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18. The otolith-ocular reflex in man
- Hong Kong University Press, HKU
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Basic and AppliedAspects o/VestibularFUN:tio1l le. Hwang, N.G. Daunton and VJ. Wilson (Eds.)© Hong Kong University Press, Hong Kong, 1988 THE OTOLITH-OCULAR REFLEX IN MAN R.W. Baloh*+, K. Beykirch+, V. Honrubia+ and R.D. Yee** Departments of*Neurology, +Surgery (Head andNeck), and **Ophthalmology, UCLA School ofMedicine, Los Angeles, CA 90024-1769, U.S.A. Abstract Horizontal and vertical eye movements were inducedinnonnal humansubjects by sinusoidal linearaccelerationon aparallel swing. The swing frequency was 0.3 Hz and the peakhorizontal andvertical accelerationranged from 0.17 to 0.48 and 0.03 to 0.34 g respectively. Eye movements were recorded with the scleral search coil technique. With the subjects seated in the dark, swing displacement along the interaural axis induced horizontal eye movements with· a mean gain (peak eye velocity/peak swing velocity) of 4.1-4.7 deg/m and a mean phase shift (eye velocityre swing velocity) of -152 to -160 deg. Vertical eye movements occurred at twice the frequency ofthehorizontal eye movements with gain and phasevalues comparable to those ofthehorizontal eyemovements. When the subjects satfacing forward so that the horizontal linear accelerations occurred in the occipitonasal axis, almost identical vertical but no consistent horizontal eye movements were induced. Ineachcase the horizontal andvertical eye movements were proportional to the horizontal and vertical displacement of the swing. Introduction Jongkees and Phillipszoon (1962) recorded compensatory eye movements with EOG as human subjects received linear acceleration in the head-to-foot axis (vertical eye movements ) or interaural axis (horizontal eye movements) while lying on a parallel swing. The induced eye movements were sinusoidal, but if the subject was asked to look to the side, behind closed eyelids, nystagmus was produced. No quantitative measurements of gain or phase of the otolith-ocular reflex were reported, but the authors suggested that the eye movements were easily induced in normal human subjects and absent in patients without labyrinthine function. Keywords: linear acceleration, horizontal eye movements, vertical eye movements, parallel swings, otolith-ocular reflex 166 Balohet al. Niven etal. (1965) used a lineartracktoproduceperiodic linearaccelerations atdifferent frequencies and in different head orientations. Linear acceleration in the interaural axis inducedcompensatory horizontal eye movements (including nystagmus),butacceleration in the head-foot axis (lying) or occipitonasal axis (sitting) did not induce vertical eye movements . Surprisingly, the horizontal eye movements induced by linear acceleration in the interaural axis were about the same whether the subjects were lying or sitting. The gain and phase of the horizontal nystagmic eye movements induced by linear acceleration (so-called L-nystagmus) were different from those associated with periodic angular acceleration ofthe canals in a comparable frequency range, so the authors considered it unlikely that these eye movements resulted from unanticipated stimulation of the horizontal canals. Buizza et al. (1980) recently confirmed the findings of Niven et ale (1965) by producing horizontal Lnystagmus in seated normal subjects during horizontal acceleration along the interaural axis in the dark (also on a linear track). To further assess the natureofL-nystagmus and its potentialas a clinicalindicatorofotolith function in patients, we produced linear accelerations with a parallel swing and precisely recorded eye movements with the scleral search coil technique. Methods Eye Movement Recordings Horizontal and vertical eye movements were recorded by a magnetic, contact lens, search coil technique described by Collewijn et al. (1975). At the start of each recording session the coil system was calibrated by having the subject refixate between targets 10 degrees apart and adjusted so that there was no visible cross-talk between horizontal and vertical channels. Our system can record eye movements as small as 20 min of arc and has a horizontal and vertical linear range of ± 30 degrees (Baloh et al., 1986). Parallel Swing The swing was constructed with rigid vertical bars and forced to swing on ball bearings so that torsional movements did not occur (documented with an angular accelerometer attached to the swing). The chair was mounted on the swing in 2 positions: (1) facing forward (horizontal motion in the occipitonasal axis) and (2) facing the side (horizontal motion in the interaural axis). The subject's head was held in place by a brace with front, back and side clamps. The angle of the swing displacement (f1 in Fig. 1) was accurately monitored with a specially designed geared potentiometer. For a rigid device constrained to move only at the axles, the dynamics of the swing are completely determined by the function.f3(t) and its derivatives. We assumed.f3(t) had the form of a cosinusoidal function with...