ABSTRACT
An experiment is reported in which subjects were presented visually with circular arrays of letters that vary in duration, number of letters, size of array, and whether or not they contain a target letter. In Pure duration conditions, subjects judged the duration of the array, and in Pure detection conditions subjects judged whether or not the array contained the target letter. In two Mixed conditions, subjects performed both duration and detection tasks, and the conditions differed according to whether the duration task was emphasized (Map 1) or the detection task was emphasized (Map 2). Decrements in duration accuracy and changes in the effects of certain variables on perceived duration were observed in going from Pure to Mixed conditions. Detection accuracy increased with stimulus duration in the Mixed but not the Pure conditions. A model assuming that duration and detection judgments are made simultaneously in the Mixed conditions provides a good explanation of the reaction time and accuracy data from Map 1 but not Map 2. It is argued that, in the Pure duration condition, judgments are based partly on a timer that starts shortly after stimulus onset and stops shortly after stimulus offset, or as soon as a critical duration is reached, whichever occurs first. When the duration task is emphasized in a Mixed condition, duration and detection judgments are done simultaneously but with a decrement in each. When the detection task is emphasized, priority is given to this task, and the duration judgment is derived mainly from the outcome of processing in the detection task.
