## 3 Results  
### Superimposition of Acoustic Startle on Habituated Touch Displacement Responses  
Unexpected touch displacements evoked responses comparable to what has been reported previously [2; 3]. The leftmost column of traces in Figure 2 depicts data from one participant who displayed a forward sway reaction (COPAP) coupled with an elbow flexion after the initial unexpected touch displacement (blue traces), but adapted to an arm-tracking behaviour (elbow extension) with minimal forward sway after the subsequent displacements (black traces). STARTLE was superimposedwith the touch displacement for the subsequent five trials, displayed in the middle column of traces in Figure 2. For this participant, the initial COMBINED stimulus (red traces) resulted in the continued expression of the arm-tracking behaviour, but with larger amplitude EMG responses, or emergence of additional EMG responses, compared with the DISPLACEMENT responses. For the final five trials, STARTLE was presented alone (rightmost column), which yielded distinct bursts of EMG activity in TA, SCM and OO, but minimal impact on the COP\({}_{\text{AP}}\) position or elbow angle, relative to the preceding DISPLACEMENT or COMBINED stimuli. The graphs in Figure 3A display average data from all participants who received the sequence of stimuli shown in Figure 2. As shown in the COP\({}_{\text{AP}}\) and elbow angle graphs, the initial unexpected touch displacement resulted in large forward sway (Trial 1 = 8.1 \(\pm\) 3.4 mm), coupled with a distinct elbow flexion (Trial 1 = 3.2 \(\pm\) 1.4\({}^{\circ}\)). On subsequent touch displacement trials, the forward sway was substantially reduced, and the elbow switched to an extension response. By the last three trials of DISPLACEMENT, the forward sway was minimal (Trials 8-10 = 0.5 \(\pm\) 0.3 mm) and the elbow extension was consistent (Trials 8-10 = 2.6 \(\pm\) 0.6\({}^{\circ}\)). With the superimposition of STARTLE on the touch displacement elbow extension was markedly increased (Trial 11 = 7.5 \(\pm\) 1.9\({}^{\circ}\)), but COP\({}_{\text{AP}}\) sway was not evident (Trial 11 = -0.2 \(\pm\) 1.6 mm). Subsequently, the augmented elbow extension abated (Trial 15 = 3.6 \(\pm\) 1.0\({}^{\circ}\)), comparable to the extension observed at the end of the DISPLACEMENT trials. The subsequent STARTLE alone trials did not evoke responses in these two metrics. EMG response amplitudes are also shown for TA, AD, SCM and OO. The initial DISPLACEMENT resulted in a burst of TA activity (Trial 1 = 7.6 \(\pm\) 3.2% MVC), but not AD (Trial 1 = 0.6 \(\pm\) 0.3% MVC), consistent with a forward sway response. With subsequent DISPLACEMENT trials the TA burst decreased (Trials 8-10 = 0.2 \(\pm\) 0.2% MVC), while AD demonstrated a clear burst (Trials 8-10 = 3.9 \(\pm\) 0.7% MVC), consistent with the occurrence of the arm-tracking behaviour. EMG activity in SCM or OO was not consistently expressed with DISPLACEMENT alone. Superimposition of STARTLE and DISPLACEMENT resulted in large bursts of EMG activity in all four muscles displayed. The first COMBINED trial resulted in a large burst in TA (Trial 11 = 21.9 \(\pm\) 8.5% MVC) that abated with repeated trials (Trial 15 = 8.0 \(\pm\) 3.1% MVC). Similar effects were observed in AD (Trial 11 = 18.4 \(\pm\) 5.7; Trial 15 = 8.9 \(\pm\) 3.2% MVC) and SCM (Trial 11 = 42.1 \(\pm\) 15.0% MVC; Trial 15 = 15.0 \(\pm\) 6.0% MVC), but not OO which displayed a burst amplitude unaffected by the repetition of the stimulus (Trial 11 = 21.7 \(\pm\) 8.4% MVC; Trial 15 = 30.7 \(\pm\) 5.7% MVC). Subsequently, with the STARTLE alone, bursts in TA and AD were markedly reduced (Trial 16 = 2.0 \(\pm\) 1.1% MVC and 1.9 \(\pm\) 1.5% MVC, respectively), whereas bursts in SCM continued to progressively decline with continued stimuli. In contrast, burst amplitudes in OO continued to be strongly expressed and did not appear to habituate with repeated STARTLE. A complete report of the descriptive statistics and the result of the associated ANOVAs is provided in Table 1A.