Visual Information and Compensatory Head Rotations During Postural Stabilisation

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