Interaction between startle and voluntary reactions in humans.

The startle reaction (SR) is usually understood as an involuntary reaction to an unexpected sensory input. However, there is evidence that the mechanisms involved in the SR contribute also to the execution of actions that we consider voluntary. We need to exert a fine control of the motor output, which usually involves inhibition of the reflex reaction but it may also imply letting the SR manifest, shaped in accordance with our willed actions. The latter occurs mainly with relatively simple ballistic movements but it has also been shown to occur in complex movements. In this review, we describe arguments published in the literature in favour of the contribution of subcortical motor tracts involved in the generation of the SR to the execution of voluntary movements. Some of these studies have brought insight on the mechanisms accounting for motor preparation and execution of voluntary movements.