An Inexpensive Eye Movement Monitor Using the Scleral Search Coil Technique

For the economical recording of eye movements using the scleral search-coil method, we developed a circuit using common components and costing only $300. The Helmholtz field coils are 51 cm in diameter and are separated by 51 cm. The horizontal and vertical field coils are driven at 50 and 75 kHz, respectively, by phase-locked square waves; no tuning is required. After amplification, the eye-coil signal is phase detected at 50 and at 75 kHz to produce the horizontal and vertical eye-position signals, respectively. For a 2 cm diameter eye coil of one turn, the noise is 1.0 min of angle rms.