Pupillary dilation elicited by attending to two disks with different luminance

Pupils become smaller when people attend to a bright disk as compared to a dark disk. However, people can divide their attention into several distinct positions, which is referred to as divided attention, and pupillary responses under such conditions have not been investigated. In this study, we examined how pupils would respond when people attended to two disks presented at two distinct positions by conducting three experiments. We found that the pupillary response when attending to two disks with different luminance was larger than when attending to a single brighter disk and was comparable to that when attending to a single darker disk, whereas the pupillary response when attending to two disks with identical luminance was not larger than when attending to a single disk (irrespective of the disk luminance). Furthermore, we found that the magnitude of pupillary dilation was determined by the magnitude of the luminance difference between two disks. These results make a useful contribution to the literature on human pupillary responses.