Motor Preparation of Spatially and Temporally Defined Movements: Evidence from Startle

The purpose of this study was to examine preparation differences in temporally versus spatially defined movements through the use of a startling stimulus. On day 1 participants performed arm extension movements to one of three spatial targets (20, 40, 60 degrees), while on day 2 participants performed an arm extension movement of 20 deg at three movement speeds (slow, moderate, fast). All movements were performed in a blocked simple RT paradigm, with trials involving a startling stimulus (124dB) interspersed randomly with control trials. The startling stimulus caused a compression of the kinematic and EMG profile of all movements such that they were performed faster; however temporally-based movements showed a greater compression on startle trials for slower movements as compared to faster movements. Furthermore, the startling stimulus decreased the time between initial agonist and antagonist bursts to a greater extent for the 20 deg movement when defined by a temporal rather than spatial goal. We attributed this result to participants utilizing an internal timekeeper for the timed movements, of which the pacemaker pulses was affected by the participant’s arousal level. These results suggest that temporally defined movements may be prepared in a different manner than spatially defined movements.