For the three dimensional recording of the head movements and compensatory head postures adopted in congenital nystagmus, there was developed an instrument which detects the movements in the x,y,z axes. This system is comprised of two pairs of coils in a Helmholtz configuration in order to generate two alternating magnetic fields of different frequency. The head movements are detected by small coils mounted on a helmet. This system was based in the magnetic field search coil technique. The induced voltages in the coils are amplified and passed through filters and sensitive phase detectors in order to have separate DC signals of each vertical, horizontal and torsional head movements in the output.
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