rely predominantly on the right hemisphere to process facial emotion. Previous studies have shown that males suffering from Asperger's syndrome show a typical right hemisphere bias for processing facial emotion (happiness and sadness) but a reduced right hemisphere bias for processing facial identity. This study looks at the lateralisation of all six basic emotions using the chimeric faces test in 64 non-clinical participants (32 males, 32 females) and correlates it with their autistic traits measured using the Broad Autistic Phenotype Questionnaire. For males only, regression analyses showed a relationship between the aloof personality trait and lateralisation for fear, happiness, and surprise. Males with high autistic scores on the aloof personality subscale (showing a lack of interest in social interaction) were more strongly lateralised to the right hemisphere for processing fear, happiness, and surprise. For males there was no relationship with anger, disgust, sadness, or non-facial stimuli, and for females there were no significant relationships at all. The autistic traits of rigidity and pragmatic language were not significant predictors of emotion lateralisation. The over-reliance on the right hemisphere for processing facial emotion in males seems to support the idea that the autistic brain could be seen as hyper-masculinised, possibly due to prenatal testosterone exposure.

Brain structure anomalies in autism spectrum disorder--a meta-analysis of VBM studies using anatomic likelihood estimation

Autism spectrum disorders (ASD) are pervasive developmental disorders with characteristic core symptoms such as impairments in social interaction, deviance in communication, repetitive and stereotyped behavior, and impaired motor skills. Anomalies of brain structure have repeatedly been hypothesized to play a major role in the etiopathogenesis of the disorder. Our objective was to perform unbiased meta-analysis on brain structure changes as reported in the current ASD literature. We thus conducted a comprehensive search for morphometric studies by Pubmed query and literature review. We used a revised version of the activation likelihood estimation (ALE) approach for coordinate-based meta-analysis of neuroimaging results. Probabilistic cytoarchitectonic maps were applied to compare the localization of the obtained significant effects to histological areas. Each of the significant ALE clusters was analyzed separately for age effects on gray and white matter density changes. We found six significant clusters of convergence indicating disturbances in the brain structure of ASD patients, including the lateral occipital lobe, the pericentral region, the medial temporal lobe, the basal ganglia, and proximate to the right parietal operculum. Our study provides the first quantitative summary of brain structure changes reported in literature on autism spectrum disorders. In contrast to the rather small sample sizes of the original studies, our meta-analysis encompasses data of 277 ASD patients and 303 healthy controls. This unbiased summary provided evidence for consistent structural abnormalities in spite of heterogeneous diagnostic criteria and voxel-based morphometry (VBM) methodology, but also hinted at a dependency of VBM findings on the age of the patients.

Structural MRI in autism spectrum disorder

Magnetic resonance (MR) examination provides a powerful tool for investigating brain structural changes in children with autism spectrum disorder (ASD). We review recent advances in the understanding of structural MR correlates of ASD. We summarize findings from studies based on voxel-based morphometry, surface-based morphometry, tensor-based morphometry, and diffusion-tensor imaging. Finally, we discuss diagnostic models of ASD based on MR-derived features.

Developmental malformation of the corpus callosum: a review of typical callosal development and examples of developmental disorders with callosal involvement

This review provides an overview of the involvement of the corpus callosum (CC) in a variety of developmental disorders that are currently defined exclusively by genetics, developmental insult, and/or behavior. I begin with a general review of CC development, connectivity, and function, followed by discussion of the research methods typically utilized to study the callosum. The bulk of the review concentrates on specific developmental disorders, beginning with agenesis of the corpus callosum (AgCC)-the only condition diagnosed exclusively by callosal anatomy. This is followed by a review of several genetic disorders that commonly result in social impairments and/or psychopathology similar to AgCC (neurofibromatosis-1, Turner syndrome, 22q11.2 deletion syndrome, Williams yndrome, and fragile X) and two forms of prenatal injury (premature birth, fetal alcohol syndrome) known to impact callosal development. Finally, I examine callosal involvement in several common developmental disorders defined exclusively by behavioral patterns (developmental language delay, dyslexia, attention-deficit hyperactive disorder, autism spectrum disorders, and Tourette syndrome).

