Patterned ballistic movements triggered by a startle in healthy humans

Abstract

1. The reaction time to a visual stimulus shortens significantly when an unexpected acoustic startle is delivered together with the 'go' signal in healthy human subjects. In this paper we have investigated the physiological mechanisms underlying this effect. If the commands for the startle and the voluntary 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 involved in the execution of voluntary movements, then reaction time would be 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 wrist, or rising onto tiptoe from a standing position. These movements generated consistent and characteristic patterns of EMG activation. In random trials, the 'go' signal was accompanied by a very loud acoustic stimulus. This stimulus was sufficient to produce a startle reflex when given unexpectedly on its own. 3. The startling stimulus almost halved the latency of the voluntary response but did not change the configuration of the EMG pattern in either the arm or the leg. In some subjects the reaction times were shorter than the calculated 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 early startle reflex adding on to a later voluntary response. This would have changed the form of the EMG pattern associated with the voluntary response. 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.

Figure 1. Raw data example and mean data from all subjects in the arm movement trials

A, EMG recordings from a single subject performing a ballistic wrist flexion movement. Upper panel: control trials in which the subject responded only to the visual cue. Lower panel: test trials in which the visual cue was accompanied by a loud startling auditory stimulus. MOV, signal from the movement transducer; BIC, biceps brachii; TRIC, triceps brachii; WF, wrist flexors; WE, wrist extensors. B, schematic representation of the mean EMG pattern of all subjects in control and test trials. The leftward extent of the bars represents the mean onset latency, whilst the horizontal line shows +1 s.d. The length of the bars represents the duration of the EMG bursts, and the horizontal lines at the right side of the bar show +1 s.d. Duration was only measured for the first agonist and the antagonist bursts. HandMOV, displacement of the wrist joint; note how the pattern of the EMG bursts (i.e. the interburst interval and the burst durations) is the same in control and test trials even though the onset latency is substantially reduced in the test trials.

Figure 2. Comparison of responses in prime moving muscles (wrist extensors and flexors), face and neck muscles

A, rapid wrist flexion movement in response to a visual stimulus on its own (control trial). B, rapid wrist flexion movement during a startling trial (test trial). C, response in the same muscles when the loud startling stimulus was unexpectedly given on its own. Note the similar latency of muscle activity in B and C. OOc, orbicularis oculi muscle; SCM, sternocleidomastoid muscle; WE, wrist extensors; WF, wrist flexors; MOV, signal from the movement transducer.

Figure 3. Raw data example and mean data from all subjects when performing the rise onto tiptoe task

A, recordings from a single subject performing a sudden rise onto tiptoes in control (upper panel) and test trials (lower panel). OOc, orbicularis oculi; SCM, sternocleidomastoid; SOL, soleus; TA, tibialis anterior; Acc, accelerometric recording from the dorsum of the foot. B, schematic representation of the mean pattern of EMG activity from all subjects observed in the leg movement task. The leftward extent of the bars represents the mean onset latency of the events, whilst the horizontal line shows +1 s.d. The length of the bars represents the duration of the events, and the horizontal lines at the right side of the bar show +1 s.d. pmsp, pre-movement silent period. Duration was only measured for the TRS pmsp and the TA burst. Note the preservation of the burst durations and interburst latencies despite the shortening of movement onset.