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Mechanism of autism
Mechanism of autism
Mechanism of autism
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The mechanisms of autism are the molecular and cellular processes believed to cause or contribute to the symptoms of autism. Multiple processes are hypothesized to explain different autistic features. These hypotheses include defects in synapse structure and function,[1][2] reduced synaptic plasticity,[3] disrupted neural circuit function, gut–brain axis dyshomeostasis,[4][5][6] neuroinflammation,[7] and altered brain structure or connectivity.[8][9][10][11] Autism symptoms stem from maturation-related changes in brain systems.[9] The mechanisms of autism are divided into two main areas: pathophysiology of brain structures and processes, and neuropsychological linkages between brain structures and behaviors, with multiple pathophysiologies linked to various autism behaviors.[10]

Evidence suggests gut–brain axis abnormalities may contribute to autism.[6][4] Studies propose that immune, gastrointestinal inflammation, autonomic nervous system dysfunction, gut microbiota alterations, and dietary metabolites may contribute to brain neuroinflammation and dysfunction.[5] Additionally, enteric nervous system abnormalities could play a role in neurological disorders by allowing disease pathways from the gut to impact the brain.[5]

Synaptic dysfunction also appears to be implicated in autism, with some mutations disrupting synaptic pathways involving cell adhesion.[2] Evidence points to teratogens affecting the early developmental stages, suggesting autism arises very early, possibly within the first eight weeks after conception.[12]

Neuroanatomical studies support that autism may involve abnormal neuronal growth and pruning, leading to brain enlargement in some areas and reduction in others.[13] Functional neuroimaging studies show reduced activation in somatosensory cortices during theory of mind tasks in autistic individuals and highlight potential imbalances in neurotransmitters like glutamate and Γ-aminobutyric acid that may underlie autism's behavioral manifestations.[14]

Pathophysiology

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Two diagrams of major brain structures implicated in autism. The upper diagram shows the cerebral cortex near the top and the basal ganglia in the center, just above the amygdala and hippocampus. The lower diagram shows the corpus callosum near the center, the cerebellum in the lower rear, and the brain stem in the lower center.
The amygdala, cerebellum, and many other brain regions have been implicated in autism.[15]

Unlike some brain disorders which have clear molecular hallmarks that can be observed in every affected individual, such as Alzheimer's disease or Parkinson's disease, autism does not have a unifying mechanism at the molecular, cellular, or systems level. The autism spectrum may comprise a small set of disorders that converge on a few common molecular pathways, or it may be a large set of disorders with diverse mechanisms.[16] Autism appears to result from developmental factors that affect many or all functional brain systems.[17] Some factors may disturb the timing of brain development rather than the final product.[15]

Listed below are some characteristic findings in ASD brains on molecular and cellular levels regardless of the specific genetic variation or mutation contributing to autism in a particular individual:

  • Limbic system with smaller neurons that are more densely packed together. Given that the limbic system is the main center of emotions and memory in the human brain, this observation may explain social impairment in ASD.[18]
  • Fewer and smaller Purkinje neurons in the cerebellum. New research suggest a role of the cerebellum in emotional processing and language.[18]
  • Increased number of astrocytes and microglia in the cerebral cortex. These cells provide metabolic and functional support to neurons and act as immune cells in the nervous system, respectively.[18]
  • Increased brain size in early childhood causing macrocephaly in 15–20% of ASD individuals. The brain size however normalizes by mid-childhood. This variation in brain size in not uniform in the ASD brain with some parts like the frontal and temporal lobes being larger, some like the parietal and occipital lobes being normal sized, and some like cerebellar vermis, corpus callosum, and basal ganglia being smaller than neurotypical individuals.[18]
  • Cell adhesion molecules that are essential to formation and maintenance of connections between neurons, neuroligins found on postsynaptic neurons that bind presynaptic cell adhesion molecules, and proteins that anchor cell adhesion molecules to neurons are all found to be mutated in ASD.[18]
  • Loss of function (LoF) mutations in genes relating to the function and development of the synapse.[19] Some of those implicated include SHANK3, SCN2A, and PTEN.[19]

Brain growth

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Neuroanatomical studies and the association between autism and teratogens strongly suggest that autism affects brain development soon after conception.[12] This anomaly appears to start a cascade of pathological events in the brain that are significantly influenced by environmental factors.[20] Just after birth, the brains of children with autism tend to grow faster than usual, followed by normal or relatively slower growth in childhood.[21] It is unknown whether early brain overgrowth occurs in all children with autism. It appears to be most prominent in the frontal and temporal lobes, which are associated with higher cognitive specializations such as social cognition, and language development.[22] Hypotheses for the cellular and molecular bases of pathological early overgrowth include an excess of neurons that causes local overconnectivity in key brain regions,[21] and disturbed neuronal migration during early gestation.[23][24]

Synapse dysfunction

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Synapse and dendritic spine growth may be disrupted in autism due to impaired neurexin–neuroligin cell-adhesion signaling[25] or dysregulated synthesis of synaptic proteins.[26][27] Disrupted synaptic development may also contribute to epilepsy, which may explain why the two conditions are associated.[28]Studies have suggested that excitatory–inhibitory networks may be imbalanced in autism.[24]

Neurotransmitters such as serotonin, dopamine, and glutamate have been implicated in autism.[1] Fragile X, the most common genetic cause of autism, is linked to dysfunction of group I metabotropic glutamate receptors (mGluR), leading some to consider their potential role in autism.[29]

Altered circuit connectivity

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A human brain viewed from above. About 10% is highlighted in yellow and 10% in blue. There is a tiny green region (~0.5%) where they overlap.
Autistic individuals tend to use different brain areas (yellow) for a movement task compared to a control group (blue).[30]

The underconnectivity theory of autism posits that autistic people tend to have fewer high-level neural connections and less global synchronization, along with an excess of low-level processes.[31] Functional connectivity studies have found both hypo- and hyperconnectivity in brains of autistic people.[32] Hypoconnectivity is commonly observed for interhemispheric (e.g. lower neuron density in corpus callosum)[33] and cortico-cortical functional connectivity.[34] Some studies have found local overconnectivity in the cerebral cortex and weak functional connections between the frontal lobe and the rest of the cortex.[35] Abnormal default mode network (task-negative) connectivity is often observed. Toggling between task-negative network activation and task-positive network activation (consisting of the dorsal attention network and salience network) may be less efficient, possibly reflecting a disturbance of self-referential thought.[36] Such patterns of low function and aberrant activation in the brain may depend on whether the brain is performing social or nonsocial tasks.[37]

Some studies have suggested that autism is a disorder of the association cortex.[38] Event-related potentials with respect to attention, orientation to auditory and visual stimuli, novelty detection, language and face processing, and information storage are altered in autistic individuals; several studies have found a preference for nonsocial stimuli.[39] Magnetoencephalography studies have observed delayed processing of auditory signals in autistic children.[40]

The mirror neuron system (MNS) theory of autism hypothesizes that disrupted development of the MNS impairs autistic people's ability to imitate others, leading to core autistic features of social impairment and communication difficulties. In animals, the MNS activates when an animal performs an action or observes another animal perform the same action. The MNS may contribute to an individual's understanding of other people by enabling the modeling of their behavior via embodied simulation of their actions, intentions, and emotions.[41][42] Several studies have tested this hypothesis by demonstrating structural abnormalities in MNS regions of individuals with ASD, delay in the activation in the core circuit for imitation in individuals with ASD, and a correlation between reduced MNS activity and severity of the syndrome in children with ASD.[43] However, individuals with autism also have abnormal brain activation in many circuits outside the MNS[44] and the MNS theory does not explain the normal performance of children with autism on imitation tasks that involve a goal or object.[45]

Common copy number variation associations have suggested similarities between the mechanisms of autism and schizophrenia. For loci such as 16p11.2, 16p13.1, 22p11, and 22q13, deletion is associated with autism whereas duplication is associated with schizophrenia. Conversely, 1q21.1 and 22p11.2 duplication is associated with autism and deletion with schizophrenia.[46]

It has been observed that people with ASD tend to have preferential processing of information on the left hemisphere compared to the right. The left hemisphere is associated with processing information related to details whereas the right hemisphere is associated with processing information in a more global and integrated sense that is essential for pattern recognition. For example, visual information like face recognition is normally processed by the right hemisphere which tends to integrate all information from an incoming sensory signal, whereas an ASD brain preferentially processes visual information in the left hemisphere where information tends to be processed for local details of the face rather than the overall configuration of the face. This left lateralization negatively impacts both facial recognition and spatial skills.[33][47]