White matter integrity in Asperger syndrome: a preliminary diffusion tensor magnetic resonance imaging study in adults

BACKGROUND

Autistic Spectrum Disorder (ASD), including Asperger syndrome and autism, is a highly genetic neurodevelopmental disorder. There is a consensus that ASD has a biological basis, and it has been proposed that it is a "connectivity" disorder. Diffusion Tensor Magnetic Resonance Imaging (DT-MRI) allows measurement of the microstructural integrity of white matter (a proxy measure of "connectivity"). However, nobody has investigated the microstructural integrity of whole brain white matter in people with Asperger syndrome.

METHODS

We measured the fractional anisotropy (FA), mean diffusivity (MD) and radial diffusivity (RD) of white matter, using DT-MRI, in 13 adults with Asperger syndrome and 13 controls. The groups did not differ significantly in overall intelligence and age. FA, MD and RD were assessed using whole brain voxel-based techniques.

RESULTS

Adults with Asperger syndrome had a significantly lower FA than controls in 13 clusters. These were largely bilateral and included white matter in the internal capsule, frontal, temporal, parietal and occipital lobes, cingulum and corpus callosum.

CONCLUSIONS

Adults with Asperger syndrome have widespread significant differences from controls in white matter microstructural integrity.

Pervasive microstructural abnormalities in autism: a DTI study

BACKGROUND

Recent studies have reported abnormal functional connectivity patterns in the brains of people with autism that may be accompanied by decreases in white matter integrity. Since autism is a developmental disorder, we aim to investigate the nature and location of decreases in white and grey matter integrity in an adolescent sample while accounting for age.

METHODS

We used structural (T1) imaging to study brain volumetrics and diffusion tensor imaging (DTI) to investigate white and grey matter integrity in people with autism. We obtained magnetic resonance images for adolescents aged 12-18 years with high-functioning autism and from matched controls. Fractional anisotropy and mean diffusivity, as well as grey and white matter volumetrics were analyzed.

RESULTS

There were 17 participants with autism and 25 matched controls included in this study. Participants with autism had lower fractional anisotropy in the left and right superior and inferior longitudinal fasciculus, but this effect was not significant after adjusting for age and intelligence quotient (IQ). The kurtosis of the white matter fractional anisotropy probability distribution was higher in this participant group, with and without adjustment for age and IQ. Most notably, however, the mean diffusivity levels were markedly increased in the autism group throughout the brain, and the mean diffusivity probability distributions of both grey and white matter were shifted toward a higher value, particularly with age and IQ adjustment. No volumetric differences in grey and white matter were found.

LIMITATIONS

We corrected for age and IQ using a linear model. The study was also limited by its sample size, investigated age range and cross-sectional design.

CONCLUSION

The findings suggest that autism is characterized by a generalized reduction of white matter integrity that is associated with an increase of interstitial space. The generalized manifestation of the white matter abnormalities provides an important new perspective on autism as a connectivity disorder.

Decreased interhemispheric functional connectivity in autism

The cortical underconnectivity theory asserts that reduced long-range functional connectivity might contribute to a neural mechanism for autism. We examined resting-state blood oxygen level-dependent interhemispheric correlation in 53 males with high-functioning autism and 39 typically developing males from late childhood through early adulthood. By constructing spatial maps of correlation between homologous voxels in each hemisphere, we found significantly reduced interhemispheric correlation specific to regions with functional relevance to autism: sensorimotor cortex, anterior insula, fusiform gyrus, superior temporal gyrus, and superior parietal lobule. Observed interhemispheric connectivity differences were better explained by diagnosis of autism than by potentially confounding neuropsychological metrics of language, IQ, or handedness. Although both corpus callosal volume and gray matter interhemispheric connectivity were significantly reduced in autism, no direct relationship was observed between them, suggesting that structural and functional metrics measure different aspects of interhemispheric connectivity. In the control but not the autism sample, there was decreasing interhemispheric correlation with subject age. Greater differences in interhemispheric correlation were seen for more lateral regions in the brain. These findings suggest that long-range connectivity abnormalities in autism are spatially heterogeneous and that transcallosal connectivity is decreased most in regions with functions associated with behavioral abnormalities in autism. Autism subjects continue to show developmental differences in interhemispheric connectivity into early adulthood.

