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Computed Sinusoidal Harmonic Acceleration

The visual and vestibular systems provide us with the ability to maintain retinal fixation of our environment in the presence of head or body movement. The velocity (and acceleration) of head movement is the critical factor in determining which of the two systems dominate. At low velocity movement, the retinal image is maintained with eye pursuit. As the velocity increases, pursuit tracking becomes less effective, and the vestibular system provides the information necessary for effective and coordinated eye movement keeping the retinal image stable. The function of the vestibulo-ocular reflex (VOR) can be demonstrated by holding your finger a foot in front of your eyes and moving it back and forth. When oscillating at a low frequency the eyes are able to follow your finger’s movement. As you increase the frequency, the images becomes blurred. Now hold your finger stationary and rotate your head back and forth. You are able to maintain a focused, stabilized retinal image both at low and high frequencies of oscillation. To evaluate the VOR, it is common practice to stimulate the labyrinth with either a caloric or a rotatory stimulus while measuring the patient’s eye movement. Caloric testing induces endolymphatic labyrinthine flow by thermal stimulation. Rotational tests stimulate the ampullae by rotating a subject in a chair that is free to turn in either direction on earth-vertical axis. When the axis of rotation is aligned with the rostral-caudal body axis, it is termed yaw oscillation. Initially, the principle of rotational testing was clinically examined by Barany in 1907 (1). With a patient seated, he turned the chair 10 rotations over a 20 sec period, stopped the chair suddenly, then observed the eye movements. This form of rotational testing is called cupulometry or impulse stimuation; in this situation the stimulus is an abrupt deceleration. By recording eye movements electrically so that the patient need not be examined when stationary, testing can also be performed with a stimulus of constant rotation in a clockwise or counterclockwise direction. The acceleration, and therefore the velocity, can progressively be increased to determine the timing and magnitude of the elicited nystagmus. Sinusoidal rotation entails a symmetric oscillation in both the clockwise and counterclockwise direction. The torsion swing, similar to Barany’s test, has chair movement that is governed by a calibrated spring. The input stimulus is a progressively dampened oscillation of the rotating chair. The intensity of the stimulus thus depends on the patient’s weight and distribution of mass in the chair. In addition, the frequency of the stimulus is fixed. This makes it difficult to establish norms for elicited responses. Sinusoidal rotation can also be performed using a motor driven chair which can be programmed to deliver a predetermined amount of intensity of stimulation with respect to acceleration, velocity, and oscillation duration and amplitude.