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Are postural responses to backward and forward perturbations processed by different neural circuits ? Startle pathways may contribute to rapid accomplishment of postural stability . Here we investigate the possible influence of a startling auditory stimulus ( SAS ) on postural responses . We formulated four specific questions : ( 1 ) can a concurrent SAS shorten the onset of automatic postural responses ? ; and if so ( 2 ) is this effect different for forward versus backward perturbations ? ; ( 3 ) does this effect depend on prior knowledge of the perturbation direction ? ; and ( 4 ) is this effect different for low- and high-magnitude perturbations ? Balance was perturbed in 11 healthy participants by a movable platform that suddenly translated forward or backward . Each participant received 160 perturbations , 25 % of which were combined with a SAS . We varied the direction and magnitude of the perturbations , as well as the prior knowledge of perturbation direction . Perturbation trials were interspersed with SAS-only trials . The SAS accelerated and strengthened postural responses with clear functional benefits ( better balance control ) , but this was only true for responses that protected against falling backwards ( i.e . in tibialis anterior and rectus femoris ) . These muscles also demonstrated the most common SAS-triggered responses without perturbation . Increasing the perturbation magnitude accelerated postural responses , but again with a larger acceleration for backward perturbations . We conclude that postural responses to backward and forward perturbations may be processed by different neural circuits , with influence of startle pathways on postural responses to backward perturbations . These findings give directions for future studies investigating whether deficits in startle pathways may explain the prominent backward instability seen in patients with Parkinson 's disease and progressive supranuclear palsy .
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Intervention_Physical
startling auditory stimulus ( SAS )
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Intervention_Physical
perturbations
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Intervention_Physical
SAS .
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Intervention_Pharmacological
SAS-only
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Intervention_Pharmacological
SAS
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Intervention_Pharmacological
SAS-triggered
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Outcome_Physical
postural responses
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Outcome_Physical
postural responses
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Outcome_Physical
postural responses
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Outcome_Physical
SAS-triggered responses
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Outcome_Physical
postural responses
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Participant_Sample-size
11
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Participant_Condition
Parkinson 's disease