Visual and oculomotor responses induced by neck vibration in normal subjects and labyrinthine-defective patients

Three-dimensional scleral search coil eye movement recordings were obtained in five normal subjects and four patients with absent vestibular function, during unilateral vibration of the neck in the supine position. The purpos e of the experiments was to investigate any role played by eye movements in the illusion that a small fixation target, viewed in an otherwise dark roo m, moves when vibration is applied to the neck (propriogyral illusion). Vib ration was applied to the right dorsal neck muscles in three visual conditi ons: total darkness, fixating a light-emitting diode (LED) in an otherwise totally dark room and LED fixation in the normally lit room. Normal subject s reported that during vibration, with LED fixation in an otherwise dark ro om, the target appeared to move predominantly leftwards and patients report ed a predominantly downward movement. Eye movements were consistently elici ted in all subjects. In normal subjects there was a slow-phase eye movement predominantly to the right, interrupted by nystagmic quick phases in the o pposite direction, whereas in the patients slow phases were predominantly u pward with quick phases downward. Eye movements were larger in the dark but the velocity of the initial slow-phase component (<200 ms) did not change with visual conditions. Mean latencies of the eye movements were typically 80 ms but in individual trials could be as short as 40-60 ms. The eye movem ents were considerably larger in the patients (e.g. mean cumulative slow-ph ase displacement in the dark 12 degrees vs 2 degrees; maximum velocity ca. 5 degrees/s vs 1 degrees/s). These results indicate that the propriogyral i llusion is secondary to vibration-induced eye movements, presumably mediate d by the cervico-ocular reflex (COR). The difference in direction of the il lusion and eye movements in the patients may be related to a predominant en hancement of the vertical COR, secondary to the prominent exposure to verti cal retinal slippage experienced by these patients during daily activities such as locomotion.