Abslracl-The autosomal dominant spinocerebellar ataxias (SCAs) are a group of neurodegenerative diseases characterized by progressive instability of posture and gait, incoordination, ocular motor dysfunction, and dysarthria due to degeneration of cerebellar and brainstem neurons. Recent studies have established that there are more than 16 genetically distinct subtypes. Clinical observations suggest that eye movements and postural stability are universally but differentially impaired in the SCAs. The aim of the present rork was to study the horizontal vestihulo-ocular reflex (VOR) in SCA6 patients to understand the pathophysiology of the VOR due to cerebellar Purkinje cell degeneration. The VOR was recorded in patients with genetically defined SCA6 during rotation in the dark. Severely affected subjects had an intact VOR, but there were quantitative differences in the gain and dynamics compared to normal controls. During angular velocity ramp rotations, there was a revecsal in the direction of the VOR that was more pronounced in SCA6 compared to controls. Modeling studies indicate that abnormal feedback of an eye position signal into the velocity storage network can account for this reversal. These and other results will help to identify features that are diagnostic for SCA subtypes and provide new information about selective vulnerability of neurons controlling vestibular reflexes. Keywords -Vestibula-ocular reflex, eye velocity storage, spinocerebellar ataxia, motor control The vestibulo-ocular reflex (VOR), initiated by stimulation of hair cells in the inner ear that are sensitive to linear and angular acceleration, helps to maintain gaze, the position of the eyes in space, during head movement. To a first approximation (neglecting the depth of focus, binocular convergence, and differences between the axes of rotation for the eyes and the head) the angular VOR causes the eyes to rotate opposite in direction and equal in magnitude to the head rotation, thereby helping to keep the visual world stable. However, when vision is absent and the head velocity is constant, the eye velocity is not maintained but decays to zero, although there is a neural network, referred to as velocity storage [I, 21, that helps to prolong the time course for decay so that the VOR continues after the neural activity in the vestibular nerve (from the inner ear to the brainstem) has returned to baseline. In addition, there can be a reversal in the direction of the eye velocity that is dependent on hrainstern pathways. The aim of the present work was to develop a model for the decay and reversal of the horizontal VOR in patients who have spinocerebellar ataxia (SCA) subtype 6 131. The pathology in SCA6 includes a degeneration of the cerebellum [4], a structure that is important for controlling the gain and direction of the VOR [SI and the spatial relationship of the VOR to a gravitoinertial reference frame 16, 71. The SCAs are a group of neurodegenerative diseases characterized by progressive instability of posture and gait, incoordination, ocular motor dysfunction, and dysarthria due to degeneration of cerebellar and brainstem neurons. Recent studies have established that there are more than 16 genetically distinct subtypes, and clinical observations suggest that eye movements and postural stability are universally but differentially impaired across the subtypes. In the present study we propose a model to characterize the horizontal VOR in SCA6. It includes an eye position signal that feeds into the velocity storage network. This is based on the model of Raphan et al [l] wherein the pathways carry physiological signals, and there are so-called direct and indirect (which contribute to the velocity storage) pathways that represent projections to the motor neurons innervating the eye muscles. There is feedback of an eye position signal into the velocity storage and this gives rise to the reversal in the direction of the VOR. Data were collected from four normal control subjects and three patients with SCA6. EOG electrodes were used to record the position of the eye in the head while the subjects sat on a chair (attached to a Contraves-Goerz 826 rate table) that was rotated in the horizontal plane. The head was held in a restraint attached to the chair (so that there was no neck movement), and the subject was in complete darkness so that only the rotation sensitive hair cells of the inner ear were stimulated. The reflex eye movement (VOR) was initiated by a short period of constant angular acceleration, IO degisec' (for 18 seconds) or 20 degisec' (for 9 seconds), causing a ramp change in angular velocity of the head. A LabVlEW (National Instruments) data acquisition program was used to sample the EOG recording of the VOR and the tachometer signal (angular velocity of the rate table) at 200 Matlab (The Mathworks) programs were used to digitally filter the sampled data, differentiate the eye position signal, and remove the quick phases from the eye velocity. The resulting slow phase eye velocity, referred to as the conipensatory part of the VOR, was used for the modeling studies. Simulink (The Mathworks) was used to set up the
The aim of the work presented in REF is to identify diagnostic features of SCA-6 patients by studying the horizontal vestibulo-ocular reflex (VOR).
Eye position feedback in a model of the vestibulo-ocular reflex for spino-cerebellar ataxia 6
2001 Conference Proceedings of the 23rd Annual International Conference of the IEEE Engineering in Medicine and Biology Society
2001 Conference Proceedings of the 23rd Annual International Conference of the IEEE Engineering in Medicine and Biology Society
Psychology