THE CONTROL OF SEQUENTIAL AIMING MOVEMENTS: THE INFLUENCE OF PRACTICE AND MANUAL ASYMMETRIES ON THE ONE-TARGET ADVANTAGE Ann Lavrysen 1 , Werner F. Helsen 1 , Luc Tremblay 2 , Digby Elliott 2 , Jos J. Adam 3 , Peter Feys 1 and Martinus J. Buekers 1 ( 1 Katholieke Universiteit Leuven, Leuven, Belgium; 2 Mc Master University, Hamilton, Ontario, Canada; 3 Maastricht University, Maastricht, The Netherlands) ABSTRACT The present experiment was conducted to explore the effect of practice on the one- target advantage in manual aiming, as well as asymmetries in intermanual transfer of training. Reaction and movement times for the first movement were longer in the 2-target than in the 1-target task, regardless of the amount of practice, hand preference and practice hand. When two movements were required, peak velocity was lower and, proportionally, more time was spent after peak velocity. Our kinematic results suggest that the one-target advantage is related to both predefined strategies as well as movement implementation processes during execution. Therefore, an integration of advance planning and on-line explanations for the one-target advantage is suggested. Regarding manual asymmetries, right-handers showed more transfer of training from the left to the right hand than vice versa. Left-handers exhibited a reversed pattern of asymmetric transfer of training to right- handers, but they were more disadvantaged using their non-dominant hand. These latter two findings have implications for models of manual asymmetry and upper limb control. Key words: manual aiming, one-target advantage, manual asymmetries, handedness INTRODUCTION The One-Target Advantage When an individual performs a fast two-component aiming movement, the time to initiate and execute the first component of the movement is generally longer than if the movement is performed in isolation. The ‘one-target advantage’ (OTA) for reaction time is generally attributed to the greater motor programming demands associated with the number of elements in a movement sequence (Henry and Rogers, 1960; Klapp, 1995; Sternberg et al., 1978; Christina et al., 1985; Fischman, 1984). 1 However, there is less agreement on the source of the movement time effect (see Adam et al., 2000; Adam et al., 1995, for a review). Possible explanations vary in the extent to which they emphasize advance planning processes, on-line control and strategic considerations. Fischman and Reeve (1992) have suggested that the additional accuracy demands associated with a two-component movement require the first movement Cortex, (2003) 39, 307-325 1 There are situations in which reaction time does not increase with complexity. In these situations, the movements are characterized by more discontinuities in the trajectory, suggesting a greater dependence on on-line control.