Fig. 4. Electrical stimulation of a single SCC nerve induces eye movements roughly in the plane of that canal (shown for stimulation of the right posterior canal). (Courtesy of A. Boehmer, Zürich.)

Modern Vestibular Testing

Semicircular Canal Function

Routine vestibular testing such as calorics and rotational testing mainly investigate the function of the lateral SCCs, while the vertical SCCs and the otoliths are basically ignored. This has changed in recent years. In the last 20 years, there has been a revival of interest in 3-D approaches to the control of eye movements. This was boosted by the fact that 3-D eye movement analysis has become practical with the development of the magnetic field search coil technique. New analytical approaches have made the mathematics of eye rotations and coordinate transformations more tractable and intuitive. Strabismus, labyrinthine dysfunction and brain disorders leading to nystagmus and other eye movement disorders are ubiquitous clinical problems and demand a 3-D approach for their understanding. This is especially true when dealing with vestibular problems. The vestibular system is intrinsically 3-D trying to stabilize the retinal image in all 3 rotational degrees of freedom. Under pathological conditions, we often find spontaneous or elicited eye movements with torsional components. The key for understanding vestibular-induced eye movements has been found in the early 60s. Since then, we know that electrical stimulation of single SCC nerves induces eye movements roughly in the plane of the canal [42, 43] (fig. 4). If more than one canal is stimulated, the different canals combine at least roughly linearly to drive the eyes. Thus, if multiple canals are stimulated, the slow phases should be in a direction that is a weighted vector sum of