When unintended movements "leak" out: a startling acoustic stimulus can elicit a prepared response during motor imagery and action observation.

Covert forms of practice, such as observation and imagery, have been shown to involve neurophysiological activation of the motor system, and a functional equivalence between covert and overt processes involved in action execution has been proposed (Jeannerod, 2001). We used a startling acoustic stimulus (SAS), which has been shown to trigger prepared movements involuntarily at short latencies via an increase in cortical activation, to probe the similarity of these processes and elicit movement responses in imagery and observation trials. Startle trials were interspersed with control trials while participants (n=16) performed or imagined a right hand key lift or observed a model perform the key lift. During physical movement trials, intended movements were triggered by the SAS at a short latency (RT=78 ms) in comparison to control trials (RT=110 ms). During imagery and observation, unimanual partial movements (assessed by force change and muscle activation) were elicited by the SAS, providing novel behavioural evidence for a functional similarity between covert and overt movement preparation processes. Examination of the magnitude of the reflexive startle response (an index of motor preparation) during imagery and observation also revealed similarities to physical movement trials. We conclude that covert and overt movements involve similarities in motor preparation and neural pathways, and propose that movements do not normally occur during imagery and observation due to low level neural activation.