Figure 1. Abnormal eye movements in patient 1 with a lesion involving the right nucleus prepositus hypoglossi.

(A) Patient 1 shows spontaneous right-beating nystagmus with a mean slow-phase velocity (SPV) of 2.9 ± 0.4°/s, which increases when fixation is eliminated, showing a decelerating waveform and an initial SPV of 12.3 ± 2.6°/s. (B) Lateral gaze produces gaze-evoked nystagmus, more intense when looking to the ipsilesional side. (C) Smooth pursuit is impaired ipsilesionally (gain ≈0.2 in response to a sinusoidal target motion at a peak velocity of 10°/s). (D) Horizontal saccades are normal. (E) Recordings of head impulse tests with the magnetic search coil technique show decreased gain of the vestibulo-ocular reflex for the left horizontal canal (HC). The gains for both anterior canals (ACs) are increased, but premature deceleration results in corrective catchup saccades. In contrast, the gains for both posterior canals (PCs) are within the normal range. The gains were measured as the ratio of mean eye velocity to mean head velocity during a 40-millisecond window centered at peak head acceleration. H = horizontal; T = torsional; V = vertical.