What differs in the brains of young children as the behaviors that characterize autism emerge? Researchers have sought to define that difference using all available tools, including functional imaging, blood-based markers, eye-tracking and scalp electrical recordings (EEG). Past discoveries have yielded new clues, but no single test has demonstrated the power to accurately indicate which infants might be at higher risk of developing autism. However, a new report published this week in BMC Medicine could change how autism risk is assessed in the future.
A team of Boston researchers have found a reliable marker of autism risk in a simple and non-invasive EEG test. Although many autism researchers measure EEG—some even from infants—the new study, partly funded by Autism Speaks, has a unique twist. Instead of thinking about EEG signals in the traditional way, the investigators have embraced subtle relationships in the wiggles of EEG waveforms. By capturing those relationships, the research team identified a signature that reflects the structure of neural connections in the brain.
“We’ve long assumed that there is much more signal in the EEG than we take for granted. Conventional approaches lack the power of these more advanced machine learning tools in detecting useful signals from noise,” says Charles Nelson, Ph.D. of Harvard and Children’s Hospital in Boston. Dr. Nelson is the senior author of the study and a member of Autism Speaks’ Scientific Advisory Board. In conjunction with long-time collaborator and Boston University Professor, Helen Tager-Flusberg, Ph.D., the researchers sought the skills of someone who could analyze the complex EEG signal. William Bosl, Ph.D., of the Informatics Program in the Children’s Hospital of Boston, brings the perspective of a physicist to this task, using a suite of tools designed to find subtle relationships among the EEG waveforms and creating a framework for identifying hidden patterns in those signals.
In the paper, the team draws links between the amount of complexity observed in the EEG signal and the self-similar patterns of connectivity observed in the brain. Self-similarity is a favorite pattern for mother nature. Just as the branching of a tree has a pattern that repeats from the trunk and large branches to the finer branches and extensions to leaves, one can think of the branching of neurons in the same way.
This pattern is, in fact, essential to the way the brain communicates. Each neuron is connected with relatively few others, given the billions of neurons in the brain. To make communication efficient in such a sparsely connected network, the principle of self-similarity must apply. Neurons communicate with neighboring neurons more frequently then they do with neurons in a distant brain region—this is typical. However, in the brains of individuals with autism, the bias toward local communication is even greater, and long-range communication is less than is expected. These patterns of communication between neurons create electrical signatures which can be measured using EEG.
Throughout development, many neural connections change. New synapses are formed and others are pruned as a child develops and experiences new things. Atypical connectivity may result when these normal processes fail to occur as they should. Experiments that use EEG could help us identify when development is starting to go off the normal course. Previous studies have shown that the brains of children with autism tend to have less synchronized activity between different EEG sensors than typically-developing children. This pattern would be expected from having too many neighboring neurons chatting and less communication with distant neurons.
The researchers compared EEG from infants aged 6 to 24 months from two groups: one group of children had an older sibling with autism and were therefore at greater risk themselves and a second group of children had no affected siblings. To ensure compliance of each little subject, a research assistant amused the infant by a blowing bubbles while an EEG cap—which resembles a space-age hairnet—was quickly situated on the baby’s head. The data for this analysis was measured during a baseline period, where the children were quietly observing their surroundings and not otherwise engaged in a specific task.
The period of the most dramatic EEG differences between risk groups—about 9 months—corresponds to a time of major milestones that form the foundation for later social and communication skills. Around this time babies develop the ability to perceive another person’s intention to act and they lose the ability to detect differences in speech sounds from languages that are not their own. In another study of high-risk infants, Sally Ozonoff, Ph.D. and colleagues found no differences in social and orienting behaviors before six months, however significant differences emerged in the following six months.
Nearly 80% of the 9 month old children were correctly categorized as high or low risk on the basis of a measure of disorder or randomness in the EEG signal compared across different scalp . For reasons that are as yet unclear, the signal varied more for girls. At 9 months, if only boys were considered, the percentage of correctly categorized infants rises to greater than 90%. The categorization degrades at later ages for all children, and in fact is best for baby girls at 6 months.
