Abstract
The extent of dilation of pupil of the eye is a reliable measure of cognitive load. We have previously shown that with appropriate luminance controls, pupillary dilation during visual tasks offers insight into the extent of higher level processing occurring during task performance (Porter, Troscianko, and Gilchrist, Perception 31 suppl, 170–171; 2002). In particular, differences were found in the dilatory pattern between difficult visual search and counting tasks despite matched reaction times and identical stimuli. Counting elicited immediate marked pupil dilation, sustained until response, whereas dilation during search increased gradually until response and was reduced in overall magnitude.
To investigate whether these patterns correspond to memory load, pupil size was measured during performance of search tasks in which the memory component was manipulated. By changing a traditional “target absent or target present?” search task to “one target present or two targets present?”, spatial memory was increased, as the need to remember one target's location, once found, was introduced. Accordingly, pupillary dilation was slightly greater in the “one or two?” task than in the target absent/present task, but only when nearing response. When target identity varied by trial, greater dilation was seen early in the search process than when target identity remained constant. This corresponded to the increased effort in encoding the target. These results suggest that pupil dilation is sensitive to both spatial and recognition memory load, and that memory for both target identity and location is differentially involved in different visual search tasks.