Inflammation

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The immune system is thought to play an important role in autism. Children with autism have been found by researchers to have inflammation of both the peripheral and central immune systems as indicated by increased levels of pro-inflammatory cytokines and significant activation of microglia.[48][49][7] Biomarkers of abnormal immune function have also been associated with increased impairments in behaviors that are characteristic of the core features of autism such as, deficits in social interactions and communication.[49] Interactions between the immune system and the nervous system begin early during the embryonic stage of life, and successful neurodevelopment depends on a balanced immune response. It is thought that activation of a pregnant mother's immune system such as from environmental toxicants or infection can contribute to causing autism through causing a disruption of brain development.[50][51][52] This is supported by recent studies that have found that infection during pregnancy is associated with an increased risk of autism.[53][54]

Some evidence suggests that gut–brain axis abnormalities may be involved by means of impaired serotonin signaling and inflammation.[6] A 2015 review proposed that immune dysregulation, gastrointestinal inflammation, autonomic nervous system malfunction, gut microbiota alterations, and food metabolites may cause brain neuroinflammation and dysfunction.[4] A 2016 review concluded that enteric nervous system abnormalities might play a role in neurological disorders such as autism.[5]

Metabolism

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Some data suggests neuronal overgrowth observed in autism may be caused by an increase in several growth hormones[55] or impaired regulation of growth factor receptors. Some inborn errors of metabolism are associated with autism, but probably account for less than 5% of cases.[56]

Brain connectivity

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Brains of autistic individuals have been observed to have abnormal connectivity and the degree of these abnormalities directly correlates with the severity of autism. Following are some observed abnormal connectivity patterns in autistic individuals:[33][18]

  • Decreased connectivity between different specialized regions of the brain (e.g. lower neuron density in corpus callosum) and relative over-connectivity within specialized regions of the brain by adulthood. Connectivity between different regions of the brain ('long-range' connectivity) is important for integration and global processing of information and comparing incoming sensory information with the existing model of the world within the brain. Connections within each specialized regions ('short-range' connections) are important for processing individual details and modifying the existing model of the world within the brain to more closely reflect incoming sensory information. In infancy, children at high risk for autism that were later diagnosed with autism were observed to have abnormally high long-range connectivity which then decreased through childhood to eventual long-range under-connectivity by adulthood.[33]
  • Abnormal preferential processing of information by the left hemisphere of the brain vs. preferential processing of information by right hemisphere in neurotypical individuals. The left hemisphere is associated with processing information related to details whereas the right hemisphere is associated with processing information in a more global and integrated sense that is essential for pattern recognition. For example, visual information like face recognition is normally processed by the right hemisphere which tends to integrate all information from an incoming sensory signal, whereas an ASD brain preferentially processes visual information in the left hemisphere where information tends to be processed for local details of the face rather than the overall configuration of the face. This left lateralization negatively impacts both facial recognition and spatial skills.[33][57]
  • Increased functional connectivity within the left hemisphere which directly correlates with severity of autism. This observation also supports preferential processing of details of individual components of sensory information over global processing of sensory information in an ASD brain.[33]
  • Prominent abnormal connectivity in the frontal and occipital regions. In autistic individuals low connectivity in the frontal cortex was observed from infancy through adulthood. This is in contrast to long-range connectivity which is high in infancy and low in adulthood in ASD.[33] Abnormal neural organization is also observed in the Broca's area which is important for speech production.[18]

Gut-immune-brain axis

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46% to 84% of autistic individuals have gastrointestinal-related problems like reflux, diarrhea, constipation, inflammatory bowel disease, and food allergies.[58] It has been observed that the makeup of gut bacteria in autistic people is different than that of non-autistic individuals which has raised the question of influence of gut bacteria on ASD development via inducing an inflammatory state.[59] Listed below are some research findings on the influence of gut bacteria and abnormal immune responses on brain development:[59]

  • Some studies on rodents have shown gut bacteria influencing emotional functions and neurotransmitter balance in the brain, both of which are impacted in ASD.[18]
  • The immune system is thought to be the intermediary that modulates the influence of gut bacteria on the brain. Some ASD individuals have a dysfunctional immune system with higher numbers of some types of immune cells, biochemical messengers and modulators, and autoimmune antibodies. Increased inflammatory biomarkers correlate with increased severity of ASD symptoms and there is some evidence to support a state of chronic brain inflammation in ASD.[59]
  • More pronounced inflammatory responses to bacteria were found in ASD individuals with an abnormal gut microbiota. Additionally, immunoglobulin A antibodies that are central to gut immunity were also found in elevated levels in ASD populations. Some of these antibodies may attack proteins that support myelination of the brain, a process that is important for robust transmission of neural signal in many nerves.[59]
  • Activation of the maternal immune system during pregnancy (by gut bacteria, bacterial toxins, an infection, or non-infectious causes) and gut bacteria in the mother that induce increased levels of Th17, a pro-inflammatory immune cell, have been associated with an increased risk of autism. Some maternal IgG antibodies that cross the placenta to provide passive immunity to the fetus can also attack the fetal brain.[59]
  • It is proposed that inflammation within the brain promoted by inflammatory responses to harmful gut microbiome impacts brain development.[59]
  • Pro-inflammatory cytokines IFN-γ, IFN-α, TNF-α, IL-6 and IL-17 have been shown to promote autistic behaviors in animal models. Giving anti-IL-6 and anti-IL-17 along with IL-6 and IL-17, respectively, have been shown to negate this effect in the same animal models.[59]
  • Some gut proteins and microbial products can cross the blood–brain barrier and activate mast cells in the brain. Mast cells release pro-inflammatory factors and histamine which further increase blood–brain barrier permeability and help set up a cycle of chronic inflammation.[59]

Social brain interconnectivity

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A number of discrete brain regions and networks among regions that are involved in dealing with other people have been discussed together under the rubric of the social brain. As of 2012, there is a consensus that autism spectrum is likely related to problems with interconnectivity among these regions and networks, rather than problems with any specific region or network.[60]

Temporal lobe

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Functions of the temporal lobe are related to many of the deficits observed in individuals with ASDs, such as receptive language, social cognition, joint attention, action observation, and empathy. The temporal lobe also contains the superior temporal sulcus and the fusiform face area, which may mediate facial processing. It has been argued that dysfunction in the superior temporal sulcus underlies the social deficits that characterize autism. Compared to neurotypical individuals, one study found that individuals with high-functioning autism had reduced activity in the fusiform face area when viewing pictures of faces.[61][verification needed]

Mitochondria

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ASD could be linked to mitochondrial disease, a basic cellular abnormality with the potential to cause disturbances in a wide range of body systems.[62] A 2012 meta-analysis study, as well as other population studies show that approximately 5% of autistic children meet the criteria for classical mitochondrial dysfunction.[63] It is unclear why this mitochondrial disease occurs, considering that only 23% of children with both ASD and mitochondrial disease present with mitochondrial DNA abnormalities.[63]

Serotonin

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Main article: Serotonin

Serotonin is a major neurotransmitter in the nervous system and contributes to formation of new neurons (neurogenesis), formation of new connections between neurons (synaptogenesis), remodeling of synapses, and survival and migration of neurons, processes that are necessary for a developing brain and some also necessary for learning in the adult brain. 45% of ASD individuals have been found to have increased blood serotonin levels.[18] Abnormalities in the serotonin transporter have also been found in ASD individuals. It has been hypothesized that increased activity of serotonin in the developing brain may facilitate the onset of ASD, with an association found in six out of eight studies between the use of selective serotonin reuptake inhibitors (SSRIs) by the pregnant mother and the development of ASD in the child exposed to SSRI in the antenatal environment.[64]

The study could not definitively conclude SSRIs caused the increased risk for ASD due to the biases found in those studies, and the authors called for more definitive, better conducted studies.[65] Confounding by indication has since then been shown to be likely.[66] However, it is also hypothesized that SSRIs may help reduce symptoms of ASD and even positively affect brain development in some ASD patients.[18]

Reduced NMDA‐receptor function

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Reduced NMDA receptor function has been linked to reduced social interactions, locomotor hyperactivity, self-injury, prepulse inhibition (PPI) deficits, and sensory hypersensitivity, among others. Results suggest that NMDA dysregulation could contribute to core ASD symptoms.[67]

Abnormal folate metabolism

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Several lines of evidence indicate abnormalities of folate metabolism in ASD. These abnormalities can lead to a decrease in 5-methyltetrahydrofolate production, alter the production of folate metabolites and reduce folate transport across the blood-brain barrier and in neurons. The most significant abnormalities of folate metabolism associated with ASD may be autoantibodies to the alpha folate receptor (FRα). These autoantibodies have been associated with cerebral folate deficiency. Autoantibodies can bind to FRα and greatly impair its function.