Neurobehavioral abnormalities in first-degree relatives of individuals with autism

CONTEXT

Studying sensorimotor and neurocognitive impairments in unaffected family members of individuals with autism may help identify familial pathophysiological mechanisms associated with the disorder.

OBJECTIVE

To determine whether atypical sensorimotor or neurocognitive characteristics associated with autism are present in first-degree relatives of individuals with autism.

DESIGN

Case-control comparison of neurobehavioral functions.

SETTING

University medical center.

PARTICIPANTS

Fifty-seven first-degree relatives of individuals with autism and 40 age-, sex-, and IQ-matched healthy control participants (aged 8-54 years).

MAIN OUTCOME MEASURES

Oculomotor tests of sensorimotor responses (saccades and smooth pursuit); procedural learning and response inhibition; neuropsychological tests of motor, memory, and executive functions; and psychological measures of social behavior, communication skills, and obsessive-compulsive behaviors.

RESULTS

On eye movement testing, family members demonstrated saccadic hypometria, reduced steady-state pursuit gain, and a higher rate of voluntary response inhibition errors relative to controls. They also showed lateralized deficits in procedural learning and open-loop pursuit gain (initial 100 milliseconds of pursuit) and increased variability in the accuracy of large-amplitude saccades that were confined to rightward movements. In neuropsychological studies, only executive functions were impaired relative to those of controls. Family members reported more communication abnormalities and obsessive-compulsive behaviors than controls. Deficits across oculomotor, neuropsychological, and psychological domains were relatively independent from one another.

CONCLUSIONS

Family members of individuals with autism demonstrate oculomotor abnormalities implicating pontocerebellar and frontostriatal circuits and left-lateralized alterations of frontotemporal circuitry and striatum. The left-lateralized alterations have not been identified in other neuropsychiatric disorders and are of interest given atypical brain lateralization and language development associated with the disorder. Similar oculomotor deficits have been reported in individuals with autism, suggesting that they may be familial and useful for studies of neurophysiological and genetic mechanisms in autism.

Volumetric and voxel-based morphometry findings in autism subjects with and without macrocephaly

This study sought to replicate Herbert et al. (2003a), which found increased overall white matter (WM) volume in subjects with autism, even after controlling for head size differences. To avoid the possibility that greater WM volume in autism is merely an epiphenomena of macrocephaly overrepresentation associated with the disorder, the current study included control subjects with benign macrocephaly. The control group also included subjects with a reading disability to insure cognitive heterogeneity. WM volume in autism was significantly larger, even when controlling for brain volume, rate of macrocephaly, and other demographic variables. Autism and controls differed little on whole-brain WM voxel-based morphometry (VBM) analyses suggesting that the overall increase in WM volume was non-localized. Autism subjects exhibited a differential pattern of IQ relationships with brain volumetry findings from controls. Current theories of brain overgrowth and their importance in the development of autism are discussed in the context of these findings.

The anatomy of the callosal and visual-association pathways in high-functioning autism: a DTI tractography study

There is increasing recognition that many of the core behavioral impairments that characterize autism potentially emerge from poor neural synchronization across nodes comprising dispersed cortical networks. A likely candidate for the source of this atypical functional connectivity in autism is an alteration in the structural integrity of intra- and inter-hemispheric white matter (WM) tracts that form large-scale cortical networks. To test this hypothesis, in a group of adults with high-functioning autism (HFA) and matched control participants, we used diffusion tensor tractography to compare the structural integrity of three intra-hemispheric visual-association WM tracts, the inferior longitudinal fasciculus (ILF), the inferior fronto-occipito fasciculus (IFOF) and the uncinate fasciculus (UF), with the integrity of three sub-portions of the major inter-hemispheric fiber tract, the corpus callosum. Compared with the control group, the HFA group evinced an increase in the volume of the intra-hemispheric fibers, particularly in the left hemisphere, and a reduction in the volume of the forceps minor (F-Mi) and body of the corpus callosum. The reduction in the volume of the F-Mi also correlated with an increase in repetitive and stereotypical behavior as measured by the Autism Diagnostic Interview. These findings suggest that the abnormalities in the integrity of key inter- and intra-hemispheric WM tracts may underlie the atypical information processing observed in these individuals.

