Slow-phase Eye Movements Research Articles

Pulse-step-sine rotation test for the identification of abnormal vestibular function

This study illustrates the use of a novel "pulse-step-sine" (PSS) rotational stimulus to identify abnormal function of the horizontal semicircular canals in human subjects with unilateral and bilateral vestibular deficits. The cyclic PSS stimulus includes a "bias component" and a "probe component". The bias component, consisting of a short duration pulse of acceleration followed by an acceleration step, is designed to produce a large change in canal afferent discharge rates that silences the neural activity in one canal during the step portion of the PSS stimulus. The pulse and step components are then repeated with opposite sign to silence afferent activity in the opposite canal. The probe component, consisting of a approximately 1 Hz sine wave superimposed on the step portions of the stimulus, is designed to test the ability of canal afferents in the excited canal to encode the probe stimulus. Various response measures are developed that characterize the horizontal slow phase eye movements evoked by the PSS stimulus. Results show that these measures can distinguish subjects with normal vestibular function from those with unilateral and bilateral vestibular loss, can identify the side-of-lesion in subjects with well compensated unilateral vestibular loss, and can possibly identify the side-of-greater-loss in subjects with asymmetric bilateral loss.

Characteristics of braking saccades in congenital nystagmus.

Several of the characteristic waveforms of congenital nystagmus (CN) contain braking saccades. We test the hypothesis that braking (including foveating) saccades, while not always satisfying the standard relationships for saccades, are normal; any differences are due to the presence of high-velocity, slow-phase eye movements. Better measurements of saccadic properties, including position- and velocity-based measures and skewness, can eliminate some of this apparent distortion. We also evoked an analogous effect in normal subjects by use of a ramp-step-ramp stimulus. Finally, we used a model to further demonstrate this distortion in the saccades of normals, deviating from their intended magnitude as a function of the magnitude of the opposing velocity. The saccadic analysis methods developed herein are applicable to all saccades made during ongoing eye movements, whether normal or pathological. The above findings support the hypothesis that the braking saccades integral to many CN waveforms have normal characteristics and are the result of a normal saccadic system's responses to a slow-eye-movement oscillation.

Cross-axis adaptation of torsional components in the yaw-axis vestibulo-ocular reflex.

The three pairs of semicircular canals within the labyrinth are not perfectly aligned with the pulling directions of the six extraocular muscles. Therefore, for a given head movement, the vestibulo-ocular reflex (VOR) depends upon central neural mechanisms that couple the canals to the muscles with the appropriate functional gains in order to generate a response that rotates the eye the correct amount and around the correct axis. A consequence of these neural connections is a cross-axis adaptive capability, which can be stimulated experimentally when head rotation is around one axis and visual motion about another. From this visual-vestibular conflict the brain infers that the slow-phase eye movement is rotating around the wrong axis. We explored the capability of human cross-axis adaptation, using a short-term training paradigm, to determine if torsional eye movements could be elicited by yaw (horizontal) head rotation (where torsion is normally inappropriate). We applied yaw sinusoidal head rotation (+/-10 degrees, 0.33 Hz) and measured eye movement responses in the dark, and before and after adaptation. The adaptation paradigm lasted 45-60 min, and consisted of the identical head motion, coupled with a moving visual scene that required one of several types of eye movements: (1) torsion alone (-Roll); (2) horizontal/torsional, head right/CW torsion (Yaw-Roll); (3) horizontal/torsional, head right/CCW torsion (Yaw+Roll); (4) horizontal, vertical, torsional combined (Yaw+Pitch-Roll); and (5) horizontal and vertical together (Yaw+Pitch). The largest and most significant changes in torsional amplitude occurred in the Yaw-Roll and Yaw+Roll conditions. We conclude that short-term, cross-axis adaptation of torsion is possible but constrained by the complexity of the adaptation task: smaller torsional components are produced if more than one cross-coupling component is required. In contrast, vertical cross-axis components can be easily trained to occur with yaw head movements.

