Pupil size reveals presaccadic attentional shifts up and downward: A possible dissociation between the where and how of attention

Humans frequently move their eyes to foveate relevant information in the world. It is dominantly assumed that attentional shifts must precede saccades to prepare the brain for postsaccadic retinal input, allowing for perceptual continuity across eye movements. A recent surge of studies have investigated visual anisotropies around the visual field, including presaccadic attention. Such studies demonstrated benefits of presaccadic attention on task performance for horizontal and downward, but not for upward saccades. This contrasts the dominant view: if attention is not moved prior to upward saccades, presaccadic attention may not be necessary to facilitate perceptual continuity. Here we capitalized on the fact that the pupil light response robustly tracks attention to investigate whether presaccadic attention moves up and downwards. We crucially manipulated whether presaccadic attention could shift toward the background brightness of the ensuing saccade target by presenting the brightness throughout the trial, or by presenting the brightness upon saccade onset. In two experiments, we observed acceleration of the onset of the pupil light response for both upward and downward saccades when the landing brightness could be prepared prior to the saccade. This shows that presaccadic attention is deployed, and can facilitate perceptual continuity along the vertical meridian. In combination with previous work, these results suggest that presaccadic attention can be shifted in space without enhancing specific facets (e.g. contrast sensitivity) of visual processing at the deployed location. The known underrepresentation of the upper visual field in early visual cortex may underlie the dissociation between where attention is deployed and how it affects visual processing. However, more work is necessary to identify when, and how, such dissociations occur.