The effect of gravity on the resting position of the cat's eye.

We measured rotation (horizontal, vertical and torisonal) and translation (horizontal and vertical) of the paralysed cat's eye in response to 45 degrees steps of orientation presented in a pseudorandom order around the roll and pitch axes (with respect to the horizontal canals). During changes of position of the animal in the roll plane, the eyes rotated towards the lowest part of the orbit (left with left ear down; top when the cat was upside down, etc.) by an average of 0.55 degree. Changing orientation in the pitch plane evoked vertical rotations of +/- 1.42 degrees (upwards eye movement during forwards head pitch) and torsional rotations of +/- 1.3 degrees. All these rotations taken together suggest that the centre of mass is in front of, below and temporal to the centre of rotation. The eyes translated temporally (thus separating by 0.72 mm) during forward pitching and there was a small vertical displacement (0.23 mm) when the animal was upside down. These findings are discussed with respect to a possible role of the extraocular proprioception system.