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Abstract:
This study investigated how human observers use visual information to stabilise posture. Participants
were required to stand as still and stable as possible on a soft foam balance pad while
fixating a small target at eye-height on a dimly lit lamp. The room was completely darkened
such that no other visual information was available. The lamp was placed at either 0.4m,
1.16m, 2,33m, 3.5m, 4.66m or 5.82m distance from the observer. So far, no other study had
investigated such a wide range of distances. Head position and orientation was measured at
120 Hz using a Vicon tracking system. Participants wore a helmet with infra-red reflecting
markers. Each trial lasted 40 seconds, and 30 sec. breaks were taken between the trials. Room
lights were switched on during the breaks in order to prevent complete dark adaptation. Postural
stability was calculated by quantifying the most frequent sway velocity that occurred at
the sampling frequency. This measure was found to be the most robust measure of postural
stability, in comparison to other measures, such as sway trajectory length. Furthermore,
RMS values for lateral and frontal sway were computed. Results showed that postural stability
significantly decreased with increasing fixation distance. At 0.4m distance, the average sway
velocity across 10 participants was 0.85 cm/s, and this value increased to 1.4 cm/s at 5.82m
fixation distance. This means that the stability of the observers decreases with increasing fixation
distance. With eyes closed, average sway velocity increased to 1.55cm/s. To investigate
the influence of the target distance on the fixation behaviour, we analysed the yaw rotation
of the head. A positive correlation between head orientation angle and head position in the
mid-lateral plane was found. This means that during lateral postural sway, the head makes
systematic compensational movements along the yaw-axis when observers aim to maintain
fixation straight ahead. The correlation significantly decreased with increasing fixation distance
and reached a plateau at about 2.5m. The decrease of postural stability at larger fixation
distances also reached a plateau at about 2.5m. No correlation between head orientation and
head position in the anterior-posterior plane was found. Further experiments which will also
include eye-tracking will investigate how afferent visual information and efferent eye-and head
movements contribute to human postural stabilisation performance.
