1. Ten participants made bimanual reaching movements with a movement time goal of 500 ms. The trajectories of the hands were recorded with an Optotrak with a sample frequency of 500 Hz. Vision of the hands was available at the start of every trial to orient the participant, but it was removed during movements to prevent vision-based on- line corrections. A fixation point was used to prevent eye movements from affecting the trajectories (Figure 1a). All movements were cued by having a target number 8 appear in the middle target box for each hand (Figure 1b). This ensured the same information processing occurred up to movement onset. Bimanual interference during on-line control to symbolically- vs. directly-cued target locations Jarrod Blinch, Brendan Cameron, Melanie Lam, Silvia Hua, Melissa Cory, Romeo Chua School of Human Kinetics, University of British Columbia jpgblinc@interchange.ubc.ca The study of bimanual coordination has found many constraints on our actions. For example, in the reaction time (RT) paradigm (Spijkers, Heuer, Kleinsorge, & van der Loo, 1997), participants simultaneously reached for a short or long target with each hand. When the targets were symbolically cued, with letters S and L for short and long movements, there was a RT cost for asymmetric movements (right hand short, left hand long or vice versa, which is cued by S L or L S) compared to symmetric movements (S S or L L). When the targets were directly cued by illuminating the targets, the cost was abolished (Diedrichsen, Hazeltine, Kennerley, & Ivry, 2001). In a similar target jump paradigm (Diedrichsen, Nambisan, Kennerley, & Ivry, 2004), participants simultaneously reached for two targets. On-line corrections were directly cued by having one of the targets jump to the left or right. These corrections were accomplished with asymmetric movements of the hands. Like the directly-cued movements in the RT paradigm, there was minimal bimanual interference with only a small, transient perturbation in the trajectory of the unperturbed hand. The advantages afforded by direct cuing are presumed to be due to the facilitation of response selection processes (RT paradigm) and to vision-dependent, automatic corrections (target jump paradigm). We compared symbolically- and directly-cued on-line corrections in a bimanual reaching task. We anticipated greater interference for symbolically-cued than for directly-cued corrections, as seen for asymmetric movements in the RT paradigm. The time to respond to symbolic cues was longer than direct cues as they rely on vision-dependent, voluntary processes rather than automatic processes. We anticipated greater interference for symbolically-cued than for directly-cued corrections. Symbolically-cued on-line corrections did result in larger lateral deviations in the unperturbed hand compared to directly-cued corrections. However, the influence of symbolic cues on the degree of interference was less than what might be expected based on RT costs for asymmetric bimanual movements. At movement onset for 50% of the trails, an on-line correction was directly or symbolically cued by a target jump (Figure 1d) or target identity change (Figure 1e), respectively. For directly-cued on-line corrections, the target 8 would jump to the left or right target box and the participant would reach to the new target location (Figure 1d). If the right target changed to a rightward facing E (Figure 1e), then the right hand (the perturbed hand) would make an on-line correction to the right target box. A leftward facing E was used for corrections to the left target box. We examined the effects of the target perturbation on the unperturbed hand when only one hand was required to change direction by comparing the lateral deviation to control trials. Figure 1. Stimulus progression for each type of trial. The fixation point appears in the middle of the target boxes for the left and right hands (a). A target number 8 appears in the middle target box for each hand as the movement cue (b). In control trials, an on-line correction is not required and the participant simply reaches to the original targets (c). Both the direct (d) and symbolic (e) cues shown above require an on-line correction of the right hand to the right target box. The red lines represent the trajectories of the hands. All participants successfully made on-line corrections to symbolically- or directly-cued target locations. Figure 2 illustrates the mean trajectories of a representative participant making directly- or symbolically-cued on-line correction to the left target box with the left hand. The time to respond to a target perturbation was significantly longer for symbolic cues than direct cues (Figure 3). The effect of the perturbation on the unperturbed hand was determined by comparing the lateral position of the unperturbed hand to control trials. We observed significantly larger mean lateral deviation in the position of the unperturbed hand in the symbolic compared to the direct condition (Figure 4). Diedrichsen, J., Hazeltine, E., Kennerley, S., & Ivry, R.B. (2001). Moving to directly cued locations abolishes spatial interference during bimanual actions. Psychological Science, 12, 493-498. Diedrichsen, J., Nambisan, R., Kennerley, S.T., & Ivry, R.B. (2004). Independent on-line control of the two hands during bimanual reaching. European Journal of Neuroscience, 19, 1643-1652. Spijkers, W., Heuer, H., Kleinsorge, T., & van der Loo, H. (1997). Preparation of bimanual movements with same and different amplitudes: Specification interference as revealed by reaction time. Acta Psychologica, 96, 207–227. Purpose Methods Results Conclusions Funding from NSERC supported this experiment. Background Figure 2. Mean trajectories of a representative participant for control (black), direct (red), and symbolic (blue) conditions. Means were calculated from trials with normalized displacement. The outside, dotted lines depict standard deviations from the mean trajectories. Figure 3. Time to respond to target perturbations for directly- and symbolically-cued corrections. Figure 4. Mean lateral position of the right, unperturbed hand by the percentage of movement. Directly-cued on-line corrections are on the left graph and symbolically-cued corrections are on the right. The position of the unperturbed hand is shown when the correction of the left, perturbed hand is right (red), left (blue), or control (black). Filled points are significantly different from control; vertical lines depict standard deviations of the means.