Abstract
Strong dual task costs are observed when people perform two tasks at the same time, and these costs appear in different tasks, effectors and modalities. Reaction time is commonly used to measure dual task costs, and inferences about underlying cognitive factors leading to dual task competition are made based on shorter or longer reaction times. Based on reaction times it has been suggested that movement goal selection is a major factor leading to dual task costs when making multiple goal directed movements at the same time. To investigate this, we asked participants to point and look at two different locations while we varied the time between the cues to start eye and hand movements. Like in previous studies, we observed that participants were slower to start their eye or hand movement if they were planning another movement at that time. Identical results were observed when participants were planning bimanual movements. Second, we measured whether spatial attention caused these observed dual task costs. Movements might have been delayed, because participants were not able to select multiple movement locations in parallel. We measured attention allocation by presenting short attentional probes at different stages of movement planning. In strong contrast to the dual task cost observed in movement latencies, participants allocated their attention to eye and hand movement goals in parallel and without any cost. The same pattern of results was evident for bimanual movements. These results demonstrate that observed dual task costs in goal directed movements do not arise from movement goal selection. The results also suggest a dissociation between movement latencies and movement goal selection, in that longer movement latencies do not equate to delayed movement goal selection.