In 2013, one study reported that 60% and 44% of 93 children with ASD were positive for FRα-blocking and binding autoantibodies, respectively. This high rate of anti-FRα autoantibody positivity was confirmed by Ramaekers et al. who compared 75 children with ASD to 30 non-autistic "controls". These controls were children who had a developmental delay, but did not have ASD. FRα-blocking autoantibodies were positive in 47% of children with ASD, but only in 3% of children without ASD.

Many children with ASD and cerebral folate deficiency have marked improvements in their clinical status when taking folinic acid.

Five children with cerebral folate deficiency and low functioning autism with neurological deficits found a complete reduction of ASD symptoms with the use of folinic acid in a child and substantial improvements in communication in two other children.[68][69][70]

Abnormal redox metabolism

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An imbalance in glutathione-dependent redox metabolism has been shown to be associated with autism spectrum disorder (ASD).[citation needed] Glutathione synthesis and intracellular redox balance are related to folate metabolism and methylation, metabolic pathways that have also been shown to be abnormal in ASD. Together, these metabolic abnormalities define a distinct endophenotype of TSA closely associated with genetic, epigenetic and mitochondrial abnormalities, as well as environmental factors related to ASD. Glutathione is involved in neuroprotection against oxidative stress and neuroinflammation by improving the antioxidant stress system.

In autistic children, studies have shown that glutathione metabolism can be improved:[citation needed]

Interestingly, recent DBPC studies have shown that N-acetyl-1-cysteine, a glutathione precursor supplement, is effective in improving the symptoms and behaviors associated with ASD.[71] However, glutathione was not measured in these studies.

Small, medium and large DPBC trials and open small and medium-sized clinical trials demonstrate that new treatments for children with ASD for oxidative stress are associated with improvements in baseline symptoms of ASD, sleep, gastrointestinal symptoms, hyperactivity, seizures and parental impression, sensory and motor symptoms. These new treatments include N-acetyl-l-cysteine, methylcobalamin with and without oral folinic acid, vitamin C, and a vitamin and mineral supplement that includes antioxidants, enzyme Q10, and B vitamins.

Several other treatments that have antioxidant properties, including carnosine, have also been reported to significantly improve ASD behaviors, suggesting that treatment of oxidative stress could be beneficial for children with ASD. Many antioxidants can also help improve mitochondrial function, suggesting that clinical improvements with antioxidants could occur through a reduction in oxidative stress and an improvement in mitochondrial function.

Some of these treatments can have frequent serious side effects such as bronchospasm.[68][72][73]

Neuropsychology

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Two major types of cognitive theories have been proposed to explain links between autistic brains and behavior.

Social cognition theories

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Social cognition theories focuses on deficits in social cognition. Simon Baron-Cohen's empathizing–systemizing theory postulates that autistic individuals can systemize, by developing internal rules of operation to handle events inside the brain, but are less effective at empathizing, by handling events generated by other agents. An extension, the extreme male brain theory, hypothesizes that autism is an extreme case of the male brain, defined psychometrically as individuals where systemizing is better than empathizing.[74] These theories are somewhat related to Baron-Cohen's earlier theory of mind approach, which hypothesizes that autistic behavior arises from an inability to ascribe mental states to oneself and others. The theory of mind hypothesis is supported by the atypical responses of children with autism to the Sally–Anne test for reasoning about others' motivations,[74] and the mirror neuron system theory of autism described in Pathophysiology maps well to the hypothesis.[43] However, most studies have found no evidence of impairment in autistic individuals' ability to understand other people's basic intentions or goals; instead, data suggests that impairments are found in understanding more complex social emotions or in considering others' viewpoints.[75]

Nonsocial cognition theories

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Nonsocial cognition theories focuses on nonsocial or general processing: the executive functions such as working memory, planning, and inhibition. In his review, Kenworthy states that "the claim of executive dysfunction as a causal factor in autism is controversial", however, "it is clear that executive dysfunction plays a role in the social and cognitive deficits observed in individuals with autism".[76] Tests of core executive processes such as eye movement tasks indicate improvement from late childhood to adolescence, but performance never reaches typical adult levels.[77] A strength of this theory is predicting stereotyped behavior and narrow interests;[78] two weaknesses are that executive function is hard to measure[76] and that executive function deficits have not been found in young children with autism.[79]

Weak central coherence theory

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Main article: Weak central coherence theory

Weak central coherence theory hypothesizes that a limited ability to see the big picture underlies the central disturbance in autism. One strength of this theory is predicting special talents and peaks in performance in autistic people.[80] Another theory, enhanced perceptual functioning, focuses more on the superiority of locally oriented and perceptual operations in autistic individuals.[81] Yet another theory, monotropism, posits that autism stems from a different cognitive style, tending to focus attention and processing resources intensely, to the exclusion of other stimuli.[82] These theories map well from the underconnectivity theory of autism.

Issues with theories

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No single type of theory is satisfactory on its own; social cognition theories poorly address autism's rigid and repetitive behaviors, while most of the nonsocial theories have difficulty explaining autism's social impairment and communication difficulties.[83] A combined theory based on multiple deficits may prove to be more useful.[84]

See also

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References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Mechanism of autism

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from Grokipedia
The mechanisms of autism spectrum disorder (ASD) encompass a multifaceted interplay of genetic predispositions, environmental influences, and neurobiological alterations that disrupt typical development, leading to persistent challenges in social communication, , and behavioral flexibility. ASD arises from this multifactorial , with estimates ranging from 50% to 80%, involving hundreds of risk genes that affect synaptic function, neuronal connectivity, and . Key genetic contributors include de novo mutations, copy number variations (such as those at 16p11.2 and 15q11-13), and epigenetic modifications, with over 1,200 genes implicated in the SFARI database, including prominent ones like SHANK3, MECP2, and . Environmental factors, particularly during prenatal and early postnatal periods, amplify these genetic risks through mechanisms like and immune dysregulation; notable exposures include advanced parental age (maternal ≥40 years or paternal ≥50 years), maternal infections (e.g., or ), , air pollutants (e.g., PM2.5), and certain pesticides, which can increase ASD risk by 1.5- to 2-fold. At the neurobiological level, ASD involves atypical brain development, including early overgrowth followed by regression, impaired , and altered neural connectivity, particularly in regions like the and , which underpin and . These changes manifest through hypotheses such as deficits in theory of mind (difficulty interpreting others' mental states), (issues with planning and attention), and weak central coherence (over-focus on details at the expense of global processing). Additional pathways include mitochondrial dysfunction (which impairs neuronal energy production, increasing vulnerability to excitotoxicity from excessive glutamate activity, leading to oxidative stress and neuronal damage), , and , which may alter via epigenetic mechanisms and contribute to the disorder's heterogeneity. Recent advances, including studies from 2024, reveal dysregulated in excitatory neurons and glial cells, highlighting the role of cellular-level disruptions in ASD pathogenesis. While no single cause explains ASD, this integrated model underscores the need for targeted interventions addressing both genetic vulnerabilities and modifiable environmental risks.

