Patterned ballistic movements triggered by a startle in healthy humans

1. The reaction time to a visual stimulus shortens significantly when an un expected acoustic startle is delivered together with the 'go' signal in hea lthy human subjects. In this paper we have investigated the physiological m echanisms underlying this effect. If the commands for the startle and the v oluntary reaction were superimposed at some level in the CNS, then we would expect to see alterations in the configuration of the voluntary response. Conversely, if the circuit activated by the startling stimulus is somehow i nvolved in the execution of voluntary movements, then reaction time would b e sped up but the configuration of the motor programme would be preserved. 2. Fourteen healthy male and female volunteers were instructed to react as fast as possible to a visual 'go' signal by flexing or extending their wris t, or rising onto tiptoe from a standing position. These movements generate d consistent and characteristic patterns of EMG activation. In random trial s, the 'go' signal was accompanied by a very loud acoustic stimulus. This s timulus was sufficient to produce a startle reflex when given unexpectedly on its own. 3. The startling stimulus almost halved the latency of the voluntary respon se but did not change the configuration of the EMG pattern in either the ar m or the leg. In some subjects the reaction times were shorter than the cal culated minimum time for processing of sensory information at the cerebral cortex. Most subjects reported that the very rapid responses were produced by something other than their own will. 4. We conclude that the very short reaction times were not produced by an e arly startle reflex adding on to a later voluntary response. This would hav e changed the form of the EMG pattern associated with the voluntary respons e. Instead, we suggest that such rapid reactions were triggered entirely by activity at subcortical levels, probably involving the startle circuit. 5. The implication is that instructions for voluntary movement can in some circumstances be stored and released from subcortical structures.