Inspection time as mental speed in mildly mentally retarded adults: analysis of eye gaze, eye movement, and orientation.

The effect of eye movements away from a target on accuracy of visual discrimination was examined. In Experiment I inspection time was measured for 10 mildly mentally retarded and 10 nonretarded adults under two conditions, with each trial initiated by the subject or under experimental control. Retarded subjects did not gain any advantage from controlling trial onset. Video records of eye movements revealed that retarded subjects glanced off-target more than did nonretarded controls, but this was not sufficient to explain appreciably slower inspection time of the retarded group. Experiment 2 supported this conclusion; the same subjects completed a letter-discrimination task with direction of gaze monitored automatically. Although retarded subjects' eye gaze was more scattered early during a trial, gaze was appropriately directed by the time that the target appeared. Results from both experiments supported the hypothesis that speed of central, perceptual processing is slower among retarded persons, over and above the influence of distractibility. Results from three experiments in Part II were consistent with this interpretation. Experiment 3 was designed to eradicate trials among retarded subjects in which gaze was not properly directed, but results showed that too few such events occurred to influence accuracy. Experiment 4 demonstrated that the preparatory procedure in the previous studies resulted in efficient eye gaze among retarded subjects. Experiment 5 confirmed that lower discriminative accuracy among 10 retarded adults (compared with 10 nonretarded controls) was not due to less-efficient orientation prior to discrimination.