We performed quantitative low and high frequency rotational testing in 17 patients with bilateral peripheral vestibular deficits. A characteristic pattern of decreased gain and increased phase lead at low frequencies of sinusoidal stimulation and decreased time constant to impulsive stimulation was observed in most patients. The results of caloric testing correlated with the results of low frequency rotational testing but not with the results of high frequency testing. Only 2 patients complained of oscillopsia at the time of testing and both of these had decreased vestibulo-ocular reflex gain at low and high frequencies of sinusoidal rotation.

A new method of local gentamicin administration was tested in the bullfrog inner ear to achieve ototoxic-induced hair cell destruction. Gelfoam pledgets soaked with known amounts of gentamicin were inserted into the perilymphatic cisterna of the bullfrog through a ventral surgical approach. A dose of 1.20 mg gentamicin, consistent with a perilymphatic concentration of 65 microg/ml, resulted in the desired ototoxic-induced hair cell damage, that is, complete hair cell destruction with minimal disruption of other components of the sensory epithelium. This study demonstrates that this is a useful and simple method to investigate the process of vestibular ototoxicity and hair cell regeneration, including aspects of hair cell destruction and repair.

We found a selective loss of vestibular hair cells in a patient followed for more than 10 years with imbalance and oscillopsia due to idiopathic progressive loss of vestibular function. Hearing function and cochlear hair cells were normal. The vestibulo-ocular reflex (VOR) gain at high frequencies was relatively maintained despite marked shortening of the dominant VOR time constant (to less than 500 ms). Ultrastructural examination of remaining hair cells showed mitochondrial abnormalities. The ultrashort VOR time constant probably resulted from changes in firing patterns of the primary afferent nerves due to loss of hair cells and impaired energy metabolism in remaining hair cells.

1. The firing rates of action potentials of abducens nerve single fibers were recorded in the cat's orbit during a variety of vestibular and optokinetic stimulations. 2. Comparison was made of the neural firing rates associated with agonist and antagonist responses during slow and fast components of vestibular and optokinetic nystagmus. It was found that the relationship between the motoneuron firing rates and the eye motion was independent of the reflex with which they were associated--vestibular or optokinetic, or the type of response--agonist or antagonist. No neurons were observed that responded only during the fast or only during the slow nystagmus phase. Motoneuron firing rates were proportional to both velocity and position of the eye in a ratio of 1 (spikes/s)/(deg/s) to 7.2 (spikes/s)/deg. The behavior of the motoneurons was compatible with the hypothesis that thier firing rates are sufficient to overcome both elastic and viscous forces by which the muscles and ligaments hold the eye in the orbit. 3. For low-frequency head rotations, eye displacement and neural responses showed a small phase angle difference. At higher frequencies, however, while the eyes maintained a fixed relationship to the head rotation, the neural responses showed an increasing phase lead. One component of this phase lead compensated for the phase lag introduced by the orbital mechanics. The other was modeled as a constant delay of approximately 70 ms, which may be accounted for by neuromuscular transmission and transduction.

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