Abstract
Fast eye movements (saccades) play an essential role in the rapid sampling of our visual environment by bringing objects of interest onto the fovea, increasing visual acuity and thus enhancing visual processing. Prior to the onset of these ballistic, overt eye movements, a center of mass (CoM) calculation occurs (Coren & Hoenig, 1972; Findlay, 1982; Kowler, 2011, Van der Stigchel & Nijboer, 2011). Does a CoM calculation occur for covert attention in the absence of eye movements? In two experiments, participants searched for either a target Q in 6 Os or an O in 6 Qs displayed for 150 ms on a 126° arc of an invisible 5°-radius circle. On target present trials, the target was located either at the center of the arc or displaced laterally. If the CoM is important for orienting attention, we hypothesized that targets at the center of the arc would be detected more quickly and accurately than targets off the CoM, but that this effect would be observed only for the attentionally-demanding feature-absent search for an O in Qs. In both experiments, participants were instructed to fixate a central cross for the duration of the trial, and the trial duration was kept short. For the second experiment, we replicated the first experiment and used an eye tracker to verify maintenance of fixation during each trial. As predicted, in both experiments, a search benefit was observed for the feature-absent search condition when the target letter appeared at the center of the search array compared to when it was laterally displaced. No CoM search benefit was observed for the feature-present condition. These results suggest that the CoM calculation is a process that contributes to the orientation of covert attention in addition to contributing to saccadic programming.