Australasian Science: Australia's authority on science since 1938

New Insights into the Autistic Brain

By Gio Braidotti

Studies of the brain have identified a physiological basis for autism’s impact on human perception, but new technology is making it possible to develop a biologically based diagnostic tool.

Differences in the way our brains process visual information have been detected among people with varying degrees of autistic behaviour, achieving something long sought-after in clinical psychology – a way to diagnose autism biologically rather than behaviourally. This would make autism detectable much sooner, allowing earlier remedial strategies that could, in turn, lead to better lifelong outcomes.

The discovery of biomarkers by Alexandra Sutherland and Prof David Crewther of Swinburne University of Technology is regarded as a groundbreaking development that could significantly improve the management of this difficult and highly stressful condition, especially for parents.

Crewther says that in the absence of biomarkers, autism has been defined exclusively on the basis of behaviour, a situation he laments because it imposes problematic limits on the age at which reliable diagnosis can be made. “Since case histories strongly indicate that early intervention leads to improved lifelong outcomes, there has been a push to identify so-called ‘biomarkers’ for autism – simple but reliable biological tests that predict the presence of autism and can be applied at infancy,” he says.

The Swinburne study found that people with higher autistic tendencies process visual information differently, relying on neurons that are more sensitive to colour and spatial texture but less responsive to motion and rapid flicker. The difference can be detected simply by recording the brain’s electrical activity over the visual cortex. Measurements can be made in infants as long as they visually fixate (stare at) something, and Crewther says that autistic children tend to do this well.

In the absence of biomarkers, autism is particularly tricky to diagnose as it is defined by a list of behaviours that occur to a greater or lesser degree in the entire population, without a distinct line separating pathological severity from a common trait.

As a result, clinicians speak of a “spectrum” in which autistic behaviours occur with increasing severity, as measured by the Autism Spectrum Quotient (AQ). The AQ was developed by UK researcher Dr Simon Baron-Cohen, and consists of 50 statements related to behaviours typical of autism, such as difficulties with social interaction, communication, motor skills, sensory processing and a tendency towards repetitive behaviour.

Test subjects are required to specify whether they “definitely” or “slightly” agree or disagree with the statements, and score one point for each question that is answered “autistically”. The higher the AQ score, the further along the spectrum an individual lies.

“Everyone has some degree of autistic tendency, expressed in terms of socialisation preference, scope of imagination, level of rigidity in opinion and whether or not we are fascinated by patterns or numbers,” Crewther says. “The average score among the population is 16, whereas scores of 32 or more indicate clinically significant levels of autistic traits. Subtler forms of the condition occur between these scores, including autism spectrum conditions (ASD) such as Asperger’s syndrome at scores below 26.”

Studies have found that high AQ scores are prevalent among engineers and computer and physical scientists. For instance, a group of 16 British Mathematical Olympiad winners scored an average of 24.

Because of this link with “savantism” – abilities that go beyond what normal people can do – Crewther thinks that a certain appeal for ASD has emerged in popular culture. Wired magazine affectionately refers to it as the “Geek Syndrome”, and actors on the TV show The Big Bang Theory are winning Emmy Awards portraying scientists with classic ASD and Asperger’s behaviours.

While these portrayals typically involve male characters, Crewther says that women can also be affected. For clinically diagnosed autism, the ratio of male and female patients is about three to one.

While behavioural studies have dominated the classification of autism, Crewther believes that the entire autism spectrum arises from brain-related functional changes that can be studied by using the techniques of cognitive neuroscience. This involves observing brain electrical activity and measuring performance while subjects undertake activities on a computer screen.

Of all the autistic traits to choose from, it was sensory processing differences that Crewther and Sutherland targeted for their study. “My interest is very much in vision, and there is evidence that autistic individuals do experience perceptual differences,” Crewther says.

“Typically affected is the visual search capacity. This involves the ability to pick out shapes embedded in more complex images, and people with autistic tendencies perform better than normal. However, the ability to detect motion can be impaired, and the brain can respond abnormally to faces, especially when the eyes and mouth are moving.”

Visual information from the retinas is processed in the visual cortex, which is located at the back of the head, where Crewther and Sutherland place electrodes on their volunteers. The aim was to delve deep into neural pathways that respond to different elements in the visual field, such as colour, motion or spatial texture.

Ultimately, they were able to test whether the reported autistic deficits and gains in visual perception could be attributed to differences in the same neurons implicated in dyslexia – the “magnocellular” pathway. Crewther says that magnocellular neurons are more sensitive to motion and rapidly flickering and low-contrast stimuli, but less sensitive to colour and fine spatial textures than visual parvocellular neurons. They are also faster.

“We designed tests that measured differences in the time required by the visual system to recover from one set of stimuli, and fire again properly,” Crewther says. “It’s called neural recovery. Our ability to directly measure it is helping us with our studies of autism.”

Volunteers were recruited online and tested with the AQ questionnaire. Those scoring low (11 or less) and high (20 to 34) were invited to take part in the study. While more members of the high AQ group were employed in technical sectors, Crewther says that all participants came from the normal population mix and had equivalent non-verbal intelligence.

Volunteers performed two psychophysical tasks. The motion test involved dots drifting across a computer screen. To overwhelm the visual system, ever more randomly drifting dots are let loose until it becomes impossible to discern which direction they are moving in. This test provides a threshold measure of motion coherence.

The second task used Navon figures to test for form recognition. These are figures like the letter “H” that are drawn using repetitions of a smaller letter (Fig. 1). Volunteers are then tested for their ability to rapidly recognise either the large “global” form or the smaller “local” form. While global forms are generally perceived more quickly, autistic people struggle to recognise these when the letters are camouflaged in one of two colours, breaking up the global form.

“With electrodes in place on our volunteers’ scalps, we did electrophysiology with computer screens set at two contrasts – black and white (96% contrast) versus grey on grey (24% contrast),” Crewther says. “At low contrast we did not find much difference between the low and high AQ group, except for a weaker initial cortical response. At high contrast we get this amazing difference.”

In the high AQ group they observed a delay in completion of the magnocellular processing. This interference occurred only after the arrival of signals from the visual parvocellular neurons to the cortex.

The delay can explain autistic deficits in the ability to see global forms instead of fragmented detail. Global forms are a prerequisite for recognising faces … and making eye contact.

“As we tested individual volunteers we could see that at low contrast everybody’s brain is processing the same way, and then at high contrast this difference jumps out,” Crewther says. “It means that people along the AQ scale perceive the world very differently. But we are not saying that perception, anywhere along the spectrum, is right or wrong.”

The test results ultimately predicted membership in the high and low AQ groups with 85% accuracy.

“What is wonderful about finding biomarkers for autism is that we can go beyond diagnostics,” Crewther says. “For example, where we saw the high AQ subjects not getting the full grouping for Navon figures this suggests ways in which we can address this apparent cognitive ‘deficit’ with training.”

In ongoing work, the Swinburne researchers are now applying their biomarker to infants to develop a diagnostic test, a process that first requires building up data about how the visual response changes as children develop. There is also scope to apply the biomarker to autistic people to better identify symptoms.

“Here we have people whose concentration is probably better in some ways than for low AQ people,” Crewther says. “However, the ability to hold and manipulate more than one concept at a time seems to be a difficulty. So there is real scope for biomarkers to help us actually train what I think could be the underlying deficit – working memory.”

With so many research fronts showing promise and research converging on clinically important developments, Crewther thinks there are interesting times ahead. “One of my dreams is to see the definition of disorders like autism and dyslexia that are currently behaviourally defined changed by studying the working brain. This research shows that we are cracking that problem.”