Neuroimaging of the functional and structural networks underlying visuospatial vs. linguistic reasoning in high-functioning autism

High-functioning individuals with autism have been found to favor visuospatial processing in the face of typically poor language abilities. We aimed to examine the neurobiological basis of this difference using functional magnetic resonance imaging and diffusion tensor imaging. We compared 12 children with high functioning autism (HFA) to 12 age- and IQ-matched typically developing controls (CTRL) on a pictorial reasoning paradigm under three conditions: V, requiring visuospatial processing; S, requiring language (i.e., semantic) processing; and V+S, a hybrid condition in which language use could facilitate visuospatial transformations. Activated areas in the brain were chosen as endpoints for probabilistic diffusion tractography to examine tract integrity (FA) within the structural network underlying the activation patterns. The two groups showed similar networks, with linguistic processing activating inferior frontal, superior and middle temporal, ventral visual, and temporo-parietal areas, whereas visuospatial processing activated occipital and inferior parietal cortices. However, HFA appeared to activate occipito-parietal and ventral temporal areas, whereas CTRL relied more on frontal and temporal language regions. The increased reliance on visuospatial abilities in HFA was supported by intact connections between the inferior parietal and the ventral temporal ROIs. In contrast, the inferior frontal region showed reduced connectivity to ventral temporal and middle temporal areas in this group, reflecting impaired activation of frontal language areas in autism. The HFA group's engagement of posterior brain regions along with its weak connections to frontal language areas suggest support for a reliance on visual mediation in autism, even in tasks of higher cognition.

Autism and anorexia nervosa: Two facets of the same disease?

We compiled data included in the Primal Health Research Database (www.primalhealthresearch.com) to test the hypothesis that when two pathological conditions or personality traits share the same critical period for gene-environment interaction, we should expect further similarities, particularly from clinical and pathophysiological perspectives. The keywords 'autism' and 'anorexia nervosa' (but not bulimia nervosa) lead to studies suggesting that for both conditions the perinatal period is critical. We take this example to look at other possible links between these pathological entities. From a clinical perspective, several teams have independently emphasized the importance of autistic traits in anorexia nervosa. Deficits in the processing of oxytocin have been demonstrated in both cases. Autistic groups have significantly lower blood oxytocin levels than normal groups, and oxytocin levels increase with age in the normal group only. In autistic groups there is a high ratio of intermediates of oxytocin synthesis (OX-T) to the nonapeptide oxytocin (OT). On the other hand, it has been reported that the level of oxytocin in the cerebrospinal fluid of anorexic women is significantly lower than the level of oxytocin in bulimic and control subjects. Scanning data reveal similar asymmetric functions with left hemisphere preponderance in autistic spectrum disorders and anorexia. A comparative study of the mirror neurons systems is another promising avenue for research. Such an accumulation of similarities from a great diversity of perspectives suggests that anorexia nervosa might be considered a female variant of the autistic spectrum. A plausible interpretation is that prenatal exposure to male hormones might protect against the expression of this disease: girls who have a twin brother are at low risk for anorexia nervosa, compared with girls who have a twin sister, and with controls; furthermore genetic linkage analyses do not detect change on the X chromosome. From an overview of the database, the perinatal period appears to be critical for all disorders related to the capacity to love (including love of oneself), to the potential for aggression (including self-destructive behaviours), or to sociability. Is the perinatal period critical for the organisation of the oxytocin system? This is an important question at a time when we learn that the widely used synthetic oxytocin can probably diffuse across the placenta. On the other hand, where the genesis of metabolic types is concerned, it is prenatal life that appears to be critical.

Interhemispheric asymmetry in EEG photic driving coherence in childhood autism

OBJECTIVE

Examination of the EEG photic driving coherence during intermittent photic stimulation in autistic patients with relatively intact verbal and intellectual functions in order to enhance the likely latent interhemispheric asymmetry in neural connectivity.

