A new and simple calibration of the electro-ocular signals for vestibulo-ocular measurements

Reliable calculation of the gain of the vestibulo-ocular reflex depends on a reliable way of calibrating the eye movements. During the new calibration manoeuvre, the subject fixes the eyes at a point about 1 m away and moves the head either in pitch or yaw approximately +/- 30 degrees. The head movements are recorded by means of an angular rate sensor. The eye movements are recorded with standard surface electrodes. By integrating the angular rate signal the precise head position angle is calculated. The fixating eyes will compensate for this angle and, thus, the signal level of the eye movements is defined. A systematic error is introduced in this procedure. Because the distance to the fixating point is finite, the eyes must deviate more than the head movement. The extra angle is dependent on the distance to the target. In yaw, the angle is independent of the direction of the head movement. In pitch, the angle is both dependent on the distance to the target and on the direction of the head movement. Correction for the error must, in most cases, be performed directly on the eye recordings before calculating the VOR gain.