Tracking working memory maintenance with pupillometry.

Phasic pupillary responses were used to track the active maintenance of information in working memory (WM). In seven experiments participants performed various change detection tasks while their pupils were continuously recorded. Across the experiments phasic pupillary responses increased as the number of maintained items increased up to around 4-5 items consistent with behavioral estimates of capacity. Combining data across experiments demonstrated that phasic pupillary responses were related to behavioral estimates of capacity. Furthermore, phasic pupillary responses demonstrated WM load-dependent relations only when active maintenance was required. When instructed to passively stare at the items or to drop items from WM, the pupil remained near baseline levels. These phasic pupillary responses also tracked the time course of maintenance demonstrating sustained responses early in the delay period, but declined thereafter. Finally, phasic pupillary responses tracked selection processes at encoding (filtering and pre-cues), but did not suggest evidence for item removal following retro-cues. These results are consistent with the notion that maintaining items in WM requires the allocation of effortful attention and further suggest that phasic pupillary responses can be used to track the active maintenance of items in WM.