• K-S Tang, F Honegger, and J H J Allum.
• Department of ORL, University Hospital, Basel, Switzerland.
• Neuroscience. 2012 Dec 6;225:140-51.

BackgroundWe investigated whether the "first trial effect" (FTE) in responses to support surface tilt has directional characteristics, or is simply due to a startle-like response. The FTE is the difference between the first (unpractised) trial response (FTR) and subsequent responses.MethodsEach group of 10 young adults received a series of identical support surface tilts (7.5°, 60°/s) in one of five leftward tilt directions or pure backward or forward. These were followed by randomly selected tilts in at least eight equally spaced directions. Only in-place responses were possible as the feet were strapped to the support surface. Body kinematics were collected and EMG activity was recorded from several trunk, leg and arm muscles.ResultsThe centre of mass (CoM) vector displacement showed a FTE in all tilt directions. It was equally large for all directions of backward tilt but smaller for forward and lateral tilts. A similar effect was noted for the CoM anterior-posterior FTE. FTRs of lateral CoM movements were small for all tilt directions except in the backward left direction. A constant amplitude trunk flexion FTE was observed in all tilt directions, and pelvis backward motion for backward tilts, preceded by a FTE in the abdominal muscles for forward (and lateral) tilts and in the soleus for backward (and lateral) tilts. Hip flexion FTEs were largest in backward left direction and preceded by increased gluteus medius and deltoid FTR activity. FTRs in sternocleidomastoïdeus muscles, generally associated with startle activity, were largest in lateral and forward tilt directions.ConclusionsFTRs appear to consist of either a forward, backward or lateral movement strategy each imposed on an adapted response strategy. Only the lateral response shows a strong directional sensitivity. We hypothesise that FTR amplitudes result from a failure of the CNS to weight properly the stimulus metrics present in lower leg proprioceptive and vestibular inputs.Copyright © 2012 IBRO. Published by Elsevier Ltd. All rights reserved.

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