This study demonstrates the use of a simple, non-invasive measurement tool and an important time window for identifying children who may be at risk of developing ASD. “The next and most critical next step is to see if our brain measures can actually predict which of our children will develop autism and which will not,” says Dr. Nelson. That information will come soon as the children who participated in this study come to an age where traditional diagnosis is highly reliable. However, as Dr. Tager-Flusberg notes, “We are still a long way from understanding the clinical significance of these brain signatures. More information is need to find a marker that predicts ASD outcomes later in life.” The team’s longer term research goals include determining how this risk marker in combination with other neural, behavioral, genetic and other risk markers may eventually lead to earlier diagnoses of ASD.
Staff Blogger, Leanne Chukoskie, Ph.D., Asst. Dir. Science Communication and Special Projects
Epilepsy is a common comorbidity in autism, occurring in as many as 30% of all cases. In overnight or 24 hour EEG studies, atypical brain activity known as spikes or epileptiform activity—that looks somewhat like brain activity during seizure but does not occur with a seizure—has been measured in as many as 60% of autism cases. What can this atypical brain activity tell us about autism? Should we think of the presence of this activity as a biomarker for autism, or is it a more generic sign of brain dysfunction?
At IMFAR, this topic was raised in an invited educational symposium focused specifically on epilepsy and autism. Sarah Spence, M.D., Ph.D., (NIMH; and also serving on the ATN and AGRE board of advisors), a neurologist specializing in both autism and epilepsy, organized and chaired the special session.
The scientific literature indicates that although there is considerable variability in the presentation of seizures among individuals with autism, there are also some consistencies. The rate of seizures in the autism community varies with the age of the individuals studied and the frequency of comorbid symptoms or syndromes and intellectual disability. Although girls are less frequently diagnosed with autism, they are more likely to experience seizures. All types of seizures have been observed in autism, but the generalized tonic clonic (or grand mal) seizure is most common. It was noted, however that it is difficult at times even for a skilled clinician to distinguish what might be absence or complex partial seizures from the behavioral arrest, periods of unresponsiveness, eye deviations and repetitive behaviors that are common in autism.
Michael Chez, M.D. (Sutter Medical Center, UC Davis School of Medicine), offered the perspective of a neurologist who sees many children with autism. Although there are many tests that are ordered for autism, the EEG is the one that most often reveals an atypical result. In a 2006 study Dr. Chez and colleagues reported on 889 children with non-syndromic autism without other comorbidities. Sixty percent of these children showed epileptiform activity even though they had never had a clinical seizure. Importantly, he and others were surprised to see no difference in the incidence of epileptiform activity between children who had regressed and those who showed no developmental regression.
All clinical seizures are treated with medication. But these data about abnormal EEGs in the absence of epilepsy suggest a clinical question: should the epileptiform activity be treated as in the same manner as frank epilepsy? A few recent studies showed benefits not only in the reduction of this aberrant activity on EEG but also some improvements in cognitive function with the use of certain anti-seizure medicines. Randomized controlled trials are needed to learn whether treating epileptiform activity itself offers more potential for improvement than harm.
In some cases of severe intractable epilepsy, steroids have been used to gain control of seizures. This appears to work for a short period of time (long term steroid use has many adverse effects) in individuals who experience seizure activity focused in the right hemisphere along the main fold (called the central sulcus) in the brain.
Jeff Lewine, Ph.D. (MIND Research Network, Albuquerque, NM) and colleagues conducted an open label trial of the anti-seizure drug, Depakote, given with a steroid, and measured how well individuals with autism produce and understand language before and after treatment. Most of the children tested showed substantial improvement in both aspects of language while on treatment. Those who responded to the steroid tended to have simpler patterns of epileptiform activity than those who did not respond to the treatment. Unfortunately, long-term steroid use isn’t a viable treatment option and 80% of the children who showed gains with the steroid lose them again after stopping the drug. A randomized clinical trial of Depakote plus steroid is needed as well as discovery of new methods to induce these improvements without the side effects.
Dr. Lewine also spoke about the typical (but by no means only) pattern of epileptiform activity in individuals with autism, which occurs in both hemispheres and is multi-focal with common activity around the sylvian sulcus. Magnetic resonance imaging (MRI) comparisons of brains of children with epileptiform activity show an increase in brain volume, especially with the white matter or “wires” in the portion of the brain where the activity is focused. The question is which comes first, the increase in white matter or the epileptiform activity? We don’t have an answer to that question yet, but it is an active area of research. We do know that when we consider the incidence of epileptiform activity across the autism spectrum, it appears that individuals with lower levels of functioning have more epileptiform activity. And as noted earlier, the overall incidence of atypical brain activity in children who experienced an autistic regression is not different from what is observed in children with general developmental delay, however some studies show that children who have regressed have greater incidence of epilepsy.
