Symbolically-cued asymmetric reaches result in spatial interference during initiation and execution Jarrod Blinch and Romeo Chua School of Human Kinetics at the University of British Columbia When both arms attempt to reach at the same time with different paths, interference occurs that forces the paths to become more similar than desired (Kelso, Putnam, & Goodman, 1983; Swinnen, Walter, & Shapiro, 1988). This spatial interference occurs during movement execution, but spatial interference can also occur during premovement planning. For example, imagine a reaction time task where each arm is cued to make either a long or short movement at the same time. When these movements are cued with symbols L or S to indicate a long or short movement (Figure 1, right), the reaction times are longer for movements with unequal amplitudes (asymmetric: L S or S L) than movements with equal amplitudes (symmetric: L L or S S). This increased reaction time for asymmetric movements is the result of interference during premovement planning, specifically the translation of two different symbolic cues (L S or S L) to movement amplitudes. Interestingly, the reaction times for asymmetric and symmetric reaches are equal when directly cueing the movements by illuminating the targets (Figure 1, left) (Diedrichsen, Hazeltine, Kennerley, & Ivry, 2001). Directly-cued movements avoid interference during initiation as they require less translation than movements cued symbolically (Hazeltine, Diedrichsen, Kennerley, & Ivry, 2003). Spatial interference during movement execution has yet to be examined in this task, but it could result in amplitude assimilation of asymmetric reaches (the short movement would become longer and vice versa). Will there be greater amplitude assimilation for symbolically- cued asymmetric reaches than directly-cued ones? Introduction Conclusions Symbolically-cued asymmetric reaches, as compared to directly-cued asymmetric reaches, have more processing demands on response selection for the cue-to-target translations (Hazeltine et al., 2003). This has been shown to result in interference during premovement planning (Diedrichsen et al., 2001). We also found that it results in greater amplitude assimilation, which is corrected by the end of the movement. Amplitude assimilation is a form of spatial interference during movement execution. Therefore, symbolically-cued asymmetric reaches result in spatial interference during initiation and execution. References Diedrichsen, J, Hazeltine, E, Kennerley, S, & Ivry, RB (2001). Moving to directly cued locations abolishes spatial interference during bimanual actions. Psychological Science, 12(6), Hazeltine, E, Diedrichsen, J, Kennerley, S, & Ivry, RB (2003). Bimanual cross-talk during reaching movements is primarily related to response selection, not the specification of motor parameters. Psychological Research, 67(1), Kelso, JA, Putnam, CA, & Goodman, D (1983). On the space-time structure of human interlimb co- ordination. The Quarterly Journal of Experimental Psychology, 35(2), Swinnen, SP, Walter, CB, & Shapiro, DC (1988). The coordination of limb movements with different kinematic patterns. Brain and Cognition, 8(3), Eighteen participants made bimanual reaching movements in six blocks with either direct (Figure 1, left) or symbolic cues (Figure 1, right). Movement trajectories were recorded with an Optotrak at a sample frequency of 500 Hz. Methods Figure 1 Results The reaction times of symbolically-cued movements were slower than directly- cued movements (Figure 2). Asymmetric reaches were initiated slower than symmetric reaches. A significant cue by reach interaction replicated Diedrichsen and colleagues (2001) and showed that symbolic asymmetric reaches were initiated 56.7 ms slower than symbolic symmetric reaches. Reaction times of direct asymmetric reaches were also slower than direct symmetric reaches, but only by 7.8 ms. The mean time-normalized forward positions are shown for a representative participant in Figure 3. Directly-cued movements are on the left and symbolically- cued movements are on the right; symmetric reaches are shown in the top row and asymmetric reaches are shown in the bottom row (overlaid on the same symmetric reaches as the top row). Solid lines are mean forward positions for the left arm, and the dotted lines are one standard deviation from the participant’s mean. Comparing the overlaid asymmetric reaches to the symmetric reaches (Figure 3, bottom), gives an indication of amplitude assimilation. Visual inspection revealed that there was greater amplitude assimilation with symbolic cues (Figure 3, bottom-right) than direct cues (Figure 3, bottom-left). Subtracting the symmetric reaches from the asymmetric reaches normalized the forward positions of asymmetric reaches. The potential amplitude assimilation was analyzed by calculating the root mean squared error of the normalized positions. Symbolically-cued movements had greater assimilation than directly-cued movements (Figure 4). Short movements also had greater assimilation than long movements. There was also a significant cue by movement amplitude interaction that revealed that short movements had a greater increase in amplitude assimilation from direct to symbolic cues than long movements. Figure 4 Figure 3. Forward positions by percent time. Figure 2 L L L S S S S L