Emphasizing on the Neurobiological Basis of Autism Spectrum Disorder: A Closer Look to a Different Brain

Abstract

Autistic Spectrum Disorders (ASD) is an umbrella term that describes a range of common neurodevelopmental disorders affecting approximately 1% of the population. ASD are characterized by multilateral shortages, such as difficulties in communication and social interaction, repetitive, stereotypical behaviors and a limited scope of interests, mobility problems and disorders of language development, as well. These deficits do not follow the norm; on the contrary, great differentiation in their profiles and severity is observed between different people. ASD have, to a great degree, a hereditary, genetic basis however up to a decade ago, the researchers focusing on their neurobiological background were few. The present review study is an attempt to shed light on the neurobiological pathway of Autism Spectrum Disorder. The last decade, there has been an intense research emphasizing on the polymorphous patterns of neurobiological mutations present in ASD. The findings pertain to such diverse differences compared to the neurotypical population in the neural, structural-functional, molecular, endocrine, electrophysiological and genetic levels and also in the connectivity and neurotransmission levels, that many investigators are led to a hypothesis of a radically different brain organization [1-3]. Based on the neurobiological findings, we are now able to explain about 10%-20% of ASD cases. In the following sections, detailed descriptions of the neurobiological differences of individuals with ASD are given compared to the neurotypical population at various levels and their association with the cognitive and behavioral profile of the observed deficits. One of the first confirmed findings is regarding the non-normotypical brain over-development of people with ASD during childhood. The majority of studies were conducted on infants or children aged one year or over, for the duration of their childhood. Thus, researchers hypothesize that this over-development might be related to the first clinical symptoms of autism, cognitive and behavioral, observed at this age [4]. For example, in a recent study, Hazlett et al. examined the growth rate of cerebral volume (white and gray matter) and cortical thickness in children aged two years [5]. They also performed a repeat measurement two years later. The results showed that children with ASD had approximately 9% greater gray and white matter volume compared to the control group at both time points, while they did not differ in cortical thickness. Despite this difference, the rate of brain development was the same between the groups, as was indicated by the lack of intergroup differences in cerebral volume between the two time points, which according to the researchers is due to an increased growth rate before the age of two. The study by Schumann et al. resulted in similar findings, the difference being that the size and growth rate of gray matter were greater in the anterior areas of the brain (frontal, temporal, parietal) compared to the posterior areas [6]. Moreover, non-normotypical over-development was mainly observed in the temporal regions. In addition, in a rare recent study, Calderoni et al. investigated the brain volume of various structures in a group consisting entirely of girls with ASD (2 to 7 years old) and a matched control group [7]. The findings showed a 5% greater overall size of intracranial structures in girls with ASD compared to the control group, and a particularly noticeable enlargement in gray matter in the superior frontal gyrus bilaterally and in the right temporo-