Autism's symptoms result from maturation-related changes in various systems of the brain. The behaviors appear to have multiple pathophysiologies. Autism appears to result from developmental factors that affect many or all functional brain systems, and to disturb the timing of brain development more than the final product. Neuroanatomical studies and the associations with teratogens strongly suggest that autism's mechanism includes alteration of brain development soon after conception. Just after birth, the brain of an autistic child grows faster than usual, followed by normal or relatively slower growth in childhood. The early overgrowth seems to be most prominent in areas underlying the development of higher cognitive specialization.
- Disturbed neuronal migration during early gestation.
- Unbalanced excitatory–inhibitory networks. or by poorly regulated synthesis of synaptic protein. Disrupted synaptic development may also contribute to epilepsy, which may explain why the two conditions are associated.
Several neurotransmitter abnormalities have been detected in autism, notably increased blood levels of serotonin. Whether these cause structural or behavioral abnormalities is unclear. Also, some inborn errors of metabolism are associated with autism but probably account for less than 5% of cases. 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 Asperger syndrome, and a correlation between reduced MNS activity and severity of the syndrome in children with ASD. However, individuals with autism also have abnormal brain activation in many circuits outside the MNS and the MNS theory does not explain the normal performance of autistic children on imitation tasks that involve a goal or object.
ASD-related patterns of low function and aberrant activation in the brain differ depending on whether the brain is doing social or nonsocial tasks.
In autism there is evidence for reduced functional connectivity of the default network, a large-scale brain network involved in social and emotional processing, with intact connectivity of the task-positive network, used in sustained attention and goal-directed thinking. In people with autism the two networks are not negatively correlated in time, suggesting an imbalance in toggling between the two networks, possibly reflecting a disturbance of self-referential thought. A 2008 brain-imaging study found a specific pattern of signals in the cingulate cortex which differs in individuals with ASD.
The underconnectivity theory of autism hypothesizes that autism is marked by underfunctioning high-level neural connections and synchronization, along with an excess of low-level processes. Evidence for this theory has been found in functional neuroimaging studies on autistic individuals and by a brain wave study that suggested that adults with ASD have local overconnectivity in the cortex and weak functional connections between the frontal lobe and the rest of the cortex. Other evidence suggests the underconnectivity is mainly within each hemisphere of the cortex and that autism is a disorder of the association cortex.
From studies based on event-related potentials, transient changes to the brain's electrical activity in response to stimuli, there is considerable evidence for differences in autistic individuals with respect to attention, orientiation to auditory and visual stimuli, novelty detection, language and face processing, and information storage; several studies have found a preference for non-social stimuli. For example, magnetoencephalography studies have found evidence in autistic children of delayed responses in the brain's processing of auditory signals.
Neuropsychology
Two major categories of cognitive theories have been proposed about the links between autistic brains and behavior.
The first category focuses on deficits in social cognition. The empathizing–systemizing theory postulates that autistic individuals can systemize—that is, they can develop 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 in whom systemizing is better than empathizing;
These theories are somewhat related to the 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 autistic children's atypical responses to the Sally–Anne test for reasoning about others' motivations, and the mirror neuron system theory of autism described in ''Pathophysiology'' maps well to the hypothesis.
The second category focuses on nonsocial or general processing. Executive dysfunction hypothesizes that autistic behavior results in part from deficits in working memory, planning, inhibition, and other forms of executive function. A strength of the theory is predicting stereotyped behavior and narrow interests; two weaknesses are that executive function is hard to measure and that executive function deficits have not been found in young autistic children. A related theory—enhanced perceptual functioning—focuses more on the superiority of locally oriented and perceptual operations in autistic individuals. These theories map well from the underconnectivity theory of autism.
Neither category is satisfactory on its own; social cognition theories poorly address autism's rigid and repetitive behaviors, while the nonsocial theories have difficulty explaining social impairment and communication difficulties. A combined theory based on multiple deficits may prove to be more useful.
