We are most aware of our place in the world when about to fall

When making orientational judgements, such as aligning picture frames or positioning for a golf swing, we maneuver rather than remaining immobile. This observation is at odds with many psychophysical studies of spatial orientation. Accurate perception of spatial orientation is of greatest importance when it is imperative to maintain precarious balance. It has been proposed [1,2] that, when balancing with respect to the gravito-inertial force vector (GIF), a subject gains information about orientation through actions and reactions (the 'dynamics of balance'). This guides the maintenance of equilibrium. We give the first experimental evidence that perception of the direction of the GIF does improve in a situation demanding a high level of balancing activity. In the classic test of perception of visual orientation, a subject attempts to align a rod to the Earth vertical. If the rod is surrounded by a tilted rectangular frame to give misleading cues to verticality, observers tend to set the rod tilted in the direction of frame tilt [3,4]. This deviation from the true vertical is called the 'frame effect' and indicates the preferential dependence on visual cues. To test the 'dynamics of balance' hypothesis, we compared subjects' abilities to perform the rod and rod and frame tests (RFT) while standing at ease and while balancing on a narrow beam (Figure 1). When they balanced on the beam, our subjects set a rod within a tilted frame 27% more accurately to vertical (Supplemental Data). The accuracy of 'rod alone' settings, without a frame, was also improved. This suggests that information from the 'dynamics of balance' improves the perception of orientation, underlining the aphorism that 'we are most aware of our orientation when about to fall'. As a corollary, the preferential dependence on a particular sensory channel can be modified by an orientation challenge. The precariousness provides the imperative to focus on establishing correct vertical orientation. Distraction or down-regulation of attention by the tilted frame, while retaining focus on the rod, could also reduce the impact of the frame tilt on the perception of orientation [5,6]. However, as 'rod alone' settings also improved with balancing, distraction of attention from the frame cannot be the only reason for the improved accuracy. It would seem paradoxical that the perception of the Earth vertical improved while the postural sway increased during balancing, because the efferent and sensory feedback signals during complex balancing movements could be noisy and difficult to interpret. However, …