Suppression of the E-effect during the subjective visual and postural vertical test in healthy subjects

In this study the influence of head-on-body and starting roll position of laser bar and chair was investigated on the perception of, respectively, the visual and postural vertical. Sixty-one subjects, between 21 and 82 years, participated in this study. Results show that head-on-body and starting roll position of laser bar or chair have an influence on the outcome for both subjective visual vertical test (SVV) and subjective postural vertical test (SPV). When head-on-body and starting roll positions are combined the E-effect (deviation of the SVV–SPV opposite to the head-on-body tilt, when the roll tilt of the head is <60°–70°) is observed in the anti-parallel condition, but is suppressed when starting roll position of laser bar or chair are relatively parallel to the length axis of the tilted head. It is suggested that in the assessment of the SVV, when the laser bar is aligned with the length axis of the head (parallel condition), the oblique effect occurs. In the assessment of the SPV, the E-effect is also suppressed in the parallel condition. It is hypothesized that in the parallel conditions, the gravitational reference frames of head and trunk are more aligned with each other providing similar information. In the anti-parallel conditions the reference frames of head and trunk are malaligned, providing contrary input making the estimation of the earth vertical more difficult. It seems that SVV and SPV measurements are influenced by head-on-body and starting roll positions and has to be taken into account in further studies.