The coordination of eyes, head and trunk in very large natural gaze saccades

In real life situations large gaze saccades may involve rotations of the trunk, as well as the eyes and head. Large saccades were studied using video methods under two circumstances: 1) with the trunk free to rotate (during food preparation in a kitchen), and 2) with the trunk not free (checking road intersections while driving). The principal result was that the rotation of the head-in-space is similar whether or not the trunk is also rotating. However, the rotation of the head on the trunk (i.e. the neck movement) is very different in the two circumstances. For this to be possible, the neck muscles must receive a command in which the rotation of the trunk is subtracted from the command to execute the head-in-space rotation. It is argued that this is achieved via a feedback loop in which the semi-circular canals monitor the rotation of the head-in-space, and the neck is driven by an error signal representing the difference between the intended head-in-space trajectory and the actual trajectory. This mechanism, which is essentially the same as the vestibulo-collic reflex, nulls out disturbances to the head-in-space trajectory, whether these are caused by active or passive trunk rotation.