Pupillary responses index cognitive resource limitations.

Cognitive task-evoked pupillary responses reliably index information-processing loads. However, previous studies have reported inconsistent findings regarding the nature of the pupillary response when processing demands approach or exceed available processing resources. This condition was examined in 22 normal undergraduates by using pupillometric recordings during a digit span recall task, with 5 (low load), 9 (moderate load), and 13 (excessive load) digits per string. Pupillary responses increased systematically with increased processing load (to-be-recalled digits) until the limit of available resources (memory capacity of 7 +/- 2 digits), when they reached asymptote and then declined with resource overload (> 9 digits). These findings suggest that pupillary responses increase systematically with increased processing demands that are below resource limits, change little during active processing at or near resource limits, and begin to decline when processing demands exceed available resources.