Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by differences in social interaction, communication, and patterns of behavior or interests. These characteristics are thought to arise from differences in how various parts of the brain form and connect. These brain differences are complex and vary considerably among individuals, meaning no single pattern applies to every person with autism.
Differences in Brain Structure
The amygdala, a brain region involved in processing emotions like fear and anxiety, shows varied differences in individuals with autism. Some studies suggest autistic children may have an enlarged amygdala early in development, with this difference potentially leveling off over time. Other research indicates that the amygdala might be smaller in some autistic individuals, particularly if they also experience anxiety. Cells within the amygdala, along with the hippocampus and limbic system, can be smaller and more densely packed in individuals with ASD.
The cerebellum, located at the back of the brain, is involved in coordinating movement, balance, and also plays a role in cognition and social interaction. Studies have found that individuals with autism often have decreased amounts of brain tissue in certain parts of the cerebellum. These structural differences may contribute to challenges in motor skills, coordination, and social communication observed in autism.
The corpus callosum, a bundle of nerve fibers connecting the brain’s two hemispheres, facilitates interhemispheric communication. Some research indicates that individuals with autism may have a larger-than-average corpus callosum, which has been linked to improved communication between the hemispheres. However, other studies suggest that reduced integrity in long-range interhemispheric fiber tracts is commonly reported.
The frontal and temporal lobes, responsible for higher cognitive functions like decision-making, problem-solving, social behavior, language processing, and memory, also show variations in autism. Children with ASD may exhibit lower gray and white matter volumes in the frontal lobe. Structural differences in the temporal lobe can affect language development, auditory processing, and the interpretation of facial expressions.
Differences in Brain Function and Connectivity
Neural connectivity refers to how different brain regions communicate with each other. In individuals with autism, there can be atypical patterns of both long-range and local connectivity. This can manifest as under- or over-connectivity, affecting how information is processed across the brain, including reduced integrity in some long-range fiber tracts.
Functional activity patterns describe how specific brain networks activate during tasks. The Default Mode Network (DMN), a brain system involved in self-referential processing and understanding others’ mental states, often shows altered functional and structural organization in ASD. Atypical integration of information about self and others within the DMN is thought to underlie social deficits in autism. Studies have found a global pattern of underconnectivity within the DMN and between the DMN and other brain regions in adolescents with ASD.
Sensory processing regions, such as the auditory, visual, and tactile cortices, may display atypical responses in individuals with autism. This can contribute to differences in how sensory input is experienced and interpreted.
The mirror neuron system is hypothesized to play a role in empathy and imitation. Functional alterations in social brain and visuospatial brain regions, including those associated with the mirror neuron system, have been observed in autism. Weaker effective connectivity paths in social brain areas have also been noted.
Neurochemical Influences
Neurotransmitters are chemical messengers in the brain, and their dysregulation is believed to influence brain function in autism. Serotonin, which regulates mood, sleep, and social behavior, has shown altered levels in individuals with ASD. Elevated serotonin levels in the blood were reported, potentially linked to repetitive behaviors and social difficulties.
Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter that calms brain activity. In autism, there may be an imbalance between GABA and glutamate, with an overabundance of excitatory glutamate and insufficient inhibitory GABA. This imbalance can lead to neuronal hyperexcitability and cognitive dysfunction.
Glutamate is the main excitatory neurotransmitter in the brain, stimulating brain function. An imbalance with GABA, where glutamate is relatively overactive, can contribute to overstimulation, sensory overload, and difficulties in processing social information.
Oxytocin, sometimes called the “love hormone,” plays a role in social bonding, empathy, and trust. Research indicates that individuals with ASD often have altered levels of oxytocin or its receptors. These differences could contribute to challenges in forming social connections and interpreting emotional cues.
Developmental Aspects of Brain Differences
Brain differences in autism are not static and can evolve over the lifespan. One hypothesis suggests an initial period of rapid brain growth in some infants who later develop autism. This early brain overgrowth, particularly in cerebral, cerebellar, and limbic structures, occurs during the first two years of life.
Following this early overgrowth, brain development in autism may show abnormally slow or arrested growth. These atypical developmental trajectories mean that brain differences change with age, influencing how symptoms present at different life stages.
The brain’s ability to adapt, known as plasticity, is particularly pronounced in early life. Early interventions can influence these developmental trajectories and potentially mitigate the severity of core autism features. Behavioral improvements from early intervention are thought to result from changes in brain structure and function, capitalizing on the brain’s capacity for change during these formative years.