Fig. 3.7.

Einstein’s equivalence principle illustrated for the otoliths. During backward linear acceleration (top left), the inertial force acts in the direction opposite to the acceleration, causing the otolithic membrane to lag behind the skull and the sensory hair cells to bend. Similarly, tilting the head forward (bottom left) causes the otolithic membrane to sag and the sensory hair cells to bend as they do during backward linear acceleration. The otoliths therefore respond to both linear acceleration of the head and head tilt relative to gravity, and cannot distinguish between the two. The same is true for forward linear accelerations and backward head tilts (right column). Although illustrated here for head pitch, the ambiguity also exists between left–right translations and roll. Graviceptive signals arising from the abdominal viscera suffer from a similar ambiguity.