Identification of candidate mechanisms for dystonic traits from an analysis of a complete system neurological / biomechanical model

Abstract The research goal is to construct a complete system model for human head movement integrating skeletal, muscular, biomechanical and neurological aspects; verifying the model by simulating ‘normal’ volitional movement. Thereafter the model was used to explore pathological scenarios by variations to model components some of which resulted in dystonic traits, including tremor and abnormal twisting. This enabled identification and analysis of the underlying mechanisms. Once research commenced it was soon apparent that a purely analytical model (a collection of mutually dependent components each mathematically expressed) was not realisable nor mathematically tractable and so a computer simulation configured for the specific case of head movement (targeting the condition of cervical dystonia but in principle generic) was constructed to allow the study to proceed. The resulting model was exercised to verify a reasonable approximation to normal volitional control of head positioning. With the model operating well in this context, a series of variations were made to see if these might induce dystonic traits. Whilst most of the variations investigated did not cause significant behavioural changes, those relating to the transmission of information to the brain for proprioception were found to be very influential on the system behaviour. Simulated behaviours of pathological interest included: movement of the head to an orientation of maximum lateral twist and locking in that state, oscillations that evidenced themselves as repetitive movement of the head and finally a ‘self healing’ capability in the case of defective proprioception which is a well established characteristic of the temporal progression of cervical dystonia. This research identifies candidate mechanisms for the dystonic traits of tremor and excess muscle tone.