Response preparation changes during practice of an asynchronous bimanual movement.

For synchronous bimanual movements, we have shown that a different amplitude can be prepared for each limb in advance and this preparation improves with practice (Maslovat et al. 2008). In the present study, we tested whether an asynchronous bimanual movement can also be prepared in advance and be improved with practice. Participants practiced (160 trials) a discrete bimanual movement in which the right arm led the left by 100 ms in response to an auditory "go" signal (either 80 dB control stimulus or 124 dB startle stimulus). The startle stimulus was used to gauge whether inter-limb timing could be pre-programed. During startle trials, the asynchronous bimanual movement was triggered at early latency suggesting the entire movement could be prepared in advance. However, the triggered movement had a shorter between-arm delay and a temporally compressed within-arm EMG pattern, results that we attribute to increased neural activation caused by the startling stimulus. However, as both startle and control trials improved over time, it does appear response preparation of interval timing can improve with practice.