On the distribution of fast-phase intervals in optokinetic and vestibular nystagmus

 Histograms of fast-phase intervals in human optokinetic and vestibular nystagmus were generated, and fitted to statistical distributions used in previous studies. The distributions did not depend on stimulation type (optokinetic or vestibular). An inverse Gaussian or a gamma distribution fitted the data better than did a reciprocal Gaussian distribution, but none fitted the data especially well. In some cases, however, the interpretation of these distributions is more physiologically satisfactory than in others. Recommendations are made on which class of models is preferred, and the experiments needed to support the particular models. Our results call into question the validity of previous studies that fit statistical distributions to data sets of a size comparable to ours. Received: 26 June 2001 / Accepted in revised form: 20 February 2002     

Spectrum analysis of vestibular nystagmus

A procedure for the spectral analysis of nystagmograms obtained by rotary stimulation with the rate of 180 degrees/s for 20 s has been developed with the aim to reveal the most typical changes in the vestibular nystagmus with one or another form of the cochleovestibular pathology. A signal recorded in the analogous form was processed by computer EC-1022. Periodograms were calculated by the method of the fast Fourier transform and periodogram smoothing by averaging of realization sites. In case of Ménière's disease the third harmonic was not recorded on periodograms when right and left labyrinths were stimulated and in the case of neurites of acoustic nerves, the third or fifth harmonic was not observed. A supposition is advanced that the third harmonic reflects the state of the peripheral area of the vestibular analyzer, while the fifth harmonic--the central one.     

Transient analysis of vestibular nystagmus

The significance of a nystagmus-dependent, transient component in the overall slow-phase response of the vestibulo-ocular reflex (VOR) is brought into focus. First, a simulated example is presented that shows how this transient component can bias current algorithms for the estimation of VOR parameters. Second, new algorithms are proposed that are able to estimate VOR parameters regardless of the presence of transients. Third, the new algorithms are applied to experimental data, and the results are compared with those from current algorithms. The results clearly show that the transient component can significantly alter the apparent VOR time constant, particularly when the reflex has been lesioned. The algorithms open new areas of research on the possible role of nystagmus in enhancing the compensatory function of the VOR.     

On the correlation dimension of optokinetic nystagmus eye movements: computational parameters, filtering, nonstationarity, and surrogate data

 We discuss the estimation of the correlation dimension of optokinetic nystagmus (OKN), a type of reflexive eye movement. Parameters of the time-delay reconstruction of the attractor are investigated, including the number of data points, the time delay, the window duration, and the duration of the signal being analyzed. Adequate values are recommended. Digital low-pass filtering causes the dimension to increase as the filter cutoff frequency decreases, in accord with a previously published prediction. The stationarity of the correlation dimension is examined; the dimension appears to decrease over the course of 120 s of continuous stimulation. Implications for the reliable estimation of the dimension are considered. Several surrogate data sets are constructed, based on both early (0–30 s) and late (100–130 s) OKN segments. Most of the surrogate data sets randomize some aspect of the original OKN, while maintaining other aspects. Dimensions are found for all surrogates and for the original OKN. Evidence is found that is consistent with some amount of deterministic and nonlinear dynamics in OKN. When this structure is randomized in the surrogate, the dimension changes or the dimension algorithm ceases to converge to a finite value. Implications for further analysis and modeling of OKN are discussed. Received: 30 August 1996/Accepted in revised form: 13 November 1996     

Fast component threshold for vestibular nystagmus in the rabbit

The existence of a threshold for the production of fast components of vestibular nystagmus was investigated in the rabbit. The characteristics (position and velocity) of reflexive eye movements were precisely monitored with the use of the search-coil method and a laboratory computer. The threshold largely depended on the eye position in the orbit during nystagmus and, to a much lesser extent, on the eye velocity. The basic characteristics of the threshold remained unchanged under vestibular stimulation in the dark and in the light, and for different frequencies and peak velocities of rotation. A pattern of vestibular nystagmus was demonstrated whereby it is possible to predict the occurrence of fast components.     

