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Effects of background event rate on sustained attention of mentally retarded and nonretarded adults.

Mentally retarded and nonretarded adults performed four, 60-minute visual vigilance tests in which single digits were presented at either a fast or slow rate. The target event was a "skipped" digit. During two tests the event rate shifted without warning to the alternate event rate after 30 minutes of the vigil. Retarded observers detected fewer targets and made more false alarms than did nonretarded observers in all test conditions. Shifts in event rate influenced the frequency of false alarms made by retarded observers. The differences in performance of retarded and nonretarded adults favor a memory deficit explanation.