Genetic and Epigenetic Factors

Heritability and genetic variants

Autism spectrum disorder (ASD) exhibits high , with twin studies estimating that genetic factors account for approximately 80% to 90% of the liability to the condition. A 2016 meta-analysis of population-based twin cohorts reinforced this, reporting heritability estimates ranging from 64% to 91%, underscoring the predominant role of over shared environmental influences in ASD . Large-scale genomic analyses in 2024 further attributed about 80% of ASD risk to heritable genetic components, highlighting the polygenic nature of the disorder while noting modest contributions from non-shared environmental factors. Genetic variants contributing to ASD encompass a spectrum of types, including de novo mutations, copy number variations (CNVs), and common inherited alleles captured by polygenic risk scores. De novo mutations, which arise spontaneously in the affected individual and are absent in parents, are enriched in ASD cases, particularly in simplex families, and are identified in approximately 20% to 50% of cases depending on familial loading. CNVs, such as deletions or duplications at the 16p11.2 locus, are recurrent and disrupt multiple genes, increasing ASD susceptibility by altering and synaptic function; for instance, 16p11.2 duplications have been linked to enhanced density in preclinical models. Polygenic risk scores, aggregating effects from thousands of common variants, explain an additional ~2.5% to 5.5% of ASD variance and often interact with rare variants to modulate phenotypic severity. Over 1,200 genes have been robustly associated with ASD through large-scale sequencing efforts, many converging on pathways critical for and neural development. Prominent examples include SHANK3, which encodes a postsynaptic scaffolding protein essential for organization; loss-of-function variants in SHANK3 disrupt morphology and glutamatergic transmission, contributing to core ASD symptoms like social deficits. CHD8, a remodeler, regulates neurodevelopmental ; leads to altered Wnt signaling and , with de novo mutations strongly implicated in early-onset ASD. The FMR1 gene, mutations in which cause —a leading monogenic form of ASD—impairs mRNA at synapses, resulting in excessive protein synthesis and elongation that underlies and repetitive behaviors. effects are evident across these loci; for example, SHANK3 duplications elevate protein levels, potentially tipping excitatory-inhibitory balance toward hyperexcitability. Recent studies indicate sex differences in , with estimates higher in males (87%) than females (76%), potentially contributing to diagnostic disparities. Recent advances have illuminated how these variants drive phenotypic heterogeneity in ASD. A 2025 study in Nature Genetics decomposed ASD into distinct genetic programs using multimodal data from over 5,000 children, revealing that de novo variants cluster with specific symptom profiles—such as social-communication deficits linked to synaptic genes versus motor delays tied to regulators—enabling subtype-specific risk prediction. These findings emphasize the interplay of rare and common variants in shaping individualized neurodevelopmental trajectories, informing precision approaches to ASD mechanisms.

Epigenetic modifications

Epigenetic modifications, which regulate gene expression without altering the underlying DNA sequence, play a pivotal role in autism spectrum disorder (ASD) by influencing the activity of risk-associated genes during neurodevelopment. Key processes include DNA methylation, where methyl groups are added to cytosine bases in CpG dinucleotides to silence genes; histone modifications, such as acetylation and methylation of histone tails that alter chromatin accessibility; and non-coding RNAs, including microRNAs and long non-coding RNAs that modulate transcription and post-transcriptional regulation. These mechanisms collectively affect ASD risk genes, such as those involved in synaptic function and neuronal differentiation, leading to dysregulated gene expression patterns observed in ASD brains. Specific epigenetic alterations have been linked to ASD phenotypes, particularly in genes overlapping with related neurodevelopmental disorders. For instance, reduced expression of methyl-CpG-binding protein 2 (MeCP2), encoded by the MECP2 gene, is frequently observed in autism frontal cortex and correlates with features shared with , an X-linked disorder with ASD overlap, where MeCP2 dysfunction disrupts -mediated . Similarly, hyper of the gene (OXTR) promoter region is associated with increased autism traits and social responsiveness deficits, as higher methylation levels correlate with reduced OXTR expression and impaired in individuals with ASD. These examples illustrate how locus-specific epigenetic changes can bridge genetic predispositions to behavioral outcomes in ASD. Environmental factors during can induce epigenetic modifications that heighten ASD risk, as demonstrated in animal models. Prenatal exposure to valproic acid (VPA), an linked to elevated ASD incidence in humans, triggers autism-like behaviors in rodent offspring, accompanied by altered and histone acetylation in genes related to neuronal migration and . These changes persist transgenerationally in some studies, suggesting heritable epigenetic reprogramming that mimics ASD without direct genetic mutations. Such interactions highlight the interplay between environmental insults and epigenetic machinery in shaping neurodevelopmental trajectories. Recent advances in methylomics have identified ASD-specific epigenetic signatures as potential biomarkers. Studies from 2024, including those supported by the Autism Research Institute (ARI), have revealed distinct patterns in peripheral blood of individuals with ASD, integrating methylomic data with protein network analyses to pinpoint signatures linked to neurodevelopmental disruptions. For example, applied to blood-derived methylation profiles has shown feasibility in distinguishing ASD cases, with patterns reflecting altered states in ASD-relevant pathways. These findings underscore the promise of non-invasive epigenetic biomarkers for early and personalized interventions. Chromatin remodeling complexes further mediate epigenetic landscapes critical for ASD etiology, particularly through chromodomain helicase DNA-binding protein 8 (CHD8). Mutations in CHD8, a high-confidence ASD risk gene, disrupt ATP-dependent , leading to aberrant modifications and altered during early neurodevelopment, which manifests as and social impairments in affected individuals. CHD8's role in establishing open states for neuronal differentiation highlights its influence on the broader epigenetic architecture, where genetic variants in remodeling factors predispose to dysregulation of ASD-related pathways.

Neural Development

Early brain growth and overgrowth

One hallmark of autism spectrum disorder (ASD) involves atypical early development, characterized by typically normal or slightly smaller volume at birth followed by accelerated growth in the first two years of life. Longitudinal (MRI) studies have demonstrated that infants later diagnosed with ASD exhibit rapid head circumference and volume increases, with total volume becoming approximately 5-10% larger than in typically developing peers by 12-24 months of age. This overgrowth peaks around toddlerhood and is evident in prospective studies tracking high-risk siblings. Regionally, this expansion affects both gray and white matter, with disproportionate enlargement in the frontal and temporal cortices, , and during . The shows increased volume as early as 6-12 months, potentially contributing to altered emotional processing, while cerebellar growth abnormalities may relate to challenges. In , some longitudinal data indicate a deceleration in growth rates, leading to partial normalization or even relative reduction in brain volume compared to early peaks, though evidence for complete normalization remains mixed. At the cellular level, early overgrowth is linked to excessive neuronal proliferation and diminished during fetal and perinatal stages, resulting in higher neuron counts in key regions like the . Postmortem analyses reveal increased neuron numbers in upper cortical layers, alongside reduced , which disrupts balanced neurodevelopment. Recent models from ASD patients confirm elevated neural progenitor survival and suppressed , supporting these prenatal origins. A 2025 mini-review synthesizes and histological data, affirming excessive volume growth in the first two years followed by a postnatal slowdown, with overgrowth magnitude correlating to core symptom severity in social communication and repetitive behaviors. Postmortem evidence further highlights cortical disorganization in the , characterized by patches of disrupted laminar architecture and a 20% reduction in the glia-to-neuron , particularly in upper layers, indicating imbalanced cellular composition. Genetic factors, such as mutations in CHD8, contribute to this overgrowth by altering regulation and promoting excess neuronal expansion.

Synaptic formation and dysfunction

, the process of formation during early development, is disrupted in autism spectrum disorder (ASD), leading to atypical dendritic spine dynamics. Dendritic spines, which serve as postsynaptic sites for excitatory , exhibit impaired formation and maturation in ASD models. Postmortem studies of cortical tissue from children with ASD reveal an excessive density of in layer V pyramidal neurons, indicating a in normal developmental refinement. This overabundance stems from reduced macroautophagy-dependent , a mechanism involving the pathway that eliminates surplus ; inhibition of signaling in animal models restores spine density to typical levels, underscoring its role in ASD pathology.00651-5) Synaptic pruning deficits further contribute to local overconnectivity, where reduced elimination of immature s persists into later development. In animal models, such as SHANK3 knockout mice, which carry mutations in a key synaptic scaffolding associated with ASD, there is evidence of age-dependent cortical overconnectivity alongside impaired social behaviors; this overconnectivity is partially reversible with interventions like IGF-1 treatment, highlighting pruning as a modifiable target. These deficits manifest as an imbalance in excitatory and inhibitory s, with upregulation of excitatory and NMDA receptors and downregulation of inhibitory s, altering the excitatory-inhibitory (E/I) ratio. For instance, neuroligin-3 (NLGN3) mutations, such as the R451C variant, increase synaptic turnover at excitatory sites while disrupting transmission, leading to elevated E/I ratios in cortical circuits. Similarly, neurexin-1 (NRXN1) deletions impair adhesion and reduce inhibitory development, exacerbating the imbalance. Mutations in synaptic adhesion proteins like neuroligins and neurexins play a central role in these disruptions by compromising trans-synaptic adhesion necessary for specification. Neuroligins bind presynaptic neurexins to organize postsynaptic densities, and their dysfunction in ASD leads to selective impairments in excitatory versus inhibitory formation; for example, NLGN1 knockout mice display increased dendritic spine density and enhanced excitatory transmission. SHANK3, which anchors neuroligin-neurexin complexes at the postsynaptic density, shows reduced expression in ASD, decreasing spine size and impairing (LTP) in hippocampal neurons. Genetic variants in these synaptic genes, including SHANK3 , are implicated in up to 1-2% of ASD cases, linking molecular defects to broader synaptic pathology. Metabolic support, such as energy provision for ATP-dependent , is also compromised in these mutants, though structural deficits predominate. Recent advances, including 2024 studies, have highlighted the involvement of calcium-binding proteins in synaptic maturation delays pertinent to ASD. Parvalbumin (PV)-expressing , which regulate inhibitory timing, show reduced density and signaling in ASD prefrontal cortex, contributing to delayed maturation of synapses; SHANK3 models exhibit lower PV signals in thalamic regions, increasing susceptibility and E/I imbalance. A 2024 review synthesizes evidence of decreased PV+ in ASD brains, linking this to impaired fast-spiking activity essential for synaptic refinement during critical periods. These findings suggest that PV interneuron dysfunction underlies persistent excitatory overdrive, offering potential biomarkers for early intervention.

