Startle response of human neck muscles sculpted by readiness to perform ballistic head movements

1. An acoustic startle stimulus delivered in place of a 'go' signal in a vo luntary reaction time (RT) task has been shown previously to advance the on set latency of a prepared distal limb movement without affecting the amplit ude of the. muscle response or movement kinematics. The primary goal of thi s study was to use muscles with a larger startle response to investigate wh ether the startling stimulus only triggered the RT movement or whether some form of, interaction occurred between a startle response and a temporally advanced RT movement. 2. Twenty healthy male or female subjects were instructed to react as quick ly as possible to an acoustic 'go' stimulus by performing a ballistic head flexion or right axial rotation. The 'go' stimulus was periodically replace d by an acoustic stimulus capable of eliciting a startle reflex. Separate s tartle-inducing stimuli under relaxed conditions before and after the movem ent trials served as control trials (CT trials). Bilateral surface electrom yography of the orbicularis oculi, masseter, sternocleidomastoid and cervic al paraspinal muscles, and head-mounted transducers were used to measure th e muscle response and movement kinematics. 3. Muscle activation times in startled movement trials (ST trials) were abo ut half those observed in RT trials, and were not significantly different f rom those observed in the startle CT trials. The duration of head accelerat ion was longer in ST trials than in RT trials and the amplitude of both the neck muscle electromyogram (EMG) and head kinematics was larger during ST trials than during RT trials. The EMG amplitude of ST trials was biased upw ard rather than scaled upward compared with the EMG amplitude of RT trials. 4. Over the 14 ST trials used in this experiment, no habituation of the ref lex response was observed in the muscles studied. This absence of habituati on was attributed to a combination of motor readiness and sensory facilitat ion. 5. The results of this experiment indicated that the neck muscle response e voked by a startling acoustic stimulus in the presence of motor readiness c ould be described as a facilitated startle reflex superimposed on a tempora lly advanced, pre-programmed, voluntary RT movement. Parallel reticular pat hways to the neck muscle motoneurones are proposed as a possible explanatio n for the apparent summation of the startle and voluntary movement response s.