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

Area of Science:

• Neuroscience
• Biomechanics
• Human Motor Control

Background:

• Postural responses (PR) to balance perturbations exhibit a 'first trial effect,' differing from subsequent trials.
• This effect may stem from the startling nature of initial perturbations, eliciting a generalized startle response (GSR).
• Startling stimuli, like acoustic stimuli (SAS), are known to cause GSRs and the StartReact effect, advancing prepared movements.

Purpose of the Study:

• To investigate whether balance perturbations function as startling stimuli.
• To determine if the StartReact effect paradigm can be used to assess the startling nature of balance perturbations.
• To elucidate the contribution of GSR to the first trial effect in postural responses.

Main Methods:

• A StartReact effect paradigm was employed using simple reaction time wrist extension tasks.
• Participants completed control trials and test trials involving either startling acoustic stimuli (TESTSAS) or toes-up support-surface rotations (TESTPERT).
• Wrist movement and muscle onsets, sternocleidomastoid activity, and PRs were measured to assess StartReact and GSR effects.

Main Results:

• StartReact effects, indicated by earlier movement and muscle onsets, were observed in the first and all repeated TESTSAS and TESTPERT trials.
• Large GSRs and PRs were noted on the first TESTPERT trial, significantly attenuating with repetition.
• These findings confirm that balance perturbations can act as startling stimuli.

Conclusions:

• The first trial effect in postural responses is likely a combination of the PR and an initially large GSR.
• The GSR component habituates with repeated exposure to the startling stimulus of balance perturbations.
• Balance perturbations possess startling properties, influencing immediate motor responses.