The coordinate systems used in visual tracking.

Tracking moving objects is a fundamental attentional operation. Here we ask which coordinate system is used to track objects: retinal (retinotopic), scene-centered (allocentric), or both? Observers tracked three of six disks that were confined to move within an imaginary square. By moving either the imaginary square (and thus the disks contained within), the fixation cross, or both, we could dramatically increase the disks' speeds in one coordinate system while leaving them unchanged in the other, so as to impair tracking in only one coordinate system at a time. Hindering tracking in either coordinate system reduced tracking ability by an equal amount, suggesting that observers are compelled to use both coordinate systems and cannot choose to track only in the unimpaired coordinate system.