Contribution of subcortical motor pathways to the execution of ballistic movements

Preparation for a voluntary ballistic movement in the context of a simple reaction time task (SRT) involves enhancement of excitability in subcortical motor tracts. However, little is known about the characteristics of that preparation, how long the excitability enhancement takes place, and whether it is present not only in SRT but also in more attention requiring tasks as those involved in choice reaction time (CRT) paradigms. In the study presented here, we used the effects induced by a startling auditory stimulus (SAS) on reaction time (the StartReact effect) as a probe for assessing motor preparation of subcortical tracts. Subjects were requested to make a ballistic wrist extension movement to the presentation of a visual imperative signal. A SAS was randomly introduced in 25% of trials at 0, -50, -100, -200, -300, -400, and -500 ms. We measured reaction time at the onset of the first burst of EMG activity in the agonist muscle, the configuration of the EMG activity in agonist and antagonist muscles, and whether the responses were congruent with the side of the stimulus. At the interval of 0 ms, the SAS induced a shortening of the reaction time similarly in CRT and SRT. The duration of the preparation time was significantly longer for SRT (421 +/- 71 ms) than for CRT (138 +/- 63 ms). Some degree of co-activation was present in all subjects in CRT trials without SAS, and increased considerably in trials with SAS. Isolated or significantly predominant preparation of the wrong hand was observed in 32% of the trials at an interval of 0 ms, and in 44% of trials in the remaining intervals. Preparation errors consisting of the presence of a well defined triphasic pattern were very variable, although more frequently seen in the dominant hand (64%) than in the non-dominant hand (36%). Our results indicate that preparation of subcortical motor pathways occurs before the execution of ballistic movements in the context of a CRT paradigm, but its duration is shorter than in SRT tasks. Preparation for CRT may be biased towards the dominant hand but factors other than laterality have also to be taken into account.