METHODS

Fourteen autistic boys, aged 6-14years, free of drug treatment, with I.Q. 91.4+/-22.8, and 19 normally developing boys were subject to stimulation of 12 fixed frequencies of 3-27Hz. The number of high coherent connections (HCC) (coherence >0.6-0.8) was estimated among 7 leads in each hemisphere.

RESULTS

In contrast to the spectral characteristics showing the right hemisphere deficit in the photic driving reactivity, the number of HCC differentiated the groups only in the left hemisphere where it was higher in autistics at the EEG frequencies corresponding to those of stimulation at 6-27Hz without asymmetry at other frequencies, the left-side prevalence increasing with frequency. No asymmetry was observed in the resting state.

CONCLUSIONS

Spectral and coherence characteristics of the EEG photic driving show different aspects of latent abnormal interhemispheric asymmetry in autistics: the right hemisphere "hyporeactivity" and potential "hyperconectivity" of likely compensatory nature in the left hemisphere.

SIGNIFICANCE

The EEG photic driving can reveal functional topographic alterations not present in the spontaneous EEG.

A meta-analysis of the corpus callosum in autism

BACKGROUND

Previous magnetic resonance imaging (MRI) studies have reported reductions in corpus callosum (CC) total area and CC regions in individuals with autism. However, studies have differed concerning the magnitude and/or region contributing to CC reductions. The present study determined the significance and magnitude of reductions in CC total and regional area measures in autism.

METHOD

PubMed and PsycINFO databases were searched to identify MRI studies examining corpus callosum area in autism. Ten studies contributed data from 253 patients with autism (mean age = 14.58, SD = 6.00) and 250 healthy control subjects (mean age = 14.47, SD = 5.31). Of these 10 studies, 8 reported area measurements for corpus callosum regions (anterior, mid/body, and posterior), and 6 reported area for Witelson subdivisions. Meta-analytic procedures were used to quantify differences in total and region CC area measurements.

RESULTS

Total CC area was reduced in autism and the magnitude of the reduction was medium (weighted mean d = .48, 95% confidence interval [CI] = .30-.66). All regions showed reductions in size with the magnitude of the effect decreasing caudally (anterior d = .49, mid/body d = .43, posterior d = .37). Witelson subdivision 3 (rostral body) showed the largest effect, indicating greatest reduction in the region containing premotor/supplementary motor neurons.

CONCLUSIONS

Corpus callosum reductions are present in autism and support the aberrant connectivity hypothesis. Future diffusion tensor imaging studies examining specific fiber tracts connecting the hemispheres are needed to identify the cortical regions most affected by CC reductions.

Differential effects on white-matter systems in high-functioning autism and Asperger's syndrome

BACKGROUND

Whether autism spectrum maps onto a spectrum of brain abnormalities and whether Asperger's syndrome (ASP) is distinct from high-functioning autism (HFA) are debated. White-matter maldevelopment is associated with autism and disconnectivity theories of autism are compelling. However, it is unknown whether children with ASP and HFA have distinct white-matter abnormalities.

METHOD

Voxel-based morphometry mapped white-matter volumes across the whole brain in 91 children. Thirty-six had autism spectrum disorder. A history of delay in phrase speech defined half with HFA; those without delay formed the ASP group. The rest were typically developing children, balanced for age, IQ, gender, maternal language and ethnicity. White-matter volumes in HFA and ASP were compared and each contrasted with controls.

RESULTS

White-matter volumes around the basal ganglia were higher in the HFA group than ASP and higher in both autism groups than controls. Compared with controls, children with HFA had less frontal and corpus callosal white matter in the left hemisphere; those with ASP had less frontal and corpus callosal white matter in the right hemisphere with more white matter in the left parietal lobe.

CONCLUSIONS

HFA involved mainly left hemisphere white-matter systems; ASP affected predominantly right hemisphere white-matter systems. The impact of HFA on basal ganglia white matter was greater than ASP. This implies that aetiological factors and management options for autism spectrum disorders may be distinct. History of language acquisition is a potentially valuable marker to refine our search for causes and treatments in autism spectrum.

