Goal-Directed Reaching: The SNARC Effect Influences Movement Planning but not Movement Execution

Abstract

The spatial-numerical association of response codes (SNARC effect) is a well-documented phenomenon wherein numerical digits spatially bias the selection of a response, with low and high numbers eliciting faster left and right responses, respectively. A host of button press tasks report expression of the SNARC effect for response planning (i.e., reaction time: RT); however, Fischer (2003) reports that the effect can selectively manifest during the movement execution phase (i.e., movement time: MT) of a goal-directed reaching task. Here we employed a kinematic technique to determine whether the reported SNARC effect is expressed in the early or late (or both) stages of a goal-directed reaching trajectory. Results for RT produced a reliable SNARC effect whereas results for MT produced a reversed-SNARC effect that was attributed to the deceleration phase of reach trajectories. More specifically, left space reaches cued by low numbers produced shorter RTs than high number counterparts and were associated with longer MTs (the converse was true of right space reaches). We have interpreted these results in terms of a spatial-numerical bias influencing a trade-off between the temporal properties of movement planning and execution mechanisms.