Circuit and connectivity alterations

In autism spectrum disorder (ASD), studies utilizing diffusion tensor imaging (DTI) have consistently revealed patterns of altered structural connectivity, characterized by hyperconnectivity within short-range cortical circuits and hypoconnectivity in long-range fronto-temporal tracts. This imbalance is thought to arise from disrupted integrity, with reduced in tracts such as the superior longitudinal fasciculus and uncinate fasciculus, potentially impairing the integration of sensory and cognitive information across distant regions. Such findings suggest that local overconnectivity may lead to excessive processing within sensory areas, while long-range underconnectivity contributes to difficulties in higher-order functions like executive control. These connectivity alterations trace back to developmental origins in the fetal cortex, where anomalous neuronal migration disrupts the formation of cortical minicolumns, resulting in their inversion or abnormal spacing. During early , disruptions in radial and tangential migration—often linked to genetic variants affecting cytoskeletal dynamics—lead to heterotopias and misplaced neurons, which in turn alter the modular organization of cortical circuits. This early miswiring establishes a foundation for later circuit imbalances, with synaptic overgrowth in proximal connections exacerbating local hyperconnectivity. Functional neuroimaging metrics further highlight these disruptions, showing reduced gamma-band synchrony in (EEG) and (MEG) recordings during sensory and cognitive tasks, indicative of impaired coordination. Additionally, altered integration within the (DMN)—evidenced by decreased coherence in posterior cingulate and medial prefrontal regions—correlates with challenges in self-referential processing and . Recent 2025 MEG studies have advanced this understanding, demonstrating delayed auditory evoked responses and reduced cross-hemispheric connectivity in children with ASD, particularly in theta and alpha bands during passive listening paradigms, which may underlie delays. during critical pruning periods has been briefly implicated in worsening these failures, though its role remains under investigation. At the behavioral level, these circuit imbalances manifest in repetitive behaviors through dysregulated basal ganglia-thalamo-cortical loops, where hyperactivity in direct pathways and hypoactivity in indirect pathways promote . Functional MRI evidence indicates increased connectivity in limbic-striatal circuits alongside reduced frontoparietal modulation, linking loop imbalances to restricted interests and stereotyped movements in ASD. This circuitry dysfunction underscores how early connectivity alterations propagate to impair action selection and formation.

Biochemical Mechanisms

Metabolic dysregulation

Metabolic dysregulation in autism spectrum disorder (ASD) encompasses widespread disruptions in cellular processing and nutrient handling that impair function and contribute to neurodevelopmental challenges. Studies indicate that individuals with ASD exhibit altered glucose in key regions, such as reduced uptake and utilization in the and temporal lobes, leading to hypometabolism that affects neural signaling and cognitive processes. Similarly, is compromised, with imbalances in pathways and oxidation in tissue, which disrupt membrane integrity and essential for early development. These core dysregulations create a state of inefficiency, where cells struggle to meet demands for growth and connectivity, potentially exacerbating ASD symptoms like sensory sensitivities and repetitive behaviors. Systemically, metabolomic analyses of plasma reveal distinct profiles in ASD, including elevated levels of branched-chain (BCAAs) such as , , and , which correlate with disease risk and severity. These elevations suggest impaired , possibly linked to genetic variants in metabolic enzymes, resulting in broader imbalances that influence synthesis and . Prenatally, maternal metabolic stress, including high BCAA concentrations in obese or diabetic pregnancies, heightens ASD risk in by altering fetal and increasing susceptibility to neurodevelopmental disruptions. Such systemic effects underscore how peripheral metabolic shifts can propagate to vulnerabilities. Recent research highlights disruptions in as a key factor in ASD, with reduced conversion to serotonin precursors contributing to sleep disturbances and behavioral issues. A 2025 holistic review of urinary in children with ASD identified altered activity, leading to decreased neuroprotective metabolites and heightened , which worsens core symptoms like social withdrawal. Therapeutically, these dysregulations influence symptom severity, as evidenced by N-acetylcysteine (NAC) supplementation, which mitigates oxidative byproducts from metabolic stress and reduces in clinical trials. This approach targets downstream effects of energy imbalances, offering potential for symptom management without addressing root causes. Mitochondrial contributions to these energy deficits may intersect briefly with cycle pathways in one-carbon , amplifying overall dysregulation.

Mitochondrial and energy metabolism

Mitochondrial disease is confirmed in approximately 5% of individuals with autism spectrum disorder (ASD), a rate significantly higher than in the general population, with broader impairments evident in up to 30-50% of cases based on biochemical markers. These impairments often manifest as secondary dysfunction rather than primary genetic mitochondrial disorders, affecting and cellular energy homeostasis. Such prevalence underscores the role of mitochondrial deficits as a contributing mechanism in ASD , particularly in subgroups with elevated lactate levels or abnormal pyruvate indicating impaired energy production. At the molecular level, mutations or deletions in (mtDNA) and nuclear genes regulating mitochondrial function, including which encodes a essential for mtDNA replication, disrupt ATP synthesis by impairing the ETC. For instance, pathogenic mtDNA variants alter respiratory chain efficiency, leading to decreased ATP output in affected cells, while nuclear gene mutations like those in exacerbate mtDNA instability and replication errors. These genetic alterations reduce the capacity for aerobic respiration, forcing reliance on less efficient glycolytic pathways and contributing to chronic energy shortages in neuronal populations. Dysfunctional mitochondrial complexes, particularly I and III, elevate reactive oxygen species (ROS) production, promoting oxidative stress that damages cellular components including proteins, lipids, and DNA. This ROS overproduction stems from electron leakage in the impaired ETC, creating a vicious cycle of mitochondrial damage and further energy depletion. In ASD models, such oxidative imbalances correlate with neuronal vulnerability, as excess ROS disrupts synaptic integrity and signaling without direct involvement in neurotransmitter pathways. Impaired mitochondrial function leads to reduced neuronal energy production, which increases vulnerability to excitotoxicity arising from excessive glutamate activity. Low ATP levels impair the function of energy-dependent glutamate transporters, resulting in elevated extracellular glutamate, prolonged receptor stimulation, calcium influx, and amplified oxidative stress. This cascade exacerbates neuronal damage and is proposed to contribute to the neurodevelopmental abnormalities in ASD, which typically manifest in early childhood through symptoms such as social and communication deficits. Recent 2024 analyses of large cohorts have identified associations between specific mtDNA haplogroups—such as haplogroup K—and modulated ASD risk or severity, suggesting evolutionary adaptations in influence phenotypic expression. These findings highlight haplogroup-specific variations in ETC efficiency and ROS handling as potential modifiers of ASD outcomes. Post-mortem studies of autistic cortex, particularly in regions like 21, demonstrate altered mitochondrial morphology and function, including decreased activities of ETC complexes I and IV despite increased mitochondrial membrane mass, leading to energy failure in high-demand cortical areas. This regional specificity contributes to synaptic and circuit-level deficits, as energy shortages impair neuronal maintenance in socially relevant brain networks. These mitochondrial issues intersect with broader metabolic dysregulation in ASD, briefly amplifying systemic imbalances via excess ROS.