Total brain volume and corpus callosum size in medication-naïve adolescents and young adults with autism spectrum disorder

BACKGROUND

Increased total brain volume (TBV) has been reported for children with autism spectrum disorder (ASD) but studies in older ASD subjects have been contradictory. Similarly, studies of corpus callosum (CC) area in ASD differ with regard to inclusion criteria, age, and IQ.

METHODS

In the present study, TBV, gray matter (GM), and white matter (WM) volume as well as midsagittal CC area were compared between 15 medication-naïve, high-functioning adolescent and young adult ASD subjects and 15 healthy control individuals, and correlations with visuomotor coordination and imitation abilities were explored. In addition, computational surface-based methods were implemented to encode callosal thickness at high spatial resolution.

RESULTS

Total brain volume, GM, and WM were increased and CC area was decreased in ASD subjects, a finding that was predominantly due to ASD subjects with lower IQ. Positive correlations of IQ with volume measures were observed only in control subjects. Autism spectrum disorder subjects showed reduced thickness in the posterior part of the CC. White matter volume showed a trend for negative correlation with dynamic balance and imitation abilities across groups.

CONCLUSIONS

This study replicates previous structural magnetic resonance imaging (MRI) findings in ASD, emphasizes the role of IQ differences, and adds some evidence for functional implications of structural findings.

Postnatal lesion evidence against a primary role for the corpus callosum in mouse sociability

The BTBR T+tf/J (BTBR) strain is an inbred strain of mice that displays prominent social deficits and repetitive behaviors analogous to the defining symptoms of autism, along with complete congenital agenesis of the corpus callosum (CC). The BTBR strain is genetically distant from the widely used C57BL/6J (B6) strain, which exhibits high levels of sociability, a low level of repetitive behaviors, and an intact CC. Emerging evidence implicates compromised interhemispherical connectivity in some cases of autism. We investigated the hypothesis that the disconnection of CC fiber tracts contributes to behavioral traits in mice that are relevant to the behavioral symptoms of autism. Surgical lesion of the CC in B6 mice at postnatal day 7 had no effect on juvenile play and adult social approaches, and did not elevate repetitive self-grooming. In addition, LP/J, the strain that is genetically closest to the BTBR strain but has an intact CC, displayed juvenile play deficits and repetitive self-grooming similar to those seen in BTBR mice. These corroborative results offer evidence against the hypothesis that the CC disconnection is a primary cause of low sociability and a high level of repetitive behaviors in inbred mice. Our findings indicate that genes mediating other aspects of neurodevelopment, including those whose mutations underlie more subtle disruptions in white matter pathways and connectivity, are more likely to contribute to the aberrant behavioral phenotypes in the BTBR mouse model of autism.

Aberrant functional connectivity in autism: evidence from low-frequency BOLD signal fluctuations

A number of recent studies have examined functional connectivity in individuals with Autism Spectrum Disorders (ASD), generally converging on the finding of reduced interregional coordination, or underconnectivity. Underconnectivity has been reported between many brain regions and across a range of cognitive tasks, and has been proposed to underlie behavioral and cognitive impairments associated with ASD. The current study employed functional connectivity MRI (fcMRI) to examine interregional correlations of low-frequency BOLD signal fluctuations in 10 high-functioning participants with ASD and 10 typically developing control participants. Whole-brain connectivity with three seed regions of interest (left middle frontal, left superior parietal, and left middle occipital cortex) was evaluated using fMRI datasets acquired during performance of a source recognition task. While fcMRI patterns were found to be largely similar across the two groups, including many common areas, effects for the ASD group were generally more extensive. These findings, although inconsistent with generalized underconnectivity in ASD, are compatible with a model of aberrant connectivity in which the nature of connectivity disturbance (i.e., increased or reduced) may vary by region. Taking into consideration methodological factors that might influence measured fcMRI effects, we suggest that ASD is associated with an inefficiency in optimizing network connections to achieve task performance.

