## 3 Results
### Comparing Initial COMBINED with Initial DISPLACEMENT  
To compare the effect of superimposing STARTLE with DISPLACEMENT with the responses observed with DISPLACEMENT alone in the initial trials of naive participants, the two cohorts of each stimulus type were collapsed. The group averaged data comparing the initial COMBINED stimuli with the initial DISPLACEMENT stimuli are presented in Figure 6. As seen in the COP\({}_{\text{AP}}\) data, COMBINED resulted in a generally larger forward sway response than for DISPLACEMENT. The ANOVA indicated a significant Stimulus x Trial interaction (\(F_{130}=4.44\), \(p=0.04\)), as the sway amplitude converged with the later trials. The first trial sway response was significantly larger for the COMBINED stimulus (Trial \(1=20.2\pm 4.8\) mm), compared with DISPLACEMENT (Trial \(1=6.8\pm 1.8\) mm). Responses at the elbow varied considerably between the COMBINED and DISPLACEMENT stimuli. In particular, with the DISPLACEMENT stimuli an initial elbow flexion was replaced with a persistent elbow extension by the second trial. In contrast, the large initial elbow flexion with the COMBINED stimuli progressively decreased in amplitude with repeated stimuli, but an extension of the arm was not observed. The ANOVA indicated a significant Stimulus x Trial interaction (\(F_{1,22}=46.85\), \(p<0.0001\)), as the amplitude of change in elbow angle converged with the later trials. The amount of elbow flexion with the first trial was significantly larger for the COMBINED stimulus (Trial \(1=22.2\pm 2.4^{\circ}\)), compared with DISPLACEMENT (Trial \(1=2.6\pm 0.8^{\circ}\)). The amplitude of the evoked burst of EMG activity in TA was generally larger with the COMBINED stimuli. The ANOVA did not identify a Stimulus x Trial interaction (\(F_{130}=3.92\), \(p=0.06\)), whereas there were significant main effects of both Stimulus (\(F_{130}=13.26\), \(p=0.001\)) and Trial (\(F_{130}=25.24\), \(p<0.0001\)). The data for burst amplitude in AD demonstrate a clear Stimulus x Trial interaction (\(F_{130}=6.80\), \(p=0.01\)) as the burst amplitudes with DISPLACEMENT are initially small (Trial \(1=1.2\pm 0.3\)% MVC), but increase with later trials (Trials 8-10 = \(3.9\pm 0.5\)% MVC). In contrast, the AD bursts evoked by the COMBINED stimuli show little adaptation with repeated trials, with an initial amplitude of \(8.5\pm 2.9\)% MVC in Trial 1 then decreasing to \(5.2\pm 1.2\)% MVC in Trials 8-10. Burst amplitude in SCM was larger with COMBINED stimuli than with DISPLACEMENT. A significant Stimulus x Trial interaction (\(F_{126}=12.26\), \(p=0.002\)) was observed as the amplitude of the burst in SCM with the COMBINED stimuli decreased from an initial amplitude of \(31.6\pm 6.7\)% MVC in Trial 1 to an amplitude of \(10.5\pm 2.1\)% MVC in Trials 8-10, in contrast to what was observed with DISPLACEMENT, which did not change from near zero values for all trials. Bursts of activity in OO were seldom evoked with DISPLACEMENT, but were consistently evoked with the COMBINED stimulus. This yielded a significant main effect of Stimulus (\(F_{130}=25.59\), \(p<0.0001\)), in the absence of a Stimulus x Trial interaction (\(F_{130}=1.46\), \(p=0.2\)) or main effect of Trial (\(F_{130}=1.17\), \(p=0.3\)).