Neurophysiology of Gaze Direction as Poly-Equilibrium

The static orientation of the eyes during visual fixation is determined by the simultaneous operation of multiple equilibria. This phenomenon is collectively referred to as poly-equilibrium, which involves multiple systems that work together to cancel each other out and establish gaze direction. While other systems, such as audio- and cervico-ocular systems, may also contribute to gaze direction, this review focuses primarily on the commands issued by the vestibulo- and visuo-oculomotor systems that determine gaze direction, as they play a key role in the poly-equilibrium process. From the visual and vestibular activities accompanying the appearance of an object in the central visual field to the recruitment of premotor neurons responsible for the generation of slow and saccadic eye movements, a delicate balance is maintained. As long as the recruited channels convey commands that counterbalance each other, no movement is initiated. This alternative viewpoint leads to reconsidering the nature of saccadic and pursuit eye movements. Rather than viewing them as the dynamic reduction in brain signals encoding kinematic parameters such as position or velocity, they can be seen as the physical expression of intracerebral processes restoring balanced activities between sensorimotor channels whose recruitment leads to mutually opposed movements.