Gaze orientation : brainstem mechanisms

This text is devoted to a review of the brain mechanisms of gaze orientation, restricted to the peripheral and brainstem levels. It is articulated around the most significant contributions of the authors to this field. This original contribution concerns the role of the Superior Colliculus in the control of eye and head orienting movements and the modalities of coordination between the orientation of these two mobile segments.
More specifically, the author has demonstrated that :
- the level of alertness influences the velocity of orienting saccades.
- there exists a copy f the oculomotor output signal “fed back” at the level of the Superior Colliculus.
- the Superior Colliculus of the cat, as opposed to the monkey, is implicated on the elaboration of an orienting motor signal that is sent simultaneously to the eyes and to the head.
- the Superior Colliculus of the cat may be divided into three functional parts corresponding to the “prime” segment involved in the orientation of the gaze: the eye, the head or the trunk.
- each of these zones corresponds to a range of retinal eccentricities that increase from the most rostral (primarily controlling eye movement) to the most caudal (primarily controlling trunk or body movement).
- depending on the “prime” moving segment, there exists at least two eye-head coordinating mechanisms, relying on two different types of eye saccades, each interacting with the compensating phase of the vestibuloocular reflex in a completely different way.
- when the head id the “prime” mover, the eye stabilising systems (mainly the vestibule-ocular reflex) are temporarily disconnected. During this transient disconnection, that is related to gaze dynamic error, eye and head coordination is performed by still largely unknown mechanisms. One of these mechanisms is most probably based on a gaze internal feedback, that involves the Superior Colliculus.
- there exists a functional link between eye position in the orbit and neck muscles activity, that may be called an oculo-collic reflex. This link is present for deviations of the eye in all directions; the eye position signal is distributed adequately to all neck muscles involved.
- the electromyographic activity of the neck muscles during an orientating gaze movement reflects the complex transcoding that has occur on the interface between the Superior Colliculus and the neck motoneurons.
- not all gaze orienting mechanisms are fully operational at birth in the human species. They progressively develop, along with the histological and functional maturation of the retinal fovea, during the first year of life.
- the orientation of gaze plays a major role in the guidance of a hand movement, not only through vision, but also thanks to “extraretinal” factors such as eye position in the orbit or head position on the trunk