Neurotransmitter systems

Imbalances in neurotransmitter systems contribute significantly to the pathophysiology of autism spectrum disorder (ASD), disrupting neural signaling and circuit function. Key systems implicated include serotonergic, glutamatergic, and GABAergic pathways, where alterations lead to excitatory-inhibitory disequilibria that manifest in core ASD symptoms such as sensory sensitivities and social deficits. Serotonin (5-HT) dysregulation is a prominent feature in ASD, with hyperserotonemia observed in approximately 30% of affected individuals, characterized by elevated whole-blood serotonin levels. This elevation is largely attributed to dysfunction in the (SERT, encoded by SLC6A4), which results in increased platelet uptake and storage of 5-HT, reducing extracellular availability in the . Gain-of-function variants in SERT, such as the Ala56 mutation, further exacerbate hyperserotonemia and are linked to autism-relevant behaviors like sensory aversion in both and models. Glutamatergic signaling, particularly through N-methyl-D-aspartate (NMDA) receptors, exhibits hypofunction in ASD, contributing to reduced excitatory transmission and deficits. This NMDA receptor hypofunction is evident in postmortem brain analyses and animal models, where genetic disruptions lead to ASD-like phenotypes including impaired social interaction. Pharmacological interventions targeting this system, such as the NMDA antagonist , have shown promise; a 2025 randomized in youths with ASD demonstrated significant improvements in social functioning after 12 weeks of treatment, with reducing excessive activation and enhancing social responsiveness, particularly in those with higher baseline impairments. GABAergic hypofunction arises from deficits in inhibitory , leading to diminished GABA release and an overall reduction in inhibitory tone across cortical regions. Postmortem studies reveal reduced numbers and altered distribution of interneurons in ASD brains, particularly in areas involved in , which correlates with intellectual impairment and stereotypic behaviors. In mouse models of ASD, such as those with Shank3 mutations, cortical interneuron dysfunction directly causes sensory hyper-reactivity due to impaired GABA-mediated inhibition. Emerging research highlights the role of trace amine-associated receptor 1 () in modulating and serotonin signaling within the gut-brain axis, offering potential therapeutic avenues for ASD. A 2025 review posits that agonists can alleviate ASD-like symptoms by regulating release and mitigating gut-derived trace amine dysregulation, which influences central serotonergic and . In preclinical models, activation attenuates and linked to prenatal valproic acid exposure, a common ASD model. These imbalances culminate in circuit-level hyperexcitability, particularly in sensory cortices, where reduced inhibition amplifies responses to stimuli. Electrophysiological studies in ASD indicate heightened neural responsiveness in auditory and somatosensory areas, driven by overdrive and underactivity, which underlies sensory hyper-reactivity reported in up to 90% of individuals with ASD.

Folate and redox metabolism

Disruptions in the cycle have been implicated in autism spectrum disorder (ASD), particularly through cerebral folate deficiency (CFD) caused by autoantibodies against folate receptor alpha (FOLR1), which impair transport across the blood-brain barrier. These autoantibodies block the uptake of 5-methyltetrahydrofolate, the active form of , leading to reduced levels in a of individuals with ASD. This deficiency disrupts one-carbon , a pathway essential for , repair, and processes, thereby contributing to neurodevelopmental abnormalities observed in ASD. In parallel, redox metabolism is altered in ASD, characterized by reduced levels of reduced glutathione (GSH), the primary cellular antioxidant, and elevated oxidized glutathione (GSSG), resulting in a decreased GSH/GSSG ratio and heightened oxidative stress. This imbalance promotes oxidative damage to cellular components, including lipids, proteins, and DNA, in brain tissues of individuals with ASD. Postmortem studies of ASD brains have confirmed lower GSH and GSH/GSSG ratios compared to controls, suggesting functional consequences for neuronal health and connectivity. Genetic variants in the (MTHFR) gene, such as the C677T polymorphism, are associated with increased levels and elevated ASD risk primarily in populations of European ancestry, though recent studies in other groups like cohorts show no significant association; the polymorphism has been linked to more severe symptoms in affected individuals where present. Elevated , a byproduct of impaired folate-dependent remethylation, correlates linearly with ASD severity and may exacerbate by depleting GSH reserves. These MTHFR variants highlight how pathway inefficiencies intersect with dysregulation to influence ASD pathophysiology. metabolism also underpins epigenetic , where deficiencies can lead to global DNA hypomethylation in neurons. At the cellular level, these disruptions manifest as DNA hypomethylation and protein oxidation in neurons, altering and in ASD. GSSG accumulation reacts with protein sulfhydryl groups, inducing oxidative modifications that impair enzymatic function and contribute to neuronal dysfunction. Such outcomes underscore the interconnected roles of and pathways in maintaining genomic stability and cellular resilience during neurodevelopment. Recent metabolomic studies, including a 2025 , have demonstrated that high-dose supplementation improves social behaviors and reciprocity in children with ASD, particularly those with abnormalities. This intervention bypasses FOLR1 blockade by providing a reduced form that enhances one-carbon flux and balance, offering a targeted therapeutic approach supported by FDA recognition in 2025 for addressing autism-related symptoms in CFD cases.

Neuroinflammation

refers to chronic inflammatory processes within the that have been implicated in the of autism spectrum disorder (ASD), involving dysregulated immune responses that disrupt neural development and function. , the resident immune cells of the , exhibit persistent activation in post-mortem examinations of autistic brains, leading to excessive release of pro-inflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α). This activation is observed across multiple brain regions, including the and , and is associated with neuronal damage and altered connectivity. In ASD, activated microglia fail to return to a quiescent state, perpetuating a cycle of inflammation that interferes with normal maturation. Prenatal triggers, particularly maternal immune activation (MIA), contribute to through mechanisms involving IL-17-producing T helper 17 (Th17) cells, which induce cortical malformations in offspring. Animal models of MIA, induced by viral mimetics like polyinosinic:polycytidylic acid (poly(I:C)), demonstrate that elevated maternal IL-17 levels disrupt cortical layering and neuronal migration, mirroring neurodevelopmental abnormalities seen in ASD. Neutralizing IL-17 during pregnancy in these models prevents behavioral deficits and structural changes in the offspring brain. Peripheral immune dysregulation is evident in ASD, with meta-analyses revealing consistently elevated levels of pro-inflammatory cytokines, including IL-6, TNF-α, and interferon-gamma (IFN-γ), in blood samples from individuals with ASD compared to neurotypical controls. These elevations correlate with symptom severity and may reflect systemic inflammation that crosses the blood-brain barrier to exacerbate central neuroinflammatory processes. Recent studies from 2024 highlight the role of in sustaining in ASD, showing that astrocytic dysfunction amplifies microglial activation and production through altered landscapes and metabolic shifts. For instance, autism-associated mutations in the CHD8 gene in lead to reactive , promoting a pro-inflammatory environment that impairs neuronal support. Additionally, glutamate-induced astrocytic responses in ASD models contribute to sustained and . The outcomes of neuroinflammation in ASD include disrupted synaptic pruning, where overactivated microglia excessively eliminate synapses or fail to refine circuits, leading to an imbalance in excitatory-inhibitory signaling. This dysregulation is linked to regressive features in some ASD cases, where acute inflammatory events, such as infections, trigger loss of previously acquired skills through heightened cytokine-mediated neuronal stress. In regressive ASD, neuroinflammation correlates with elevated immune activation markers and behavioral decline, suggesting a role in episodic worsening of symptoms. Gut-derived inflammatory signals may briefly amplify this central inflammation, while metabolic stress from energy deficits can further intensify glial responses.

