A new solution to the problem of the subjective vertical

Abstract

Based on new evidence and the extensive literature, this report develops an outline of a comprehensive theory of the subjective vertical (SV) in humans. Traditionally the large deviations of the SV from veridicality are attributed to a failure on the part of the gravity systems to correctly perform the necessary coordinate transformation on the visual system. It is experimentally demonstrated, however, that in the control of posture the gravity systems do in fact work close to perfection in positions where the deviations of the SV from the physical vertical are almost largest. Since also the visual system is known to process angular information veridically in the respective range, the intervention of a third agent is suggested, namely a tendency to shift the SV towards the person's own longitudinal axis (“idiotropic vector”). The predictions of the theory are confirmed experimentally, proving that not only the visual but also the haptic zenith is shifted towards the long axis by strongly correlated amounts, when the head is pitched backwards. The theory is also shown to be compatible with, or amenable to typical properties of the SV response characteristic [1], quantitative neurological data on primate gravity receptors [2], a theory of postural control worked out earlier [3], and a particular type of non-linear interaction also found in other orientation systems [4].