The vestibular system senses motion of the head for the purposes of maintaining stability of images on the fovea of the retina and stability of postural control during that head motion. In normal function, the vestibular receptors provide an elegant and accurate representation of the motion of the head in three dimensions. This information is then carried along central vestibular pathways to control the reflexes and perceptions that are mediated by the vestibular system. Disorders of vestibular function result in abnormalities in these reflexes and lead to sensations that reflect abnormal information.
The output of the vestibular system can grossly be considered a motor response reflected in an ocular or postural behavior. The ocular signature of the vestibular system is typically measured from the vestibulo-ocular reflex (VOR), and the postural behavior is typically measured using force plate platform equipment. In this chapter, we discuss the common testing techniques of the vestibular system, categorized by ocular or posturographic behavior. Further delineations are made that incorporate testing the behavioral outcome of the vestibular function.
In the 1800s, it was realized that a voltage of the eye existed and varied with eye rotation.1 Because the cornea is positively charged with respect to the retina, a dipole (i.e., battery) is formed (Fig. 11.1). This corneo-retinal potential (CRP) has a baseline (eyes stationary, positioned in neutral) potential difference of 1 millivolt and provided the first technique useful to record eye motion, called electrooculography (EOG).2,3 By the early 1900s, the CRP had become the standard method to record eye motion.4 Using horizontally and vertically positioned surface electrodes, the CRP can be detected as the polarity of the eye rotates toward or away from those electrodes. Because a torsional eye rotation (being purely clockwise or counterclockwise) does not cause a vertical or horizontal deflection of the globe within the orbit, the electrodes cannot detect a relative change in the CRP. Thus, torsion eye rotations cannot be monitored with EOG.
Corneo-retinal potential (CRP). The cornea of the eye is positive and the retina is negative. As the eyes move horizontally or vertically toward surface electrodes positioned both horizontally and vertically, the CRP can be detected. The electrodes are recording the relative change in CRP, not potentials from ocular muscles. Torsional (roll) eye rotations are not recorded, because the CRP does not change relative to the electrode position.
In addition to a method for recording eye rotation, ENG also refers to a battery of tests. As a test battery, the ENG includes measures of oculomotor function (including gaze stability, smooth pursuit, and saccades), positional and positioning testing, and the caloric exam. With the development of video-based recording of eye movements, the test battery is often referred to as VNG, video nystagmography.