Differences in visual orienting between persons with Down or fragile X syndrome.

The voluntary and reflexive orienting abilities of persons with Down syndrome and fragile X syndrome, at average MA levels of approximately 4 and 7 years, were compared with an RT task. Reflexive orienting abilities appeared to develop in accordance with MA for the participants with Down syndrome but not for those with fragile X syndrome. However, both groups showed delayed voluntary orienting. The group differences in reflexive orienting at the low MA level reinforce the practice of separating etiologies and highlight the contribution of rudimentary attentional processes in the study of individuals with mental retardation.