Rebuttal from Brian C. Horslen, Christopher J. Dakin, J. Timothy Inglis, Jean-Sébastien Blouin and Mark G. Carpenter

The topic of debate is whether fear influences vestibular-evoked balance responses. Opposing arguments from Reynolds et al. (2015) rely on the reported lack of changes in the early ( 800 ms) responses. The evidence drawn from the lack of early response change is unconvincing due to an increased likelihood of type II error caused by a small sample size and poor resolution resulting from the following factors: (i) use of centre of mass position and velocity estimates to detect vestibular-evoked responses in leg muscles, especially as tandem-stance requires multijoint control; (ii) the limited number of stimulations; and (iii) small early responses due to low (1 mA) stimulus intensity (Ali et al. 2003). Likewise, interpretation of late responses is confounded by the potential influence of fear on reactions to non-vestibular sensory feedback related to the vestibular perturbation (Day & Guerraz, 2007) and conscious strategies (Reynolds, 2010) within this time period. We disagree with Reynolds et al. (2015) that fear-related increases in gain and coupling of ground reaction forces (GRFs) to stochastic vestibular stimulation (SVS) are irrelevant to balance. Early responses are clearly distinguishable using GRFs (Marsden et al. 2002), with less mechanical filtering of the neuromuscular response affecting GRFs than kinematics (Forbes et al. 2015). In our study, SVS was engineered to highlight changes in early balance responses to vestibular stimulation that are not confounded by later perturbation-induced sensorimotor responses (Horslen et al. 2014). The significant SVS gain changes observed between 2 and 16 Hz coincides with natural head frequencies recorded during gait and whole-body perturbations (for review, see Forbes et al. 2015). After establishing threat-related effects on early vestibular-evoked balance responses, we confirmed these effects in GRFs and EMG from posturally engaged muscles using an SVS stimulus that included the 0–2 Hz bandwidth and with vestibular-evoked myogenic potentials (see Horslen et al. 2015). The notion that we have confused statistical and functional significance is false; we view evidence of a true effect, beyond chance or bias, as a necessary prerequisite to considering functional significance. Furthermore, functional significance should not be discounted because of small absolute changes, which would be expected given the small virtual rotational velocity induced by the stimulus (Peters et al. 2015) and are consistent with magnitudes reported elsewhere (Reynolds, 2010; Mian & Day, 2014). In contrast, we believe that the relative change in gain of early vestibular-evoked responses provides more meaningful insight into functional significance. Thus, fear-related increases in gain (81–231%) are non-trivial and functionally significant, particularly when translated to real-life head accelerations.