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The number of saccades, vestibular gain and visual suppression as a function of smooth pursuit gain were determined in 10 cerebellar cases. Visual suppression increased and the number of saccades decreased with increasing smooth pursuit gain while a high vestibular gain was found in most cases but without relation to the gain of smooth pursuit. An eye-motor model is presented according to which all findings could be related to a disturbance of proper cerebellar processing of optic impulses and of impulses to the vestibular nuclei in the paramedian reticular formation.

In 5 healthy individuals the postrotatory, exponential constants were estimated for each of the four nystagmus qualities, viz. the velocities and durations of fast and slow components. The residual spread (Sr) of each quality in a semilogarithmic plot was also determined. The value of the constant for postrotatory decay indicated a decrease of the velocity and an increase for the duration of the slow component, both with a significant difference between individuals, while for the corresponding values of the fast components no systematic change in the postrotatory reactions was indicated, nor any difference between individuals. The constant that corresponds to the value at maximum stimulus showed differences between individuals for each of the four nystagmus qualities, while Sr presented differences between individuals only for the two slow phase qualities. This technique to describe postrotatory nystagmus reactions by the two exponential constants for each of the four nystagmus qualities together with the residual spread for each of the qualities is intended for future 'on-line' nystagmus analysis.

Postrotatory responses of nystagmus were analysed in an exponential model by utilizing linear regression analysis. Four nystagmus qualities (velocity and duration of slow and fast phases) were studied in 10 patients with vestibular peripheral lesions, 10 patients with frontal lobe lesions and 10 patients with brain-stem lesions, together with 10 control subjects. In addition, pauses during the responses were quantified. Patients with frontal lobe lesions differed from other groups by scoring higher values of slow phase velocity and by exhibiting more pauses. The time constant was significantly shorter in patients with brain-stem lesion than in any other group. As regards other qualities, e.g. slow phase duration and fast phase velocity, or duration, no differences were observed. The pathological dysrhythmia may therefore be presented as changes in the gain and time constant of slow phase velocity as well as in pauses during nystagmus. Since all these changes may be encountered in normal subjects, one should be cautious in interpreting these changes as being pathological in each individual case.

The maximum velocity gain of smooth pursuit and optokinetic, vestibular, and optovestibular slow phases was examined in 15 patients with pontine, 10 with medullary, 10 with cerebellar, and 5 with combined cerebello-brain stem disorders. Marked dissociations were observed between smooth pursuit and optokinetic slow phases, especially in medullary disease. A cerebellar deficit enhanced slow phase velocity gain during rotation in darkness, whereas the corresponding gain during rotation in light was normal.

The eye tracking test is today a well established part of clinical otoneurological examination. In the present paper some applications of the test in evaluating of the level of lesion within the vestibular system are reviewed.

Eye movements in 30 patients with brain-stem lesions were evaluated with electro-oculography, and the findings were analyzed with regard to the site of the lesion in the brain stem. Eye velocities generally were reduced more severely in pontine than in medullary lesions. In medullary lesions, there even was a slight enhancement of the velocities in the fast component of optokinetic and postrotatory nystagmus. In contrast to the slow-phase velocity of OKN, which often was preserved in medullary lesions, the smooth-pursuit velocity was reduced in lesions at all levels in the brian stem. This indicates that these two types of visually induced eye movements use different mechanisms.

Optokinetic nystagmus (OKN) was studied in ten patients with vestibular neuritis, and in seventeen patients with unilateral and thirteen patients with bilateral infratentorial lesions and compared with OKN in fifty healthy subjects. Mean and maximum slow phase velocity of OKN was calculated as well as the asymmetry of responses. The efficiency of the different variables in OKN, in discriminating between different lesion sites, was tested in a linear discriminant function analysis. Slow phase velocity of OKN could satisfactorily separate patients with infratentorial lesions from healthy subjects or subjects with vestibular neuritis. Asymmetry of OKN did not contribute further to the correct diagnosis.