The Ambiguous Anatomy of Autism: Explorations in Current Neurobiological Research

The ever expanding fields of neuroscience and behavioral science often intersect in an attempt to discover the cause of many psychological disorders, a process which subsequently helps direct improvements in treatment techniques. Today, researchers have expressed an emerging interest regarding the source of one of the most commonly diagnosed developmental disorders known as autism which, since its discovery in 1943, continues to possess many mysterious qualities that have yet to be unearthed by empirical research. In particular, one presently debated argument regarding autism is aimed at confirming specific brain abnormalities which may subsequently cause or have an effect on the disorder. Though many credible hypotheses have been presented, no conclusion has yet been widely agreed upon, suggesting both a multitude of causal factors and an obvious need for increased research.

Autism is a pervasive developmental disorder (PDD) affecting 2-5 infants per 10,000 births (4), or nearly 400,000 individuals in the US (5). A number of core features are expressed by a majority of autistic individuals and are generally characterized by 12 diagnostic criteria, each of which are divided among 3 clusters of symptoms including communication impairment, hindered social reciprocities, and atypical, repetitive, and stereotypic behavior and interests. Other expressions of unusual developmental and behavioral features include an observable resistance to change in environment or routines which frequently results in an insistence on sameness. Additionally, autistic individuals possess specific, isolated interests, and may demonstrate proficiency in an insignificant task that is incessantly repeated (4). Many autistic children are also mentally retarded with a vast array of cognitive impairments and other neurobiological abnormalities which may result in seizure (4). All of the evidence above clearly demonstrates that autism involves many pervasive developmental implications, but surprisingly research has yet to yield definitive findings regarding the source of the disorder.

As current research delves more deeply into autism etiology, several different and sometimes contradicting theories have been posited in hopes of defining a route cause of the disorder, and treatment has subsequently greatly improved in the past decade. Unfortunately there is presently no cure for the disorder and outcome is frequently poor (4), especially for individuals with extreme symptomology. Currently in existence are several modes of treatment including drug therapy, behavioral modification, nutritional treatments, and education interventions, however no such treatments have produced major changes in the course of the disorder (4) and increased research is called for to more definitively assess the effectiveness of such treatments. Clinicians suggest that early and continuous behavioral modification in concurrence with parental involvement in therapeutic settings predict the best outcome for autistic children, thereby reducing the potential for the acquisition of subsequent disorders (4). With treatment, nearly 1 in 3 autistic children is eventually able to achieve some level of personal independence and self sufficiency as an adult (4). Attainment of communicative speech by age 5 and the child’s IQ level also serve as strong predictors of outcome. Additionally related to improved outcome for autism is the need for increased research regarding early detection and superior screening techniques to reduce the number of untreated cases and misdiagnoses.

Thus far, the behavioral and cognitive implications of autism have been touched upon, but what remains ambiguous is the exact cause of the disorder. Autism has a 90% heritability rate, with reports of frequency in siblings nearing 3% (7). Both of these statistics suggest a biological or genetic contributory component. In fact, approximately 2-10 genes are estimated to be involved with autism (3). Additionally, autism is approximately 4-5 times more likely to be expressed by males than females, though girls often exhibit more extreme symptomology. This example further supports the hypothesis that autism etiology is related to brain malformation because of its relation to structural differences already inherent to the brains of males and females. Other current findings which support a neurological basis of autism focus on such brain structures and atypical development, though frequently such data is not based on gender differences. For example, according to research, enlarged head circumference, or microcephaly, occurs frequently among children with autism (7), and may be partly due to a deficiency of neuron pruning during brain development. In a typically developing brain, pruning is a process which functions to eliminate faulty neuron connections, thereby optimizing coordinated neuron inputs and outputs, but impairment in this area of development may ultimately lead to poor functioning or slowed processing of certain affected neuronal circuits (3) since weak neuron synapses are not weeded out. Perhaps these defective neurons may cause maladaptive behaviors associated with autism because inputs and outputs are not sufficiently coordinated.

Other significant studies have suggested that autism results from faulty gray matter and white matter production. Gray matter is composed of neuron cell bodies which process information originating from sensory organ input. White matter, on the other hand, is composed of axons, which serve as tracts for such messages to be sent to the neuron cell bodies. Regarding autism, this area of research has yielded many discrepancies. For example, one study found increased white matter in the cerebellum, and increased cerebral gray matter volume, possibly due to an amplified number of neurons resulting from excessive sparing (1). Conversely, McAlonan et al, (6) found opposing evidence of a significant decrease in total gray matter volume and a significant decrease in white matter. The results of both hypothetical brains are plausible, but would undoubtedly cause different expressions of behavior, so which data is more accurate? Unfortunately the correct answer to this puzzle is still unknown, but obviously such conflicting results demonstrate a need for increased research regarding an understanding of the autistic brain.

Furthermore, additional studies have looked at other structures of the brain in an effort to distinguish among other potentially effected areas. In one study, autistic children displayed decreased cerebellum size including a 31% decrease of Purkinje cells, or the largest type of neurons, which function in the output of motor coordination. Obviously any reduction in neuron quantity is going to influence behavior, but the question is how so? Perhaps this particular malformation plays a role in producing the atypical motor behaviors of autistics. Similarly, another study found a decreased number of neurons in the amygdala (8), or the emotion center of the brain. Could this theory partially explain an aspect of the anti-social or nonreciprocal tendencies of autistics? Lastly, though autopsy studies are infrequent, postmortem studies of autistic brains have additionally detected abnormalities which further support a neurological basis of the disorder.

