## 3 Results
### Initial Superimposition of Acoustic Startle and Touch Displacement  
Presenting naive participants with the STARTLE superimposed on DISPLACEMENT in the first 10 trials evoked pronounced forward sway, coupled with elbow flexion in the initial trial (Figure 4, red traces). Subsequent exposures to the COMBINED stimuli continued to evoke a forward sway with elbow flexion (black traces), but of smaller amplitude. The participant displayed in Figure 4 continued to express a modest elbow flexion throughout the 10 COMBINED trials and never adopted an arm-tracking behaviour. Subsequent STARTLE trials did not evoke a discernible response in this  
Figure 3: Trial-by-trial mean (SE) mechanical and muscle response amplitudes for participants who habituated to the DISPLACEMENT stimuli initially. (**A**) Participants subsequently received the COMBINED stimuli, followed by the STARTLE. (**B**) Participants subsequently received the STARTLE, followed by the COMBINED stimuli. Asterisks indicate differences identified by [PERSON]’s HSD comparisons (\(p<0.05\)). Blue squares, DISPLACEMENT; Red circles, COMBINED; Green triangles, STARTLE. Closed symbols indicate data points used in the statistical analysis. TA, tibialis anterior; AD, anterior deltoid; SCM, sternocleidomastoid; OO, orbicularis oculi; COP\({}_{\text{AP}}\), center of pressure anterior–posterior; MVC, maximum voluntary contraction. participant, outside of a persistent blink reflex (OO). The DISPLACEMENT trials at the end of the sequence of stimuli resulted in a modest elbow extension on the first exposure (blue traces), that persisted thereafter (black traces) in this participant. Group averaged data for the cohorts that received the COMBINED stimulus initially are displayed in Figure 5. Figure 5A depicts the cohort that received the same sequence of stimuli as the participant in Figure 4. As can be seen, the initial pronounced forward sway (Trial 1 = 18.8 \(\pm\) 6.0 mm) and elbow flexion (Trial 1 = 17.1 \(\pm\) 3.3\({}^{\circ}\)) was a consistent outcome for all participants who received the COMBINED stimulus in the first trial. Subsequently, the forward sway persisted, but decreased (Trials 8-10 = 5.9 \(\pm\) 2.7 mm), while the elbow flexion decreased towards no discernible response (Trials 8-10 = 0.8 \(\pm\) 0.5\({}^{\circ}\) of flexion). Pronounced EMG bursts were evoked in each of the muscles depicted with the initial COMBINED stimuli. Where the response amplitudes in TA and SCM decreased by 2/3 of their first trial amplitude on average, AD burst amplitude decreased more modestly and OO burst amplitudes were unchanged with repeated exposure to the COMBINED stimuli. The subsequent STARTLE alone trials evoked a blink reflex in OO that was not different in amplitude from the response evoked in the preceding COMBINED trials, but little else of note in the other measures. Thereafter, the DISPLACEMENT trials resulted in modest forward sway (Trial 16 = 3.0 \(\pm\) 2.1 mm) that decreased with repeated trials (Trial 20 = 0.8 \(\pm\) 1.0 mm), and a minimal elbow extension (Trial 16 = 0.7 \(\pm\) 0.4\({}^{\circ}\)) that persisted with repeated trials (Trial 20 = 1.0 \(\pm\) 0.3\({}^{\circ}\)). Small, persistent bursts were observed in AD, while little activity in TA, SCM or OO was evoked with DISPLACEMENT trials. A complete report of the descriptive statistics and the result of the associated ANOVAs is provided in Table 2A. Figure 4: Sample data traces from a single participant who received the COMBINED stimuli initially. Each cluster of traces represents the complete series of trials for that stimulus mode, with the coloured traces representing the first trial for that stimulus mode (Red, COMBINED; Green, STARTLE; Blue, DISPLACEMENT). The thin black lines of each cluster represent the subsequent trials. The vertical dashed line is aligned to the onset of the stimuli. Positive deflections in the COP\({}_{\text{AP}}\) traces represent forward, elbow traces represent extension, and ankle traces represent plantar flexion.