ABSTRACT
This chapter is dedicated to intellectual disability. The author presents a short definition and a discussion of the most controversial issues concerning the disorder, followed by a comprehensive overview of the empirical evidence for an atypical cerebral lateralization reflected in non-right-handedness and lack of or reversed language lateralization in both syndromic intellectual disability (S-ID) and non-syndromic intellectual disability (NS-ID).
This chapter also describes the author’s personal empirical research on functional laterality in intellectual disability triggered by the high inconsistency of results coming from studies of subjects with NS-ID, as opposed to studies of subjects with S-ID, especially with Down syndrome, which exhibit largely consistent results.
Presuming that the presence of an atypical lateralization in subjects with intellectual disability could be associated with comorbid language disorders rather than with the intellectual disability, the author examines the handedness and the hemispheric asymmetry for dichotic speech perception both in children with mild NS-ID and comorbid language and literacy disorders and in age- and gender-matched controls.
The results confirm that the ID group exhibits a reduced right hemispheric asymmetry for speech perception and high frequency of mixed handedness.