Reverse asymmetry of brain hemispheres has also been hypothesized to explain neurobiological origins of autism. Theories infer right hemisphere dominance rather than the typical expression of left hemisphere dominance, and researchers have focused particularly on a possible reallocation from right to left speech processing, which could potentially serve to explain the communicative impairments so frequently associated with autism. In following with this idea, Dawson and colleagues (2) were predominantly interested in the relationship between hemispheric specialization and the language abilities of autistic children. Subsequently, they found that 7 out of 10 autistic subjects showed greater right than left hemisphere activation during language processing tasks, confirming their hypothesis that autism is in fact somehow associated with a deviant pattern of hemispheric specialization related to speech processing and production (2). The question is what causes this brain abnormality? Or is it an effect of autism rather than a cause? Lastly, additional studies have reported evidence of pronounced sensorimotor and perceptual impairments detected in the left hemisphere, which may explain some of the maladaptive, withdrawn behavioral patterns associated with autism.

Not surprisingly, indirect similarities in asymmetrical brain structure have also been found among children with Selective Language Impairment (SLI). In fact, autistic and SLI children express similar neurocognitive phenotypes including enlarged left cortical regions including the perisylvian region, the plenum temporale, and Heschel’s gyrus (7), all of which are areas which have historically been associated with language production. It is important to acknowledge that enlarged brain structures do not result in advanced functioning of the affected areas, as one might reasonably assume. The results are instead quite the opposite and can have devastating affects on the disordered individual. For example, Tager-Flusberg and Joseph’s research (7) comparing autistic children with typically developing controls also found that autistic brains displayed 27% larger inferior lateral frontal cortex in the right hemisphere including the pars opercularis which is associated with Broca’s area, or the structure of the brain involved in language processing, speech production, and comprehension. The autistic subjects manifested difficulties in communication and language, whereas the control subjects expressed no impairment in the same areas. All of the above data is extremely persuasive in explaining the origins of the communicative and language impairments expressed by autistic individuals, but many questions still remain unanswered.

As can be inferred by the multitude of theories discussed here, though a vast array of studies have been conducted to explain the neurobiological component of autism etiology, no concrete conclusive statements have been made, especially since much of today’s literature is often either questionable or contradictory. Nevertheless, research has yielded a remarkable body of evidence since the disorder’s first discovery, and as more data is collected science comes closer to untangling autism’s many complexities. All of the above findings provide significant information for the direction of further research which is decidedly extremely necessary in order to obtain a greater understanding of the cause(s) and effects of autism, which could subsequently lead to improved treatment.

Works Cited

1.) Akshoomoff Natacha, Pierce, Karen, Courchesne, Eric, (2002). The Neurobiological Basis of Autism from a Developmental Perspective. Development and Psychopathology, Vol. 14, pp. 613-634.
http://journals.cambridge.org/download.php?file=%2FDPP%2FDPP14_03%2FS0954579402003115a.pdf&code=c85f77950c34c11374f024857fd4885c

2.) Dawson, Geraldine, Finley, Charles, Phillips, Sheila, Galpert, Larry., (1986). Hemispheric Specialization and the LanguageAbilities of Autistic Children. Child Development, Vol. 57 (No. 6), pp. 1440-1453.
http://www.jstor.org/view/00093920/ap030231/03a00130/0

3.) Gillberg, Christopher, (2004). Neurobiology of Autism.

http://www.awares.org/pkgs_files/librarydoc_435(1).ppt.

4.) Klin, Ami, Volkmar, Fred R., (1999). Autism and Other Pervasive Developmental Disorders. S. Goldstein & C. Reynolds (Eds.) Handbook of Neurodevelopment and Genetic Disorders in Children, pp. 247-274.

http://books.google.com/books?id=r6z7CT5v8L4C&pg=PA195&lpg=PA195&dq=klin+and+volkmar+1999&source=web&ots=vK-30Vnyr_&sig=2fTXIIYiBSyDpVxgECZPCIu8bqs

5.) National Institute on Deafness and Other Communicative Disorders, Autism and Communication. (2007).

http://www.nidcd.nih.gov/health/voice/autism.asp

6.) McAlonan, Grainne, Cheung, Charlton, Suckling, John, Lam, Grace., (2005). Mapping the Brain in Autism: A Voxel-Based MRI Study of Volumetric Differences and Intercorrelations in Autism. Brain, Vol. 128 pp. 268-276.

http://brain.oxfordjournals.org/cgi/reprint/128/2/268

7.) Tager-Flusberg, Helen Joseph, Robert M., (2003). Identifying Neurcognitive Phenotypes in Autism. Philosophical Tranactions: Biological Sciences, Vol. 358 (No. 1430), pp. 303-314.

http://www.ncbi.nlm.nih.gov/pubmed/12639328

8.) Sample, Ian, (2006). Research Links Autism to Brain Abnormalities http://www.guardian.co.uk/medicine/story/0,,1823520,00.html