Effects of target probability and memory demands on the vigilance of adults with and without mental retardation.

The vigilance of young adults with mild mental retardation and without mental retardation was compared in two experiments in which observers performed two memory demanding, cognitively based, 60-minute tests. In Experiment 1, target probability was 5% or 30% with a 1.5-second event rate. Detection by observers with mental retardation declined more rapidly than did that of observers without mental retardation in both target probability conditions; further, their response criterion became increasingly more conservative. In Experiment 2, target probability was 10% or 30% with an 8-second event rate. Observers with mental retardation detected fewer targets under both target probability conditions. Subjects' response criterion differed as a function of level of intelligence from the onset of the vigil and did not change with time on task. Intelligence-related differences in vigilance were explained in terms of subjects' information-processing abilities.