An assessment of the vestibulo-ocular reflex (VOR) in persons with Down syndrome.

Down syndrome (DS), the most common genetically defined cause of intellectual disability, is the phenotypic consequence of a supernumerary chromosome 21. Persons with DS commonly display deficits in visuomotor integration, motor coordination, and balance. Despite the key roles of the optokinetic and vestibular systems in these submodalities of motor function, a systematic investigation into the optokinetic nystagmus (OKN) and vestibulo-ocular reflex (VOR) in persons with DS was lacking in the literature. Accordingly, this study generated quantitative data on oculomotor function in persons with DS under vestibular stimulation (an accompanying work describes results on the analysis of optokinetic function in the same cohort of participants). Experiments involved 32 participants with DS (14-36 years old, equally divided by gender) and 32 chronological age- and gender-matched typically developing controls. Eye movements were recorded by binocular video-oculography, and a servo-controlled rotary chair produced vestibular stimulation. Participants were assessed for VOR during step, ramp, and sinusoidal stimulations; inhibition of the VOR by visual target fixation; and VOR adaptation to conflicting visual input. Individuals with DS displayed small alterations in the VOR gain and dynamics compared to controls. In contrast, the number of VOR nystagmus beats and the ability to inhibit the VOR by visual target fixation were markedly and robustly smaller in persons with DS. Significantly increased VOR adaptation was observed in men with DS. These findings may have implications to the understanding of the neurological basis of the motor dysfunction that affects performance in many practical tasks persons with DS encounter in their everyday lives.