Prepared movements are elicited early by startle

A startle stimulus has been shown to elicit a ballistic response in a reaction time (RT) task at very short latencies without involvement of the cerebral cortex (J. Valls-Sole, J. C. Rothwell, F. Gooulard, G. Cossu, & E. Munoz, 1999). The present authors examined the nature of the startle response. A simple RT task was used in which 8 participants performed arm extension movements to 3 target distances (20 degrees, 40 degrees, and 60 degrees ) in a blocked design. An unpredictable startling acoustic stimulus (124 dB) replaced the imperative stimulus in certain trials. The authors verified the presence of a startle response independent from the prepared response by observing electromyographic (EMG) activity in sternocleidomastoid and orbicularis oculi muscles. Findings indicated that when the participant was startled, the intended voluntary response was produced at significantly shorter response latencies. Furthermore, the kinematic variables of the observed response during startle trials for all 3 target distances were mostly unchanged. The EMG characteristics of the responses were not modified, indicating that the response produced was indeed the prepared and intended response.