The role of the otoliths in perception of the vertical and in path integration.

The role of the otoliths in essential performances of human orientation is analyzed. The following interactions of the otoliths are considered: 1. The otoliths cooperate with graviceptors in the trunk in the perception of body posture. The truncal graviceptors turn out to yield on average 60% of the total gain. 2. The otoliths cooperate with proprioceptors in the head-to-trunk coordinate transformation. However, under static conditions, proprioceptors in the legs, although effective in the control of posture, neither affect the perception of posture nor of the visual vertical. 3. In contrast to the perception of posture, the perception of the visual vertical (SVV) receives the necessary gravity information exclusively from the otoliths. However, their output appears to be affected by a central nervous component that tends to rotate the SVV into the z-axis of head and trunk. A theory of vectorial summation of this component, the "idiotropic vector," with the otolithic vector is able to explain the cause of the A- and E- effects, the increase of the variance of the SVV with the tilt angle, and the asymmetrical effect of rotatory visual flow. 4. Finally, it is shown that the otoliths, by the separation of the effects of tilt from those of translation, play an essential role in navigation by path integration.