Suppression of the E-effect during the subjective visual vertical test.

The subjective visual vertical was measured in 38 healthy subjects. The head was alternately roll-tilted to the right and to the left, and the start position of the light bar was alternately set clockwise and counterclockwise. When the head is tilted less than 60-70 degrees a deviation of the subjective visual vertical in the opposite direction to the head tilt is expected. This phenomenon has been reported in various studies as the E-effect. The present study demonstrates however that the E-effect is suppressed if the start position of the light bar is presented relatively parallel to the length axis of the tilted head.