Dwell time mediates the preparation of single vs. multiple component movements

Targeted reciprocal aiming movements are pervasive in everyday life (e.g. video games), but it is unclear how timing parameters between elements affects the preparation of these movements. In order to probe the preparatory state of the motor system a loud (>124 dB) startling acoustic stimulus (SAS) can be used since it has been shown that the prepared and intended movement is "triggered" involuntarily by the SAS (Carlsen et al. 2012). In the present experiment, participants performed ballistic 20 deg. extension-flexion movements using the right wrist to a fixed target region in which duration of a pause (dwell time) at the target was manipulated (50ms, 200ms, or 500ms). A SAS was presented randomly during 20% of trials before either moving toward or moving away from the target region to determine how motor preparatory mode was affected by task requirements. Results indicate that for the shorter dwell times (50ms, 200ms), the SAS presented prior to the initial outward movement led to significantly earlier onsets of both the outward and return components (p's < .05). In contrast, for the long dwell time (500ms), only the outward component of the movement was elicited early. Similar results were seen for SAS delivered prior to exiting the target region. These findings suggest that for long dwell times the movement was planned as two separate components while for shorter dwell times (