Rigidity in Autism Spectrum Disorder (ASD): A Unified (Evolutionary) Account of Salient Linguistic and Non-Linguistic Characteristics

Autism spectrum disorder (ASD) typically exhibits stereotyped or repetitive behavior that can be described as rigid, but also certain characteristics in the domain of language that can equally be characterized as rigid. These include heightened, enhanced sensitivity to the rules of grammar (morpho-syntax), often resulting in hyper-systemizing, as well as rigidity interpreting metaphorical, non-literal language. Human cognition and behavior (including language) believably resulted from an intense feedback loop between an increase in cognitive flexibility (primarily through a gradual emergence and complexification of language/grammar) and a reduction in reactive aggression and impulsivity (this resulting in more prosocial behaviors). Neurobiologically, this feedback loop contributed to a relatively recent evolution of denser connectivity between some cortical structures and the striatum, which is found affected in ASD. In this paper, we propose that the seemingly disparate features of ASD, including linguistic and behavioral rigidity, result from a common cause: an enhanced striatal function, linked to a reduced control of the striatum by selected cortical structures. The striatum is associated with both impulsiveness (including reactive aggression) and with automated, rigid, ritualized responses. Our proposal is specific enough that it can be experimentally tested, with implications for clinical approaches to autism.