The visual analysis of emotional actions

Is the visual analysis of human actions modulated by the emotional content of those actions? This question is motivated by a consideration of the neuroanatomical connections between visual and emotional areas. Specifically, the superior temporal sulcus (STS), known to play a critical role in the visual detection of action, is extensively interconnected with the amygdala, a center for emotion processing. To the extent that amygdala activity influences STS activity, one would expect to find systematic differences in the visual detection of emotional actions. A series of psychophysical studies tested this prediction. Experiment 1 identified point-light walker movies that convincingly depicted five different emotional states: happiness, sadness, neutral, anger, and fear. In Experiment 2, participants performed a walker detection task with these movies. Detection performance was systematically modulated by the emotional content of the gaits. Participants demonstrated the greatest visual sensitivity to angry walkers. The results of Experiment 3 suggest that local velocity cues to anger may account for high false alarm rates to the presence of angry gaits. These results support the hypothesis that the visual analysis of human action depends upon emotion processes.

From loci to networks and back again: anomalies in the study of autism

Recent developments in functional imaging as well as the emergence of new anatomical imaging techniques suited for the study of white matter have shifted investigational paradigms from a localized to a more holistic network approach. Aside from detecting local activity, functional MRI can be applied to the study of connectivity. However, the concept of "functional connectivity" remains broad, and specific designs and analyses may affect the results. In addition, connectivity cannot be viewed in isolation. Rather, from a developmental perspective, connectivity and local cortical architecture are intimately related. Therefore, combined approaches examining local organization and connectivity are the most promising avenues for elucidating disturbances of neurofunctional organization in developmental disorders. Here this approach is illustrated via data obtained from autism research that suggest impaired local cortical architecture and reduced long-range connectivity between cerebral regions.

Neurodevelopment and executive function in autism

Autism is a neurodevelopmental disorder characterized by social and communication deficits, and repetitive behavior. Studies investigating the integrity of brain systems in autism suggest a wide range of gray and white matter abnormalities that are present early in life and change with development. These abnormalities predominantly affect association areas and undermine functional integration. Executive function, which has a protracted development into adolescence and reflects the integration of complex widely distributed brain function, is also affected in autism. Evidence from studies probing response inhibition and working memory indicate impairments in these core components of executive function, as well as compensatory mechanisms that permit normative function in autism. Studies also demonstrate age-related improvements in executive function from childhood to adolescence in autism, indicating the presence of plasticity and suggesting a prolonged window for effective treatment. Despite developmental gains, mature executive functioning is limited in autism, reflecting abnormalities in wide-spread brain networks that may lead to impaired processing of complex information across all domains.

Voxel-based morphometry study on brain structure in children with high-functioning autism

Earlier studies have suggested abnormal brain volumes in autism, but inconsistencies exist. Using voxel-based morphometry, we compared global and regional brain volumes in 17 high-functioning autistic children with 15 matched controls. We identified significant reduction in left white matter volume and white/gray matter ratio in autism. Regional brain volume reductions were detected for right anterior cingulate, left superior parietal lobule white matter volumes, and right parahippocampal gyrus gray matter volume, whereas enlargements in bilateral supramarginal gyrus, right postcentral gyrus, right medial frontal gyrus, and right posterior lobe of cerebellum gray matter in autism. Our findings showed global and regional brain volumes abnormality in high-functioning autism.

EEG photic driving: right-hemisphere reactivity deficit in childhood autism. A pilot study

In 14 autistic boys, aged 6-14 years, free of drug treatment, with relatively intact verbal functions and without severe or moderate mental retardation (I.Q. 91.4+/-22.8), intermittent photic stimulation at 11 fixed frequencies of 3-24 Hz revealed latent deficiency of the right hemisphere in the photic driving reactivity, predominantly at the fast alpha and beta frequencies of stimulation. The left-side prevalence was observed: 1) in the total number of driving peaks evaluated for the first four harmonics in the EEG spectra of 14 cortical areas and 2) in the driving amplitude in the spectra of the 2 occipital areas. As compared to 21 normally developing boys matched on age who did not show interhemispheric asymmetry in the driving reactivity, the autistic patients had significantly lower driving characteristics only in the right hemisphere. There were no significant differences between the autistic and control groups in the spontaneous EEG spectra of the occipital areas in the resting state.