Using sensory weighting to model the influence of canal, otolith and visual cues on spatial orientation and eye movements

The sensory weighting model is a general model of sensory integration that consists of three processing layers. First, each sensor provides the central nervous system (CNS) with information regarding a specific physical variable. Due to sensor dynamics, this measure is only reliable for the frequency range over which the sensor is accurate. Therefore, we hypothesize that the CNS improves on the reliability of the individual sensor outside this frequency range by using information from other sensors, a process referred to as "frequency completion." Frequency completion uses internal models of sensory dynamics. This "improved" sensory signal is designated as the "sensory estimate" of the physical variable. Second, before being combined, information with different physical meanings is first transformed into a common representation; sensory estimates are converted to intermediate estimates. This conversion uses internal models of body dynamics and physical relationships. Third, several sensory systems may provide information about the same physical variable (e.g., semicircular canals and vision both measure self-rotation). Therefore, we hypothesize that the "central estimate" of a physical variable is computed as a weighted sum of all available intermediate estimates of this physical variable, a process referred to as "multicue weighted averaging." The resulting central estimate is fed back to the first two layers. The sensory weighting model is applied to three-dimensional (3D) visual-vestibular interactions and their associated eye movements and perceptual responses. The model inputs are 3D angular and translational stimuli. The sensory inputs are the 3D sensory signals coming from the semicircular canals, otolith organs, and the visual system. The angular and translational components of visual movement are assumed to be available as separate stimuli measured by the visual system using retinal slip and image deformation. In addition, both tonic ("regular") and phasic ("irregular") otolithic afferents are implemented. Whereas neither tonic nor phasic otolithic afferents distinguish gravity from linear acceleration, the model uses tonic afferents to estimate gravity and phasic afferents to estimate linear acceleration. The model outputs are the internal estimates of physical motion variables and 3D slow-phase eye movements. The model also includes a smooth pursuit module. The model matches eye responses and perceptual effects measured during various motion paradigms in darkness (e.g., centered and eccentric yaw rotation about an earth-vertical axis, yaw rotation about an earth-horizontal axis) and with visual cues (e.g., stabilized visual stimulation or optokinetic stimulation).

Self-motion perception: assessment by real-time computer-generated animations

We report a new procedure for assessing complex self-motion perception. In three experiments, subjects manipulated a 6 degree-of-freedom magnetic-field tracker which controlled the motion of a virtual avatar so that its motion corresponded to the subjects’ perceived self-motion. The real-time animation created by this procedure was stored using a virtual video recorder for subsequent analysis. Combined real and illusory self-motion and vestibulo-ocular reflex eye movements were evoked by cross-coupled angular accelerations produced by roll and pitch head movements during passive yaw rotation in a chair. Contrary to previous reports, illusory self-motion did not correspond to expectations based on semicircular canal stimulation. Illusory pitch head-motion directions were as predicted for only 37% of trials; whereas, slow-phase eye movements were in the predicted direction for 98% of the trials. The real-time computer-generated animations procedure permits use of naive, untrained subjects who lack a vocabulary for reporting motion perception and is applicable to basic self-motion perception studies, evaluation of motion simulators, assessment of balance disorders and so on.

The effect of gravity on the horizontal and vertical vestibulo-ocular reflex in the rat.

Horizontal and vertical eye movements were recorded in alert pigmented rats using chronically implanted scleral search coils or temporary glue-on coils to test the dependence of the vestibulo-ocular reflex (VOR) upon rotation axis and body orientation. The contributions of semicircular-canal versus otolith-organ signals to the VOR were investigated by providing canal-only (vertical axis) and canal plus otolith (horizontal axis) stimulation conditions. Rotations that stimulated canals only (upright yaw and nose-up roll) produced an accurate VOR during middle- and high-frequency rotations (0.2-2 Hz). However, at frequencies below 0.2 Hz, the canal-only rotations elicited a phase-advanced VOR. The addition of a changing gravity stimulus, and thus dynamic otolith stimulation, to the canal signal (nose-up yaw, on-side yaw, and upright roll) produced a VOR response with accurate phase down to the lowest frequency tested (0.02 Hz). In order to further test the dependence of the VOR on gravitational signals, we tested vertical VOR with the head in an inverted posture (inverted roll). The VOR in this condition was advanced in phase across all frequencies tested. At low frequencies, the VOR during inverted roll was anticompensatory, characterized by slow-phase eye movement in the same direction as head movement. The substantial differences between canalonly VOR and canal plus otolith VOR suggest an important role of otolith organs in rat VOR. Anticompensatory VOR during inverted roll suggests that part of the otolith contribution arises from static tilt signals that are inverted when the head is inverted.

Visual and oculomotor responses induced by neck vibration in normal subjects and labyrinthine-defective patients.

