Dichotic stimulation and mental retardation

Abstract

Beginning with an overview of the complexities and current thinking with respect to the non-invasive behavioural technique of dichotic stimulation as it relates to hemispheric lateralization, the present paper examines the findings for dichotic stimulation in the area of mental retardation. A survey of the extant literature reveals that the application of dichotic stimulation with the mentally retarded has been relatively rare. However, given the paucity of studies, three conclusions can be drawn. First, when retarded subjects (excluding Down syndrome individuals) are compared to nonretarded subjects matched for mental are (MA), the ear asymmetries are invariably in the expected direction for the type of stimulus employed, however, the magnitude is variable. Second, under dichotic monitoring mentally retarded subjects (inclusive of Down syndrome individuals) demonstrate the same pattern of intrusion errors from the unattended right ear as do non-mentally retarded subjects matched for MA. Third, a majority of the dichotic stimulation studies have focused upon the “atypical” speech lateralization of Down syndrome individuals. The dichotic listening data are equivocal regarding the lateralization of speech in these individuals. However, there are two data sets that argue against their right hemispheric lateralization for speech. The first examines the intrusion errors demonstrated by Down syndrome individuals on dichotic monitoring tasks and the second examines the performance of Down syndrome subjects employing the dual-task interference paradigm.