Gaze direction as poly-equilibrium: Balancing structural and functional asymmetries while looking at a visual target
Résumé
It is proposed that the static orientation of the eyes during visual fixation is the outcome of multiple and simultaneous equilibria (poly-equilibrium) of commands that cancel each other out (Goffart 2019, 2025). From the visual activities evoked by the presence of an object in the central visual field to the recruitment of premotor neurons responsible for the horizontal and vertical components of saccadic (fast) and slow movements of the eyes, no movement is initiated as long as the recruited channels convey commands that counterbalance each other. From this novel viewpoint, a saccade toward a visual target is not anymore the dynamic reduction of motor error, namely a command resulting from comparison between neuronal signals “encoding” the current direction of gaze (eye + head) and the target location relative to gaze direction. These notions remain useful, but they belong to the physical domain. Rather, it is proposed that a saccade is the physical expression of intracerebral processes that restore multiple balances between sensorimotor channels whose activities are mutually opposed. This novel viewpoint extends to the kinetics of saccades insofar as their curvature is the signature of bilateral commands that are more or less unbalanced. Assuming unequal number of synapses, unequal number of neurons and unequal number of emitted action potentials between opposing channels, asymmetries can yet lead to relatively straight saccades if a process adjusts their balance. Activities in the medio-posterior part of the cerebellum are proposed to be involved in this adjustment. Goffart L. (2019) Kinematics and the neurophysiological study of visually-guided eye movements. Progress in Brain Research 249: 375-384. Goffart L. (2025) Orienting gaze toward a visual target: Neurophysiological synthesis with epistemological considerations. Vision 9(6): 1-43
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