Low sociability is associated with reduced size of the corpus callosum in the BALB/cJ inbred mouse strain

The behavioral manifestations of autism, including reduced sociability (reduced tendency to seek social interaction), may be related to underdevelopment of the corpus callosum (CC). The BALB/cJ inbred mouse strain is a useful model system for testing the relationship between reduced sociability and CC underdevelopment. BALB/cJ mice show low levels of sociability, on average, but substantial intrastrain variability in sociability, as well as striking variability in CC development. This study tested the hypothesis that sociability is positively correlated with CC size within the BALB/cJ inbred strain. 30-day-old BALB/cJ and C57BL/6J mice were tested for sociability towards gonadectomized A/J stimulus mice in a social choice task. The size of the corpus callosum was measured histologically at the midsagittal plane. BALB/cJ mice showed a significant positive correlation between the tendency to sniff the stimulus mouse and size of the CC relative to brain weight. C57BL/6J mice showed consistently high levels of sociability and normal corpus callosum development. These results suggest that abnormal white matter structure is associated with deficits in sociability in BALB/cJ mice. Additional studies are warranted to elucidate the relationship between brain connectivity and sociability in this model system.

Atypical frontal-posterior synchronization of Theory of Mind regions in autism during mental state attribution

This study used fMRI to investigate the functioning of the Theory of Mind (ToM) cortical network in autism during the viewing of animations that in some conditions entailed the attribution of a mental state to animated geometric figures. At the cortical level, mentalizing (attribution of metal states) is underpinned by the coordination and integration of the components of the ToM network, which include the medial frontal gyrus, the anterior paracingulate, and the right temporoparietal junction. The pivotal new finding was a functional underconnectivity (a lower degree of synchronization) in autism, especially in the connections between frontal and posterior areas during the attribution of mental states. In addition, the frontal ToM regions activated less in participants with autism relative to control participants. In the autism group, an independent psychometric assessment of ToM ability and the activation in the right temporoparietal junction were reliably correlated. The results together provide new evidence for the biological basis of atypical processing of ToM in autism, implicating the underconnectivity between frontal regions and more posterior areas.

Evolutionary perspectives on psychoses and autism: Does genomic imprinting contribute to phenomenological antithesis?

Crespi & Badcock (C&B) have presented a novel view that the influence of genomic imprinting causes diametrically opposite disorders: namely, psychoses and autism. I propose an extended hypothesis that while genomic imprinting is likely to have an influence on the pathogenesis of psychoses and autism, it might contribute to phenomenological antithesis between as well as within these disorders.

Psychosis and autism as diametrical disorders of the social brain

Autistic-spectrum conditions and psychotic-spectrum conditions (mainly schizophrenia, bipolar disorder, and major depression) represent two major suites of disorders of human cognition, affect, and behavior that involve altered development and function of the social brain. We describe evidence that a large set of phenotypic traits exhibit diametrically opposite phenotypes in autistic-spectrum versus psychotic-spectrum conditions, with a focus on schizophrenia. This suite of traits is inter-correlated, in that autism involves a general pattern of constrained overgrowth, whereas schizophrenia involves undergrowth. These disorders also exhibit diametric patterns for traits related to social brain development, including aspects of gaze, agency, social cognition, local versus global processing, language, and behavior. Social cognition is thus underdeveloped in autistic-spectrum conditions and hyper-developed on the psychotic spectrum.;>We propose and evaluate a novel hypothesis that may help to explain these diametric phenotypes: that the development of these two sets of conditions is mediated in part by alterations of genomic imprinting. Evidence regarding the genetic, physiological, neurological, and psychological underpinnings of psychotic-spectrum conditions supports the hypothesis that the etiologies of these conditions involve biases towards increased relative effects from imprinted genes with maternal expression, which engender a general pattern of undergrowth. By contrast, autistic-spectrum conditions appear to involve increased relative bias towards effects of paternally expressed genes, which mediate overgrowth. This hypothesis provides a simple yet comprehensive theory, grounded in evolutionary biology and genetics, for understanding the causes and phenotypes of autistic-spectrum and psychotic-spectrum conditions.