Attention in the eye of the beholder: pupil dilation in peripheral vision

Previous work has shown that attention decreases with increasing distance to eye fixation. In this paper, we take the first step in developing a paradigm that measures attention in peripheral vision without involvement of a task directly linked to peripheral vision. This would allow us to test peripheral attention in the absence of a person’s conscious awareness. We built on the well-established attention-window task (AWT, Hüttermann et al., 2013) in which participants typically evaluate simultaneously two stimuli presented in visual periphery. The AWT lends itself for investigating a participant’s maximal attention span. We conducted two eye-tracking experiments investigating whether the AWT can be used to test peripheral attention outside participants’ conscious awareness. In each trial of Expt1, objects were presented peripherally for 300ms in one of various distances (12.5°, 20°, 27.5°, 35°, 42.5°) to both sides of fixation. Objects included zero to four triangles and participants indicated the number of triangles. Replicating previous work, participants’ performance decreased with increasing distance, p < .001. More importantly, participants’ pupil size increased with increasing distance, p < .001. In Expt2, we excluded the possibility that pupil size in Expt1 increased as a mere function of visual distance. Before each trial, participants were prompted to either indicate whether or not an object appeared in their visual periphery (perception condition) or count triangles within the objects (attention condition). Pupil size increased much more strongly for the attention than the perception condition, and the increase was significantly steeper as a function of centerobject distance in the attention than the perception condition, p = .027. Taken together, we found that shifting attention to the periphery involves increasing pupil size. These results indicate that peripheral attention can be measured in absence of an explicit task linked to visual periphery.