A physiological measure of processing load or "mental effort" required to perform a cognitive task should accurately reflect within-task, between-task, and betweenindividual variations in processing demands. This article reviews all available experimental data and concludes that the task-evoked pupillary response fulfills these criteria. Alternative explanations are considered and rejected. Some implications for neurophysiological and cognitive theories of processing resources are discussed.

During a short-term memory task, pupil diameter is a measure of the amount of material which is under active processing at any time. The pupil dilates as the material is presented and constricts during report. The rate of change of these functions is related to task difficulty.

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The magnitude of task-evoked pupillary dilations during mental activity has previously been shown to index the cognitive capacity utilized in the performance of the mental task. To determine the relation between "intelligence" and capacity demands during mental activity, task-evoked pupillary dilations were measured while two groups of university students differing in their scores on the Scholastic Aptitude Test solved mental arithmetic problems. Over three levels of problems difficulty, more intelligent subjects showed smaller task-evoked pupillary dilations than did their less intelligent counterparts. Thus, the more intelligent appear to possess more efficient cognitive structures of information processing. These data provide evidence that physiological differences between individuals of differing psychometric intelligence emerge during mental activity.

Pupillary movements were monitored as 11 university students listened to a series of 1K Hz 50‐msec tone bursts presented at 3.2‐sec intervals for a period of 48 min. Their task was to report target tones (‐3dB), which were presented randomly with a probability of 0.12. Under these conditions, monitoring performance deteriorated as a function of time on the task. This vigilance decrement was attributed to both a decrease in listener sensitivity and a conservative shift in decision criterion, as determined by a signal detection analysis. The amplitude of the phasic task‐evoked pupillary response reflected these changes in performance, decreasing significantly over time on the task. Tonic or baseline pupillary diameter exhibited no such relationship with performance. These results are interpreted in the context of an activation theory of attention that suggests that a common neurophysiological system mediates both intensive and sustained attentional processes.