Gut-immune-brain axis

The gut-immune-brain axis in autism spectrum disorder (ASD) involves bidirectional communication between the , , and , where alterations in composition—known as —play a central role in influencing brain function and behavior. Studies have consistently identified microbiome in individuals with ASD, characterized by reduced abundance of species and increased levels of , alongside lower overall microbial diversity that promotes the overgrowth of potentially harmful . This is associated with elevated production of microbial metabolites such as p-cresol, a derived from like difficile, which correlates with increased and behavioral severity in children with ASD. Key mechanisms underlying this axis include microbial metabolites like short-chain fatty acids (SCFAs)—including , propionate, and butyrate—that are produced through bacterial of dietary fibers and modulate blood-brain barrier (BBB) permeability to regulate and neuronal signaling. Children with ASD often exhibit altered SCFA levels, with some studies reporting reductions in butyrate, which may impair BBB integrity and disrupt gut-brain signaling via the , a primary that transmits microbial signals to the and influences autonomic and emotional regulation. Additionally, contributes to an immune bridge through increased , or "leaky gut," which allows luminal contents to enter the bloodstream, triggering and the production of autoantibodies, such as those targeting folate receptor alpha (FRα), thereby hindering transport to the and exacerbating neurodevelopmental deficits; however, the causality of leaky gut in ASD remains a with mixed . Recent advances highlight the therapeutic potential of targeting this axis. A 2025 review proposes modulating trace amine signaling within the gut using models such as larval or germ-free conditions to address ASD , underscoring the role of microbial influences in neuro-immune . Clinical interventions like microbiota transfer therapy (MTT), involving fecal microbiota transplantation from healthy donors, have shown promise in reducing gastrointestinal () symptoms and improving behavioral outcomes in children with ASD, with benefits persisting for up to two years post-treatment through increased microbial diversity and decreased pathogenic metabolites. These gut alterations are also linked to co-occurring behavioral features in ASD, such as heightened anxiety and issues, where dysbiotic profiles correlate with amplified emotional reactivity and sensory responsivity independent of discomfort.

Regional Brain Abnormalities

Temporal lobe structures

The temporal lobe, particularly structures such as the superior temporal sulcus (STS) and planum temporale, exhibits structural abnormalities in individuals with autism spectrum disorder (ASD). Magnetic resonance imaging (MRI) studies have identified enlargement of the right superior temporal gyrus, which borders the STS, in children and adolescents with ASD compared to typically developing peers. This enlargement may reflect early developmental perturbations in cortical folding and gyral formation within the STS region, a key area for integrating sensory and social information. Additionally, reduced leftward asymmetry in the planum temporale—a language-related area on the superior temporal gyrus—has been consistently observed in ASD, contrasting with the pronounced left-lateralization typical in neurotypical brains; this asymmetry reduction is evident from childhood and persists into adolescence. Autopsy studies reveal cellular-level disruptions in temporal lobe architecture, contributing to these macrostructural changes. Minicolumnar disarray, characterized by narrower minicolumn width (approximately 20% reduction), increased cell packing density, and disrupted alignment of pyramidal neurons, has been documented in Layer III of the superior temporal cortex (Brodmann areas 21 and 22) in postmortem ASD brains. Pyramidal neuron morphology is also altered, with evidence of smaller somata, reduced dendritic spine density, and compromised mitochondrial function in temporal lobe pyramidal cells, suggesting impaired energy metabolism and synaptic connectivity at the cellular level. These findings indicate a neuropathological basis for disrupted local processing in temporal regions, potentially stemming from aberrant neurogenesis or migration during early brain development. Functionally, these structural anomalies manifest as hypoactivation in areas during social tasks. Functional MRI (fMRI) evidence shows reduced activation in the posterior, middle, and anterior STS, as well as the (FFA), when individuals with ASD process faces. Recent 2024 meta-analyses of voxel-based morphometry studies confirm temporal gray matter reductions, particularly in the right , across age groups in ASD, highlighting a potential shift from early overgrowth to later volume loss. These disruptions are linked to core ASD symptoms in and social domains. Abnormalities in the and STS contribute to deficits in prosody perception and production, as these regions process intonational cues essential for communicative intent; individuals with ASD often exhibit flattened or atypical prosodic patterns tied to temporal hypoactivation. Similarly, —repetitive echoing of speech—may arise from impaired integration of auditory and linguistic inputs, leading to rote without semantic comprehension, as supported by of networks in ASD.

Social brain networks

Social brain networks in autism spectrum disorder (ASD) encompass interconnected regions critical for processing social stimuli, such as faces, emotions, and intentions, and exhibit atypical patterns of activation and connectivity that contribute to social impairments. These networks integrate inputs from multiple cortical and subcortical areas to support adaptive , but in ASD, disruptions lead to inefficient information flow and reduced . studies have consistently identified imbalances in these networks as a core neurobiological feature of ASD, distinguishing them from typical development. Key nodes within these networks show characteristic alterations in ASD. The amygdala, a central hub for emotional salience detection, demonstrates hyperresponsivity to social stimuli, particularly faces with direct gaze, which may underlie heightened arousal and avoidance behaviors in social contexts. This elevated amygdala response is potentiated during tasks involving eye contact, suggesting an exaggerated threat detection mechanism that disrupts normal social processing. In contrast, the fusiform face area (FFA), specialized for face recognition, exhibits hypoactivation during face perception tasks, correlating with difficulties in identifying facial expressions and identities. Structural analyses further reveal fewer and smaller neurons in the fusiform gyrus in ASD, contributing to this reduced responsiveness. Additionally, the medial prefrontal cortex (mPFC), involved in self-referential and social inference processes, shows underconnectivity with distant brain regions, impairing the integration of social cues with personal perspective-taking. These node-specific changes highlight a pattern of localized hyperactivity alongside broader hypoconnectivity in social networks. Network dynamics in ASD are marked by reduced synchrony, particularly in mirror neuron systems that facilitate action understanding and , though the role of the mirror neuron system in ASD remains debated. During imitation tasks, individuals with ASD display diminished activation and coupling between premotor areas and parietal regions, leading to poorer synchronization of observed and executed movements. This reduced mirror neuron system (MNS) engagement is evident in functional MRI studies, where atypical connectivity patterns hinder the automatic mapping of others' actions onto one's own motor representations, contributing to imitation deficits. Such dynamics extend to real-time social interactions, where desynchronized neural responses impair reciprocal behaviors. In ASD, social brain regions like the superior temporal sulcus and mPFC show reduced long-range connectivity in adolescence and adulthood, resulting in decoupled network function. These trajectories suggest that disruptions in social circuit refinement contribute to persistent imbalances. Recent advances in 2025 EEG studies have illuminated reduced mu rhythm suppression during social observation in ASD, providing a non-invasive marker of MNS dysfunction. In tasks involving action or gesture observation, individuals with ASD show attenuated desynchronization of the 8-13 Hz mu rhythm over sensorimotor areas, correlating with impaired social attention. These findings, from high-density EEG analyses, underscore context-dependent alterations in oscillatory synchrony and offer potential for early biomarkers. One such study revisited mu suppression profiles, confirming blunted responses in ASD during dynamic social scenes, independent of aperiodic EEG components. These network imbalances directly correlate with (ToM) deficits in ASD, where impaired connectivity between mPFC, , and hinders mental state attribution. Disrupted synchrony in these circuits predicts poorer performance on ToM tasks, such as false-belief understanding, with underconnectivity explaining up to 30% of variance in scores. regions, including the , contribute briefly to face recognition within these networks, but the primary deficit lies in their integration. Overall, such correlations link neurobiological disruptions to observable social symptoms, informing targeted interventions.

Cognitive Mechanisms

Social cognition

Social cognition in autism spectrum disorder (ASD) encompasses deficits in processing social information, including the recognition of , understanding others' mental states, and engaging in reciprocal social interactions. These impairments are central to the social communication challenges observed in individuals with ASD and are evident from . Core deficits include difficulties in recognizing from facial expressions and vocal tones, which hinder the interpretation of . For instance, individuals with ASD often show reduced accuracy in identifying basic such as or in static faces or dynamic expressions. A hallmark of deficits in ASD is impaired (ToM), the ability to attribute mental states like beliefs and intentions to oneself and others. Seminal work demonstrated that children with ASD frequently fail false-belief tasks, such as the Sally-Anne test, where they struggle to predict behavior based on another's mistaken belief rather than reality. This ToM impairment persists into adulthood and correlates with real-world social difficulties, though some individuals develop compensatory strategies over time. Neural correlates of these social cognition deficits involve atypical engagement of key brain regions. Functional neuroimaging studies reveal underactivation in the during emotion processing tasks, contributing to reduced emotional salience detection in social stimuli. Similarly, the (TPJ), implicated in mentalizing, shows diminished activity in individuals with ASD when inferring others' intentions, supporting a disrupted social brain network. These findings underscore how structural and functional anomalies in these areas underlie behavioral impairments. Developmentally, social cognition deficits in ASD often trace back to early disruptions in joint attention, the ability to coordinate focus on an object or event with another person around 9-12 months of age. Longitudinal studies indicate that delays in initiating or responding to joint attention bids predict later severity of social impairments and ToM failures, highlighting a cascading effect from infancy. Early interventions targeting joint attention can mitigate some trajectories, emphasizing the importance of timely detection. Recent advances have illuminated gender differences in , with 2024 research showing that autistic females often mask deficits more effectively than males through camouflaging behaviors, leading to underdiagnosis. This masking, involving conscious imitation of neurotypical social responses, may explain why females exhibit subtler impairments despite similar neural underengagement in the and TPJ. These findings suggest the importance of gender-sensitive assessments to capture these nuances. Assessment of in ASD commonly employs tools like the (ADOS), particularly its modules evaluating social reciprocity. The ADOS-2 social affect domain scores impairments in emotional sharing, conversational turn-taking, and , providing standardized metrics for and intervention planning. These tools integrate behavioral observations with ToM probes to quantify deficits reliably across ages. Briefly, these social cognition challenges overlap with in tasks requiring integrated , while relying on the broader social brain network for coordinated processing.

