Time to reconfigure balancing behaviour in man: changing visual condition while riding a continuously moving platform.

While balancing on a continuously anteroposterior
(A-P) translating platform (10 cm, 0.5 Hz),
the head normally oscillates with the platform without
vision but is stabilized in space with vision. We estimated
the time to shift from one to the other balancing
behaviour when visual condition changed at some
stage during the balancing trials. Ten subjects performed
randomly 50 balancing trials (each lasting 18 s):
10 trials with eyes open (EO), 10 with eyes closed
(EC), 15 in which participants started with EO and
closed their eyes (condition EO!EC) in response to
an acoustic signal delivered during the trial, and 15
starting with EC and closing their eyes (EC!EO) in
response to the same signal. No other speciWc instruction
was given. Displacements of malleolus, hip and
head, and EMG from leg and axial muscles were
recorded. Indexes of amplitude of A-P head and hip
oscillation and of amplitude of EMG activity were
computed. All variables were larger with EC than EO.
On changing visual condition during the trial, the pattern
of head and hip movement and of muscle activity
turned into that appropriate for the new visual condition
in a time-interval ranging from about 1 to 2.5 s.
For each subject, the mean latency of the change in the
balancing behaviour was assessed by statistical methods.
On average, the latencies of kinematics and EMG
changes proved to be longer for the EO!EC condition
than viceversa. Further, the latencies of the
changes were also measured across all EO!EC and
EC!EO individual trials. These values were clustered
around particular epochs of the Wrst few oscillation
cycles following the shift in visual condition. The
results show that subjects can rapidly adapt their balancing
behaviour to the new visual condition. However,
they appear to refrain from releasing the new
behaviour were this unWt, and unfastened it at appropriate
time in the next platform translation cycle.
These Wndings reveal the temporal and spatial features
of the automatic release of the new balancing strategy
in response to a shift in the ongoing sensory set, and
emphasize the swiftness in the change in balancing
behaviour when subjects pass from a non-visual to a
visual reference frame.