Abstract
Attention shifts that precede goal-directed eye and hand movements are regarded as markers of motor target selection. Recent studies found parallel allocation of visuospatial attention to saccade and reach targets during simultaneous eye-hand movements, arguing in favor of independent, effector-specific target selection mechanisms. This raises the question whether the overall attention capacity increases with the number of active effectors. In a modified Theory of Visual Attention (TVA; Bundesen, 1990) paradigm, participants reported briefly presented letters during eye, hand, or combined eye-hand movement preparation to centrally cued locations. Modeling the data according to the TVA allowed us to assess both the overall attention capacity as well as the deployment of visual attention to individual locations in the visual work space. In two experiments, we show that attention was predominantly allocated to the motor targets – without competition between effectors. The parallel benefits at eye and hand targets, however, had concomitant costs at non-motor locations, and the overall attention capacity was not increased by the simultaneous recruitment of both effector systems. Moreover, premotor shifts of attention dominated over voluntary deployment of processing resources, yielding severe impairments of voluntary attention allocation. We conclude that attention shifts to multiple effector targets without mutual competition, given that sufficient processing resources can be withdrawn from movement-irrelevant locations.








