title: The nonlinearity of passive extraocular muscles.
abstract: Passive extraocular muscles (EOMs), like most biological tissues, are hyperelastic, that is, their stiffness increases as they are stretched. It has always been assumed, and in a few occasions argued, that this is their only nonlinearity and that it can be ignored in central gaze. However, using novel measurement techniques in anesthetized paralyzed monkeys, we have recently demonstrated that EOMs are characterized by another prominent nonlinearity: the forces induced by sequences of stretches do not sum. Thus, superposition, a central tenet of linear and quasi-linear models, does not hold in passive EOMs. Here, we outline the implications of this finding, especially in light of the common assumption that it is easier for the brain to control a linear than a nonlinear plant. We argue against this common belief: the specific nonlinearity of passive EOMs may actually make it easier for the brain to control the plant than if muscles were linear.
authors.affiliation: Laboratory of Sensorimotor Research, National Eye Institute, National Institutes of Health, Bethesda, MD 20892-4435, United States.
authors.forename: Christian
authors.initials: C
authors.lastname: Quaia
authors.forename: Howard S
authors.initials: HS
authors.lastname: Ying
authors.forename: Lance M
authors.initials: LM
authors.lastname: Optican
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D004548:Elasticity; D005133:Eye Movements / Q000502:physiology; D000882:Haplorhini; D006801:Humans; D008954:Models, Biological; D008959:Models, Neurological; D009119:Muscle Contraction / Q000502:physiology; D009126:Muscle Relaxation / Q000502:physiology; D017711:Nonlinear Dynamics; D009801:Oculomotor Muscles / Q000294:innervation / Q000502:physiology*; D009802:Oculomotor Nerve / Q000502:physiology