Bimanual reaches with symbolic cues exhibit errors in target selection

We examined the movement trajectories of symmetric and asymmetric bimanual reaches to targets specified by direct spatial cues and by indirect symbolic cues. Symbolically cued asymmetric reaches have been shown to exhibit longer reaction times compared with symmetric reaches, whereas no such reaction time cost is observed when targets are spatially cued--a pattern thought to implicate increased demands on response selection (Diedrichsen et al. in Psychol Sci 12(6):493-498, 2001). As symbolically cued reaches impose greater demands on cognitive visuomotor translation than spatially cued reaches (Diedrichsen et al. in Cereb Cortex 16(12):1729-1738, 2006), we asked whether bimanual movements exhibit more spatial coupling with symbolic cues than with spatial cues. Participants made bimanual symmetric and asymmetric reaches to short- and long-distance targets cued either symbolically or spatially. We replicated the reaction time cost for symbolically cued asymmetric movements. A subset of these asymmetric reaches also showed large trajectory modulations. It appeared that this subset had been incorrectly prepared and the movements required of the left and right arms had been switched. No such errors in target selection were observed when targets were spatially cued. In contrast to the reaction time cost and errors in selection for symbolically cued movements, we observed little evidence of increased spatial coupling with symbolic cues when movements were initiated towards the correct targets. We conclude that cognitive visuomotor translation demands during response selection increases bimanual coupling at the level of response selection (reaction time cost, errors in target selection) but not at the level of movement execution (spatial coupling).