First Trial Postural Reactions to Unexpected Balance Disturbances: A Comparison With the Acoustic Startle Reaction

Abstract

Unexpected support-surface movements delivered during stance elicit "first trial" postural reactions, which are larger and cause greater instability compared with habituated responses. The nature of this first trial reaction remains unknown. We hypothesized that first trial postural reactions consist of a generalized startle reaction, with a similar muscle synergy as the acoustic startle response, combined with an automatic postural reaction. Therefore we compared acoustic startle responses to first trial postural reactions. Eight healthy subjects stood on a support surface that unexpectedly rotated backwards 10 times, followed by 10 startling acoustic stimuli, or vice versa. Outcome measures included full body kinematics and surface EMG from muscles involved in startle reactions or postural control. Postural perturbations and startling acoustic stimuli both elicited a clear first trial reaction, as reflected by larger kinematic and EMG responses. The ensuing habituation rate to repeated identical stimuli was comparable for neck and trunk muscles in both conditions. Onset latencies in neck muscles occurred significantly later for first trial perturbations compared with startle responses, but earlier in trunk muscles. Our results show that platform tilting initially induces reactions larger than needed to maintain equilibrium. For neck and trunk muscles, these first trial postural reactions resembled acoustic startle reflexes. First trial postural reactions may be triggered by interaction of afferent volleys formed by somatosensory and vestibular inputs. Acoustic startle reactions may also be partially triggered by vestibular inputs. Similar muscle activation driven by vestibular inputs may be the common element of first trial postural responses and acoustic startle reactions.

First trial and StartReact effects induced by balance perturbations to upright stance

“…IN A REPEATED SEQUENCE OF trials involving discrete balance perturbations, amplitudes of postural responses (PRs) evoked by the first trial are significantly larger compared with subsequent trials involving the same postural stimulus (Bloem et al 1998a;Chong et al 1999;Hansen et al 1988;Keshner et al 1987;Oude Nijhuis et al 2009, 2010. This observation has come to be known as the first trial effect (FTE) (Allum et al 2011).…”

“…This observation has come to be known as the first trial effect (FTE) (Allum et al 2011). Although it is common to observe FTEs following many types of balance perturbations to both sitting and standing postures (Allum et al 2011;Blouin et al 2006Blouin et al , 2007Oude Nijhuis et al 2009, 2010Siegmund et al 2008;Tang et al 2012), the underlying mechanisms that contribute to the FTEs remain largely unknown.…”

“…Arguments in support of a GSR contribution to FTEs draw on similarities observed between GSRs and FTEs. For example, both GSRs and PRs are known to habituate rapidly after the first exposure to a series of repeated stimuli (Oude Nijhuis et al 2010). Secondly, excitation of muscles normally associated with GSRs [i.e., masseter and sternocleidomastoid (SCM)] is frequently observed during the first response to a balance perturbation, but to a much lesser degree in subsequent perturbations (Oude Nijhuis et al 2010).…”

“…For example, both GSRs and PRs are known to habituate rapidly after the first exposure to a series of repeated stimuli (Oude Nijhuis et al 2010). Secondly, excitation of muscles normally associated with GSRs [i.e., masseter and sternocleidomastoid (SCM)] is frequently observed during the first response to a balance perturbation, but to a much lesser degree in subsequent perturbations (Oude Nijhuis et al 2010). Finally, coherence between muscle responses in frequency bandwidths typically associated with GSRs has been observed in the first response to a seated perturbation, but not during subsequent balance responses (Blouin et al 2006).…”

“…For example, SCM muscle activity onsets evoked by known startling stimuli [i.e., startling acoustic stimuli (SAS)] differ significantly from those induced by balance perturbations (Oude Nijhuis et al 2010). Thus it is plausible that perturbation-induced SCM activity may represent a response that is characteristically distinct from GSRs.…”