On automatic diagnosis of pathological saccadic eye movements

A syntactic technique is described for the recognition of saccadic eye movements to distinguish normal saccades from those distorted by brain stem lesions. A digitalized eye movement signal is transformed into a sequence of symbols. Eye movements are then found from this sequence by using a parser. This recognition method appropriately enlarged could be applied as a classifier of saccades to aid in diagnosis     

The effect of a long stay under microgravity on the vestibular function and tracking eye movements

Pre-and postflight examinations of cosmonauts participating in missions ISS-3 to ISS-9 on the International Space Station were performed using a computer-aided method of integrated assessment of the oculomotor system. The role and significance of the vestibular system in the eye tracking were determined; the individual and general characteristics of spontaneous oculomotor reactions and oculomotor reactions induced by visual and vestibular stimuli after a long-term stay at zero gravity (126–195 days) were determined; and the changes in the indices of oculomotor reactions were monitored. Studies of the vestibular function, intersensory interactions, and the tracking function of the eyes in the crew members were performed on the second, fifth (sixth), and ninth (tenth) days of the readaptation period. The results of the postflight examinations showed a significant change in the accuracy, velocity, and temporal characteristics of eye tracking and an increase in the vestibular reactivity. It was shown that the structure of visual tracking (the accuracy of fixational eye rotations and smooth tracking) was disturbed (the appearance of correcting saccades, the transition of smooth tracking to saccadic tracking) only in those cosmonauts who, in parallel to an increased reactivity of the vestibular input, also had central changes in the oculomotor system (spontaneous nystagmus, gaze nystagmus). With one exception, recovery of the indices of the accuracy of tracking eye movements in cosmonauts to the background level in the selected period of examination was not observed, although a positive trend was recorded.     

Modeling the interaction among three cerebellar disorders of eye movements: periodic alternating,gaze-evoked and rebound nystagmus

A woman, age 44, with a positive anti-YO paraneoplastic cerebellar syndrome and normal imaging developed an ocular motor disorder including periodic alternating nystagmus (PAN), gaze-evoked nystagmus (GEN) and rebound nystagmus (RN). During fixation there was typical PAN but changes in gaze position evoked complex, time-varying oscillations of GEN and RN. To unravel the pathophysiology of this unusual pattern of nystagmus, we developed a mathematical model of normal function of the circuits mediating the vestibular-ocular reflex and gaze-holding including their adaptive mechanisms. Simulations showed that all the findings of our patient could be explained by two, small, isolated changes in cerebellar circuits: reducing the time constant of the gaze-holding integrator, producing GEN and RN, and increasing the gain of the vestibular velocity-storage positive feedback loop, producing PAN. We conclude that the gaze- and time-varying pattern of nystagmus in our patient can be accounted for by superposition of one model that produces typical PAN and another model that produces typical GEN and RN, without requiring a new oscillator in the gaze-holding system or a more complex, nonlinear interaction between the two models. This analysis suggest a strategy for uncovering gaze-evoked and rebound nystagmus in the setting of a time-varying nystagmus such as PAN. Our results are also consistent with current ideas of compartmentalization of cerebellar functions for the control of the vestibular velocity-storage mechanism (nodulus and ventral uvula) and for holding horizontal gaze steady (the flocculus and tonsil).

     

Interaction between vision and vestibular nystagmus]

In order to assess the visuo-vestibular interaction and its behaviour in connection with the age, we have submitted 20 healthy subjects from two age groups ranging between 21-30 and 65-75 years of age to two rotation tests (with closed eyes and with opened eyes with fixation of a light). The results show a variation of the nystagmic response with fixation of about 60% in the young people and of about 40% in the elderly. These results demonstrate that the extent of the inhibition is dependent upon age and is less effective in the elderly.     

A model of Alexander's law of vestibular nystagmus

The observation that the amplitude of vestibular nystagmus grows as gaze is increased in the direction of the nystagmus fast phase and diminished with gaze in the opposite direction is known as Alexander's law. We have developed an analog computer model to simulate Alexander's law in nystagmus secondary to dysfunction of a semicircular canal. The model utilizes relevant brainstem anatomy and physiology and includes gaze modulation of vestibular signals and push-pull integration to create eye positition commands. When simulating normally functioning semicircular canals, the model produced no nystagmus. When simulating total impairment of the canal on one side with gaze directed maximally in the opposite direction, the model produced a large amplitude nystagmus with linear slow phases directed toward the affected side. As gaze was changed from far contralateral to ipsilateral, the nystagmus gradually diminished to zero. When simulating partial impairment of one canal, the nystagmus was smaller in amplitude and absent in ipsilateral gaze.