One of the genetic syndromes that presents with a high proportion (approximately 40%) of individuals with autism is Tuberous Sclerosis (TSC). TSC is a rare disorder affecting the brain, the skin and other organs. Individuals with TSC frequently experience seizures (90% likely with a lifetime) whether or not they have autism. The pathological features of TSC in the brain involve the presence of tubers in the cerebral cortex. Within these tubers scientists have observed giant brain cells that are 10 times greater than the size of a typical neuron. This seemingly odd result makes sense with what is known about the effects of the TSC enzyme complex in controlling the growth of cells. TSC acts by putting a break on the cellular machinery that creates new proteins for cell growth.
Dr. Mustafa Sahin, M.D., Ph.D. (Harvard; Children’s Hospital, Boston) presented data on TSC along with an intriguing hypothesis: miswiring of the projections that connect neurons (called axons) results in the start of the disease process in TSC. Animal models of TSC support this hypothesis—TSC mutants have more axons. Scientists examining these animals’ early development also see errors in how axons weave their way through the immature brain to reach the location where they will begin making adult-like connections. Treatment with the drug rapamycin helped animals with dysfunctional TSC genes in terms of improving the insulation on the neural wires (the insulation that covers axons is called myelin), which makes communication more effective in axons that travel between distant brain regions. By improving the process of adding myelin to these animals’ axons, seizures stopped and learning also seemed to improve. Dr. Sahin and colleagues are now beginning Phase II of a randomized controlled trial using rapamycin in patients with TSC to look for neurocognitive improvements with the drug. By learning more about how this drug works in people with TSC, we increase our opportunities for creating therapies for individuals with non-syndromic forms of autism.
Atypical sensory perceptions are commonly experienced in autism and some have noted altered perceptions just prior to the onset of seizures. The immature visual system offers a unique opportunity for understanding how early epileptiform activity in the brain can alter development and perception. Michaela Fagilioni, Ph.D. (Harvard; Children’s Hospital, Boston) and colleagues showed how the outcome of visual development could be controlled in elegant experiments designed to nudge what is known as the “critical period” for determining how much real estate in the visual brain will be dedicated to the right versus left field of vision. The critical period is a time of great adaptability of brain circuits. Administration of drugs, like diazapam (more commonly known as Valium) very early in mouse postnatal development will shift the start and end of the critical period to earlier times. Keeping the animal in the dark and suppressing inhibitory activity in the local brain circuits are two ways Dr. Fagilioni and colleagues have found to keep the critical period open longer.
Both excitatory and inhibitory neurons are important in establishing the critical period, with the inhibitory (GABA-secreting) cells serving as the trigger for flexibility in this system. The balance of activity between excitatory and inhibitory neurons is very much at the heart of the challenge to understanding epilepsy. Recent work suggests that changes that affect the critical period also change an individuals’ vulnerability to epileptiform activity. These results are important for autism because they undercover important functional anomalies in mouse models of autism that we may also expect to observe in some individuals with autism. For example in the Neuroligin 3 mutant mouse, the critical period for establishing eye dominance is completely absent. The competition for neural real estate, which is typically seen only very young animals, can be induced throughout the life of the Neuroligin 3 mouse. Similar atypical findings with respect to this early sensory flexibility or plasticity occurs in other animal models for autism. Lastly, but importantly, while the visual system’s plasticity may be well-established and produce easily observable effects, Dr. Fagilioni and her colleagues are examining the critical period in other sensory modalities like audition.
Considering these results together offers a greater appreciation of the complexities—and also the excitement—of investigations at the interface of epilepsy and autism. We need research investments at both ends of the discovery path. Greater investment in the basic research will help relates early sensory plasticity, and altered sensory experiences, to mechanisms of epilepsy and autism. We also need a better understanding of the drugs that can be made available now to deliver safe and effective treatment options for achieving optimal outcomes in individuals with autism. Autism Speaks is currently exploring some of theses issues in a partnership with the International League Against Epilepsy.