Moving to directly cued locations abolishes spatial interference during bimanual actions

Interference is frequently observed during bimanual movements if the two ha nds perform nonsymmetric actions. We examined the source of bimanual interf erence in two experiments in which we compared conditions involving symmetr ic movements with conditions in which the movements were of different ampli tudes or different directions. The target movements were cued either symbol ically by letters or directly by the onset of the target locations. With sy mbolic cues, reaction times were longer when the movements of the two hands were not symmetric. With direct cues, reaction times it-ere the same for s ymmetric and nonsymmetric movements. These results indicate that directly c ued actions can be programmed in parallel for the two hands. Our results ch allenge the hypothesis that the cost to initiate nonsymmetric movements is due to spatial interference in a motor-programming stage. Rather the cost a ppears to he caused by stimulus identification, response-selection processe s connected to the processing of symbolic cues, or both.