A technique is described for the assessment of vestibular sensation. The two main goals of the study were (i) to compare the perception of angular velocity with the eye velocity output of the vestibulo-ocular reflex and (ii) to study vestibular function in patients with congenital nystagmus; this was needed since most previous studies, based on eye movement recordings, have been inconclusive. Subjects indicated their perceived angular velocity by turning by hand a wheel connected to a tachometer. The vestibular stimuli used consisted of sudden deceleration from rotation at a constant horizontal velocity of 90 degrees /s ('stopping' responses). Eye movements were recorded simultaneously with electro-oculography. In normal subjects the perceived angular velocity decayed from the moment of deceleration in an exponential fashion. The mean time constant of sensation decay was approximately 16 s. Eye movement velocity decayed with a similar exponential trajectory (time constant 16 s). Congenital nystagmus patients showed markedly shortened vestibular sensation (mean time constant 7 s). The following conclusions can be drawn: (i) the similarity of the eye velocity and perceptual responses suggests that these two systems receive a vestibular signal which has been similarly processed; (ii) the time constant of the responses indicates that this vestibular signal probably originates in the same brainstem 'velocity storage' integrator; (iii) the technique described is useful for clinical assessment of vestibular function, particularly in patients with ocular motility disorders; (iv) patients with congenital nystagmus have short vestibular time constants, which is probably due to changes induced in velocity storage processing by the persistent retinal image motion present in these patients.

Congenital or acquired periodic alternating nystagmus (PAN) is characterized by nystagmus occurring in a cycle. The cycle consists of a left-beating nystagmus, a transition phase, a right-beating nystagmus, and a further transition phase. The purpose of this review is to assist the clinician in the recognition of periodic alternating nystagmus (PAN), either as a type of congenital nystagmus or in its acquired form, and to highlight why such identification is important. Recent studies using eye movement recordings are reviewed to point out the frequency of congenital PAN in samples of patients with congenital nystagmus, and to describe the characteristics of the waveforms and the influence of foveation time on the alternation of head turns. Classical and new surgical alternatives are reported. The identification of congenital PAN is essential when surgical treatment is being considered for the correction of anomalous head postures. Acquired PAN is usually due to cerebellar disease and causes oscillopsia. Unlike other forms of acquired nystagmus, it responds well to drug treatment.

Baseline data are useful to differentiate normal changes occurring with age from early signs of disease. AdditIonally, disease progression and effects of treatment can be monitored.

Labyrinthine defective subjects (LDS) experience oscillopsia during head movements due to the absence of the vestibulo-ocular reflex (VOR). The purpose of this study was to compare horizontal and vertical visual motion detection in LDS during (i) body-stationary and (ii) horizontal whole-body oscillation conditions. Twelve LDS and controls detected the onset of drift direction of a grating that moved with accelerating velocity. Thresholds were raised in the patient group in both conditions. The loss of the VOR per se cannot explain raised thresholds in the body-stationary condition nor during whole-body (horizontal) oscillation with vertical grating motion. Findings indicate changes in visual processing that make LDS less sensitive to visual motion. It is postulated that these changes are due to adaptive mechanisms involved to reduce oscillopsia.