Autism and DLD Lateralization

 

Lateralization of the two hemispheres is an important process in human brain development. Even so, it is hypothesized that excessive asymmetry can affect higher-order association mechanisms, such as those responsible for language processing. Similarities of asymmetry in autism and developmental language disorder (DLD) provide supporting evidence that above normal lateralization can affect average functioning in these individuals. This discussion will focus on a study conducted to examine the similarities and differences of brain matter and lateralization in children with autism and DLD compared to control groups. It should be noted that this is the first comprehensive whole-brain survey of volume lateralization in DLD and high-functioning autism (Herbert, Ziegler, Deutsch, O'Brien, Kennedy, Filipek, et al., 2005).

Disorder Summary and Hypothesis

Autism and DLD are disorders that materialize early in life and involve language impairment. Previous studies have shown evidence of large brain volume (primarily white matter) in children with autism. In addition, even though most DLD studies have leaned toward examination of language regions in the brain, entire brain analyses have exposed an increase in full brain volume as well. Researchers hypothesized that cortical components of neural systems with superior interconnectivity are more liable to be affected by large areas of white matter, and that these two disorders should exhibit volume asymmetry in the higher-order association cortex. Regions of the association cortex correlating to language functions were one distinct area of study, being that asymmetry reversal in children with autism has been noted in the language-associated cortex (Herbert, et al., 2005).

Subjects

The subjects of the study included 46 boys between 5 and 12 years of age whose primary language was English. Fifteen candidates were autistic, 15 had DLD, and 15 were controls. To make the study more evenly balanced, the three groups had an almost equal percentage of right-handed children (around 28 % a piece) and of left-handed children (around 6% a piece). Individuals with autism and DLD had performance IQs of 80 or above, but normal children were chosen on factors such as normal school performance, neurological exams, and development; and lack of seizures, head injuries, brain lesions, hearing or sensorimotor impairments, anticonvulsant medications, etc. opposed to IQ. This is one downfall of the study, but because the similarities of results in the autism and DLD groups (whose IQs were not dissimilar), and the fact that the control group displayed normal development and school performance, the results of the study should not be particularly biased (Herbert, et al., 2005).

Techniques

MRIs were conducted on participants and data was analyzed on Sun Microsystems workstations. Neuroanatomical segmentation was used to divide the brain into grey matter and white matter. The neocortical (dorsal region of the cerebral cortex) ribbon was parcellated (divided) into 48 primarily gyral-based sections (elevated areas) per hemisphere, also known as PUs. Sulcal patterns (grooves) were identified and labeled in multiplaner views to allow for markers to be tracked three-dimensionally. Anatomical markers for other anterior-posterior divisions of large gyral areas were also tracked. Volumes (amount of space occupied) were formulated by adding the voxels (quantity of 3D data) in each cortical PU (Herbert, et al., 2005).

Asymmetry analyses included examination of total brain volume, segmented divisions, lobes of the cerebral cortex, and individual PUs of the cerebral cortex. General linear models with multiple variables (multivariate) for correlated data were used to test for differences in asymmetry in the three groups. PUs were categorized according to their functional nature. For example, forty PUs were classified in the following groups: primary sensory and motor cortexes, unmoral association cortex, and higher-order association cortex. Subtypes of PUs were also identified, such as the planum temporale, inferior frontal gyrus, and the insula, to name a few (Herbert, et al., 2005).

Results

No significant dissimilarities in asymmetry were noted for any of the three groups when solely comparing total hemispheric volume. Even so, noteworthy differences in asymmetry between the groups were displayed when the hemispheres were segmented. Surprisingly, these increased morphological (structural) asymmetries were actually found in the control groups. These findings might lead one to believe that children with autism or DLD actually have greater lateralization in the brain, but asymmetry in cortical PUs was much higher in boys with autism and DLD opposed to control groups. Cortical volume was the largest in the autism group (41.7%), followed by the DLD group (32.6%), and the smallest in the control group (20.1%). The greatest right-asymmetrical volume was once again the most considerable in the autism group, followed by the DLD group, and then the control group. As far as the left-asymmetrical cerebral cortex goes, the DLD group had the highest volume reduction, followed by the autism group and control group, respectively. A highly noteworthy result of this study was the significant difference in asymmetry of the language-related cortical areas, which is the basis of asymmetry in the unimodal association cortex PU. While this has many implications, results of various studies have not consistently linked asymmetry in language regions to diagnosis. Atypical lateralization was discovered in many other PUs of the brain, with the autism and DLD group consistently showing greater asymmetry compared to the control group (Herbert, et al., 2005).

Discussion

The founders of this study hypothesized that the asymmetrical aspects of the brain in patients with autism and DLD are not the direct cause for language and functional disorders, but more of an underlying basis for the real root of the problem- the lack of connectivity between the hemispheres. Both groups have noted increased brain volume, but also corpus collossi that either grows slowly or decreases in size. Due to the fact the corpus collosum serves as the primary transportation mechanism between the hemispheres, this proportional abnormality may in fact result in interhemispheric constraints of information transfer. A collosal transfer insufficiency has already been recognized in cases of DLD. This deficiency increases the chance for greater functional lateralization and anatomical asymmetry. Perhaps the reason that both autistic and DLD children have language deficits, but do not have routine regional anatomical abnormalities is due to the fact that volume and white matter are links to underlying processing anomalies. The volume and white matter grow postnatally, and the increase is greatest in areas that myelinate later in life. This gradual growth increases the asymmetry and amplifies it over time. The arrangement of the nerve-cell bodies in the language regions of the cerebral cortex (cytoarchitecural organization) also appears to be associated with asymmetries in cortical processing deficits (Herbert, et al., 2005).

Conclusion

Asymmetry and rightward cerebral cortical lateralization has been discovered in autism and DLD. Researchers found supporting evidence that volume increases in white matter of the brain may be an underlying factor for the lack of interhemispheric connectivity leading to dysfunction in higher-order association areas, such as those involved in language processing. Some processes may be affected more so than others, as demonstrated in normal functionality of certain areas in individuals with autism and DLD (Herbert, et al., 2005). This study adds to examples in the text of individuals who have had their corpus collosum severed. Once the interhemispheric connectivity was broken, individuals often suffer from communication deficiencies. Also, Carlson notes that most verbal deficiencies occur when the left side of the brain is damaged (2004). The rightward lateralization in patients with autism and DLD may contribute to the language problems in these disorders (Herbert, et al., 2005). The posterior areas of the cerebral hemispheres are chiefly responsible for our having something to say, as stated in the text (Carlson, 2004). In this study, the DLD group had the greatest volume reduction in the left-asymmetrical areas of the cerebral cortex, which could also add to the language deficits in this disorder (Herbert, et al., 2005). The study described in this discussion contributes to our text by adding supporting evidence of the effects lateralization can have on language disorders.
References:

1. Carlson, Neil R. (2004). Human Communication. Physiology of Behavior, 8, 481-514.

2. Herbert, M., Ziegler, C., Deutsch, L., O'Brien, D., Kennedy, P., Filipek, A, et al. (2005). Brain asymmetries in autism and developmental language disorder: a nested whole-brain analysis. Brain, Vol. 128, No. 1, Pgs. 213-226.