The coordination of rotations of the eyes, head and trunk in saccadic turns produced in natural situations

Abstract

In real life situations large gaze saccades may involve rotations of the trunk, as well as the eyes and head. When this happens 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 similar head-in-space rotations to occur, the neck and trunk movements cannot simply add independently: they must be coordinated. 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.