Altered triggering of a prepared movement by a startling stimulus

An experiment is reported that investigated the effects of an auditory startling stimulus on a compound movement task. Previous findings have shown that, in a targeting task, a secondary movement can be initiated based on the proprioceptive information provided by a primary movement. Studies involving the presentation of a startling stimulus have shown that in reaction time (RT) tasks, prepared ballistic movements could be released early when participants are startled. In the present study we sought to determine whether the secondary component in an ongoing movement task, once prepared, could also be triggered by a startling stimulus. Participants performed a slow active elbow extension (22 degrees /s), opening their hand when the arm passed 55 degrees of extension from the starting point. An unexpected 124 dB startle stimulus was presented 5, 25, or 45 degrees into the movement. Findings showed that, when participants were startled, the secondary component was triggered despite incongruent kinesthetic information. However, this only occurred when the startle was presented late in the primary movement. This suggests that the secondary movement was not prepared prior to task initiation, but was "loaded" into lower brain structures at some point during the movement in preparation to be triggered by the CNS. This occurred late in the movement sequence, but >/=400 ms prior to reaching the target. These findings indicate that, in addition to ballistic RT tasks, a startle can be used to probe response preparation in ongoing compound movement tasks.