A Low-Dimensional Model of Coordinated Eye and Head Movements
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Eye and head movement data, were recorded under head fixed and head-free conditions, and compared with theoretical results obtained using a nonlinear model of eye-head coordination. The model explains slow, or pursuit movement correlated closely to target movement, and saccades, or quick phases of eye movement. Eye movement under head-fixed conditions was modeled by an externally forced Duffing equation, whilst properties of head movement are described by a second externally forced Duffing equation with lower eigen frequency. In the more natural, head-free conditions where both eye and head movements are used synergetically to pursue a visual target, the vestibulocular reflex (VOR) is represented by coefficients defining the mutual coupling between these two oscillatory systems. In the present model, the oscillator that models eye movement has an inhibitory influence on head movement; head to eye coupling coefficients are included to model the influence of the VOR mechanism. Individual eye and head movement patterns in different subjects can be adequately modeled by altering the coupling coefficients. In order to adequatcly simulate those changes introduced by microgravity conditions, the coefficients defining eye-head coordination (mutual coupling) must be changed. It may be hypothesized tlmt such changes in the neurovestibular system could introduce the instability in eye-head coordination, which is known to lead to space sickness.