On the Action of Linear Accelerations upon the Otoliths

Abstract

SUMMARY In 10 rabbits, examined within 48 hours after unilateral labyrinthectomy, we found three with a direction-fixed positional nystagmus to the intact labyrinth and seven with a direction-changing positional nystagmus. All the rabbits with direction-changing positional nystagmus showed in one or more positions a nystagmus of both eyes in opposite directions (i.e. in the left eye a nystagmus to the left and in the right eye one to the right). All 10 rabbits showed in the lateral positions a stronger nystagmus when they were lying on the operated side than when they were lying on the side of the normal ear. In these rabbits the nystagmus was also stronger in the prone position than in the supine position. In nine rabbits we studied the Bechterew nystagmus. No influence of the above-mentioned positions was found upon this nystagmus. This makes it highly probable that the changing of the size of the positional nystagmus in different positions after unilateral labyrinthectomy has to be sought in the action of the gravitational force upon the otoliths of the intact labyrinth. In 20 normal rabbits we investigated the influence of position upon eye-movements provoked by linear accelerations on the parallel-swing. The reactions in the prone position and in both lateral positions were smaller than the eye-movements which resulted from swinging the rabbit in the supine position. After destruction of one labyrinth this phenomenon was still present. Now, however, a distinct difference in the size of the provoked eye movements was to be observed between the two lateral positions. The reactions were definitely larger when the animal was swung while lying on the side of the intact labyrinth than when it was lying on the operated side. In our opinion the following conclusions can be drawn. In resting positions a difference in stimulus from the otoliths upon positional nystagmus after unilateral labyrinthectomy is evident when the otoliths are given constant pressure or traction upon their underlayers as a result of the action of gravity. During movements provoked by sinusoidally changing linear accelerations on the parallel-swing, the shearing forces are maximal when the otoliths are hanging freely in the endolymphatic space. The fact that there is a difference in both lateral positions in rabbits after a onesided labyrinthectomy, as regards the positional nystagmus and the eye-movements provoked by linear accelerations, makes it highly probably that the saccule has a vestibular function.