Nonsocial cognition and executive function

Individuals with autism spectrum disorder (ASD) often display impairments in nonsocial cognitive processes, including that support planning, problem-solving, and self-regulation independent of social contexts. These functions encompass , which involves adapting to changing rules or stimuli; , which enables suppression of irrelevant responses; and , which maintains and manipulates information temporarily. A comprehensive review of 26 studies found consistent deficits in these areas among children with ASD compared to typically developing peers, with particular challenges in leading to perseverative errors on tasks like the (WCST), where ASD participants exhibited slower reaction times and higher error rates. Inhibition deficits were less pronounced in pure ASD than in comorbid ASD-ADHD cases, while impairments manifested as increased errors across verbal and visuospatial tasks in both groups. Attention profiles in ASD are uneven, featuring strengths in detail-oriented processing and hyperfocus on specific elements but notable weaknesses in attentional shifting. Autistic individuals demonstrate enhanced sustained attention on local details, often outperforming peers in tasks requiring focused perceptual analysis, yet they struggle with disengaging from one focus to redirect toward new information, resulting in delayed response times during set-shifting paradigms. This pattern aligns with electrophysiological evidence of prolonged attentional capture by non-social stimuli, contributing to rigid behavioral patterns without directly impairing overall cognitive flexibility. Memory performance in ASD reveals a dichotomy, with superior rote memorization of isolated facts contrasting impairments in semantic tasks that demand integrative processing. Early studies documented exceptional recall of verbatim details, such as word lists or dates, in autistic individuals, attributed to strong item-specific encoding. However, challenges arise in central coherence-dependent tasks, where semantic relatedness hinders free recall or organization of information into meaningful wholes, leading to poorer performance relative to verbal ability-matched controls. Recent advances highlight the role of bilingualism in modulating executive function variability among children with ASD. A 2025 scoping review of 15 cross-sectional studies involving 982 ASD children found that bilingual participants exhibited enhanced accuracy on tasks, improved on Dimensional Change Card Sort tasks comparable to typically developing peers, and superior on Stroop-like measures, with faster response times than monolingual ASD counterparts. These benefits suggest bilingualism may amplify intra-individual variability in executive performance, potentially buffering core deficits. Heterogeneity in nonsocial and executive function within ASD is closely tied to IQ disparities, with high-functioning individuals showing distinct profiles that include preserved or even superior in structured, non-social contexts despite overall variability. Across IQ levels, executive impairments persist but vary in severity, with higher-IQ ASD subgroups demonstrating relative strengths in metacognitive on tasks like the , though linked to reduced adaptive outcomes. This variability underscores domain-specific differences, where and show greater individual fluctuations than flexibility, influencing functional independence.

Theoretical models

The weak central coherence theory posits that individuals with autism spectrum disorder (ASD) exhibit a favoring local processing of details over global integration of , leading to superior performance on tasks requiring to specifics, such as the embedded figures test. This theory, originally proposed by , suggests that the reduced drive for central coherence underlies core ASD features like restricted interests and repetitive behaviors by impairing the ability to derive overarching meaning from sensory input. from meta-analytic reviews supports this, showing consistent deficits in global processing across visual and linguistic domains in ASD populations. The theory proposes that impairments in higher-order cognitive processes, particularly planning, , and , serve as a unifying mechanism for ASD symptoms, linking both social and nonsocial deficits to inefficiencies. This framework, drawing from neuropsychological parallels, attributes difficulties in adapting to change and organizing to core executive deficits rather than isolated gaps. Meta-analyses confirm broad executive impairments in ASD, with moderate effect sizes for flexibility and planning tasks stable across developmental stages. The empathizing-systemizing theory, developed by Baron-Cohen, argues that ASD arises from an extreme cognitive profile of reduced empathizing (intuitive understanding of others' ) combined with intact or enhanced systemizing (rule-based analysis of patterns), which explains social communication challenges and strengths in logical domains. This imbalance, potentially influenced by prenatal testosterone, accounts for biases in ASD , with males showing more extreme systemizing. deficits, such as impairments, align with the reduced empathizing component of this model. Meta-analytic evidence validates reductions in ASD, particularly cognitive , with effect sizes moderated by age and measurement type. Recent theoretical advances integrate these models with frameworks, proposing that ASD involves an over-reliance on bottom-up sensory data due to weakened top-down predictions, resulting in heightened perceptual detail focus and reduced contextual inference. This synthesis reframes weak central coherence and executive issues as manifestations of imbalanced predictive hierarchies, where prediction errors from sensory inputs dominate over prior expectations. Such integrations highlight how atypical weighting in hierarchical brain processing could unify disparate ASD symptoms. Overall empirical support for these theories stems from meta-analyses demonstrating convergent behavioral validation, including consistent local processing advantages and executive impairments across large ASD samples, though heterogeneity in task paradigms underscores the need for unified testing.

Perceptual processing issues

Individuals with autism spectrum disorder (ASD) frequently exhibit atypical sensory sensitivities, manifesting as hyper- or hyposensitivity across modalities, with pronounced effects in auditory and visual domains. Approximately 90% of individuals with ASD display such sensory reactivity differences, including over-responsivity to everyday sounds like vacuums or lights, which can prompt avoidance behaviors, and hyposensitivity leading to seeking intense stimulation such as spinning objects. In the auditory domain, neurophysiologic studies reveal delayed evoked responses and altered discrimination of speech versus non-speech sounds, contributing to heightened sensitivity or muted reactions. Visually, enhanced detection of motion in high-contrast stimuli has been documented, with autistic children demonstrating significantly faster reaction times—up to 20% quicker—compared to neurotypical peers, suggesting superior low-level perceptual acuity for dynamic elements. Categorical perception in ASD is marked by deficits in prototype formation, leading to piecemeal recognition of faces and objects rather than holistic abstraction. High-functioning individuals with autism are less likely to select mean prototypes in tasks involving subtle spatial distortions of faces, with adults showing four times lower odds of doing so compared to controls, indicating reliance on detailed, exemplar-based processing over generalized categories. Reviews of prototype-distortion paradigms confirm that autistic participants struggle more with abstracting central tendencies, often fixating on local features and ignoring distortions, which impairs efficient categorization. This detail-oriented style aligns with a single-sentence reference to weak central coherence theory, positing a perceptual toward local processing that enhances feature detection but hinders integration. The neural underpinnings involve atypical activation in perceptual brain regions, including overactivation in temporal areas such as the (STS) during processing of sensory details, which may drive the focus on local elements. Functional neuroimaging shows abnormal STS engagement in response to visual and auditory inputs, with patterns of hyperactivation in posterior temporal regions linked to enhanced detail in non-social stimuli. Recent 2025 literature reviews on savant abilities in ASD emphasize this enhanced perceptual functioning, attributing exceptional skills—like prodigious memory for calendars or art—to automatic, detail-centric processing without typical global integration, observed in about 59% of savants who also have ASD. Behaviorally, these perceptual processing issues contribute to difficulties in generalization during learning, as impaired category formation limits the transfer of to novel contexts and fosters rigid, idiosyncratic representations. For instance, atypical learning of perceptual categories from random dot patterns or abstract shapes results in reduced flexibility, exacerbating challenges in such as adapting to new social or environmental demands. This can perpetuate restricted interests and hinder broader skill acquisition over time.

References

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