THERE ARE several different methods available to record movements of the eyes. The most widely used for testing of patients with neurological disease is electrooculography (EOG). EOG makes use of a naturally occurring potential difference between the ront and the back of the eye (cornea and retina). When the eyes turn horizontally, an electric field due to the cornea–retinal potential moves in a fairly linear fashion. Surface electrodes are applied to the skin at the inner and outer corners (canthi) of each eye, and the difference between these electrodes is referenced to a ground electrode (placed over the mid-forehead) and amplified. EOG is widely used for clinical testing because it allows measurement of the full range of horizontal movement and is relatively inexpensive. However, it suffers from several limitations, including inability to reliably measure vertical eye movements, low sensitivity (due to muscle artifact and other noise sources), baseline drift, and limited bandwidth due to the filtering required to remove noise from the signal. Several other methods are in general use for measuring eye movements. The magnetic search coil technique is generally regarded as the most reliable and versatile method, and it is used widely to measure eye movements in human and many animal species. The subject wears a ‘‘contact lens’’ (a silastic annulus in which are imbedded coils of fine wire); this annulus (the search coil) is placed after applying topical anesthetic eye drops. The subject sits in a weak magnetic field (the field coil), which induces a voltage in the search coil. When the eye turns, the voltage in the search coil changes and using special amplifiers it is possible to measure eye rotations around all three axes, with a sensitivity of greater than 5 min of arc, a potential linear range of 3601, a bandwidth of 0–500 Hz, minimal drift, and an unlimited field of view (Fig. 1). One disadvantage is the need for the subject to wear a contact lens, but with proper care it is well tolerated and without significant risk. The search coil technique is especially valuable for measuring eye movements in patients who cannot reliably point their eyes at calibration targets (e.g., due to ocular oscillations) since the scleral annulus that the patient wears can be precalibrated on a protractor device. Photoelectric methods track the scleral–iris edge of the eye by measuring the amount of scattered light from infrared sources. This approach is generally more sensitive and reliable than EOG, but it provides a limited linear range, especially vertically. Another approach is to measure movement of images reflected by the eye as it rotates; a stationary source of infrared light can be used. Because the center of curvature of the corneal bulge differs from the center of rotation of the globe, eye movements cause displacement of the corneal, or first Purkinje, image. Alternatively,