Pupillary dilation as an index of task demands.

To analyze how pupillary responses reflect mental effort and allocation of processing resources under several load conditions, the pupil diameter of 18 participants was recorded during an auditory digit-span recall task under three load conditions: Low (5 digits), Moderate (8 digits), and Overload (11 digits). In previous research, under all load conditions a significant linear enlargement in pupil diameter was observed as each digit was presented. Significant dilations from the end of the presentation phase to the beginning of the recall phase were also observed but only under low and moderate loads. Contrary to previous research, under the Overload condition, no reduction in pupil diameter was observed when resource limits were exceeded; however, a plateau was observed when the ninth digit was presented until the beginning of the recall phase. Overall, pupillometric data seem to indicate that participants may keep processing actively even though resources are exceeded.