Unexpected external perturbations o

Unexpected external perturbations of body
equilibrium elicit compensatory postural reflexes.The
reflex patterns change only minimally, even after repetitive
perturbations.This study addressed the question of
whether classical conditioning can alter the reflex
patterns.In the first session 27 healthy subjects were
tested when standing on an unexpectedly tilting platform.
Electromyographic (EMG) activity from different leg
muscles and the vertical ground forces, from which the
centre of vertical pressure (CVP) was computed, were
recorded.In a subsequent session subjects were tested
using the classical conditioning paradigm with the tilting
platform as the unconditioned stimulus (US) and a prior
auditory signal as the conditioning stimulus (CS).The
decay of the unconditioned response (UR) observed in the
first session was similar and small in all subjects.During
conditioning, 22% of the subjects established conditioned
responses (CR) in all muscles recorded (strategy 1).UR
amplitudes of the anterior tibialis (TA) decayed more than
in the first session.The resulting CVP excursions were
similar to those observed in US-alone trials.The remaining
subjects exhibited CR only in the gastrocnemius
muscle but developed a substantial decay of UR, resulting
in very small CVP excursions (strategy 2).Our data
suggest that processing of US-preceding conditioning
stimulus leads to different strategies in the control of
postural adjustment with assumed underlying associative
and non-associative plastic processes.