Influence of perceived egocentric coordinates on the subjective visual vertical.

The majority of previous studies which have explored the mechanisms underlying perception of the direction of gravity in static roll tilt have proposed that the tendency to estimate the subjective visual vertical (SVV) as tilted towards body tilt ('Aubert effect') arises from an underestimation of perceived body tilt. The present study has evaluated an alternative assumption that erroneous estimates of verticality may be related to the ability to estimate the orientation of external objects with respect to the observer's perceived body Z-axis. Experiments showed that Aubert effects and the overestimation of 30-90 degrees angles from the body Z-axis in the roll plane were both related to errors made in adjusting a visual rod parallel to the body Z-axis. The results suggest that errors in providing visual estimates of the observer's own body Z-axis reference are implicated in Aubert effect.