Perceptual Deficit during a Mental Task.

Subjects monitored for a visual signal while engaged in a demanding mental task. The probability of detecting the signal depends on the time of its presentation during the 8 seconds of the task. A similar time course is observed for failures to detect and for changes of pupil size. Momentary variations in the load that the task imposes on the subject are reflected in both indices. Detection failures are not explained by the pupillary changes.