Three-dimensional scleral search coil eye movement recordings were obtained in five normal subjects and four patients with absent vestibular function, during unilateral vibration of the neck in the supine position. The purpose of the experiments was to investigate any role played by eye movements in the illusion that a small fixation target, viewed in an otherwise dark room, moves when vibration is applied to the neck (propriogyral illusion). Vibration was applied to the right dorsal neck muscles in three visual conditions: total darkness, fixating a light-emitting diode (LED) in an otherwise totally dark room and LED fixation in the normally lit room. Normal subjects reported that during vibration, with LED fixation in an otherwise dark room, the target appeared to move predominantly leftwards and patients reported a predominantly downward movement. Eye movements were consistently elicited in all subjects. In normal subjects there was a slow-phase eye movement predominantly to the right, interrupted by nystagmic quick phases in the opposite direction, whereas in the patients slow phases were predominantly upward with quick phases downward. Eye movements were larger in the dark but the velocity of the initial slow-phase component (<200 ms) did not change with visual conditions. Mean latencies of the eye movements were typically 80 ms but in individual trials could be as short as 40- 60 ms. The eye movements were considerably larger in the patients (e.g. mean cumulative slow-phase displacement in the dark 12 degrees vs 2 degrees; maximum velocity ca. 5 degrees /s vs 1 degrees /s). These results indicate that the propriogyral illusion is secondary to vibration-induced eye movements, presumably mediated by the cervico-ocular reflex (COR). The difference in direction of the illusion and eye movements in the patients may be related to a predominant enhancement of the vertical COR, secondary to the prominent exposure to vertical retinal slippage experienced by these patients during daily activities such as locomotion.

Ocular responses to radial optic flow and single accelerated targets in humans

Self-movement in a structured environment induces retinal image motion called optic flow. Optic flow on one hand provides information about the direction of self-motion. On the other hand optic flow presents large field visual motion which will elicit eye movements for the purpose of image stabilization. We investigated oculomotor behavior in humans during the presentation of radial optic flow fields which simulated forward or backward self-motion. Different conditions and oculomotor tasks were compared. In one condition, subjects had to actively pursue single dots in a radial flow pattern. In a second condition, subjects had to pursue single dots over a dark background. These dots accelerated or decelerated similar to single dots in radial optic flow. In a third condition, subjects were asked to passively view the entire optic flow stimulus. Smooth pursuit eye movements with high gain were observed when dots were actively pursued. This was true for single dots moving over a homogeneous background and for single dots in the optic flow. Passive viewing of optic flow stimuli evoked eye movements that resembled an optokinetic nystagmus. Slow phase eye movements tracked the motion of elements in the optic flow. Gain was low for simulated forward self-motion (expanding optic flow) and high for simulated backward movement self-motion (contracting optic flow). Thus, voluntary pursuit and passive optokinetic responses yielded different gain for the tracking of elements of an expanding optic flow pattern. During passive viewing of the optic flow stimulus, gaze was usually at or near the focus of radial flow. Our results give insights into the oculomotor performances and needs for image stabilization during self-motion and in the role of gaze strategy for the detection of the direction of heading.

Principles Underlying Real-Time Nystagmus Analysis of Horizontal and Vertical Eye Movements Recorded With Electro-, Infra-Red-, or Video-Oculographic Techniques

New methods for separating fast and slow phases of human nystagmus in real time are presented that are adaptive to the time-dependent noise properties of the input eye movement signal and therefore applicable to different recording techniques and directions. The methods employ a statistical filter technique to track slow-phase eye movements, uninfluenced by fast phases and blink artifacts, and fuzzy-logic techniques to identify fast-phase eye movements. Because these two techniques are decoupled from one another, highly accurate phase separation and slow-phase velocity profiles are achieved. In addition, the tracking of the variance of slow-phase and fast-phase eye movement recording permits a quality control of the analysis for different recording techniques and a variety of ocular nystagmus responses. Because blinks impose different eye velocity profiles on the recordings, depending on the type of recording technique and direction (horizontal, vertical), blink detection and its effect on fast-phase amplitude must be individually adjusted to each recording technique. Results are illustrated in the context of simultaneously recorded video-oculographic, infra-red, and electro-oculographic recordings of vestibulo-ocular reflex and optokinetic reflex responses causing horizontal or vertical eye movements.

Vestibular caloric responses and behavioral state in the fetal sheep

Functional activity of the vestibular system in relation to behavioral state of fetal sheep in utero was studied by cooling and heating of the fetal middle ear and skin (control) with implanted copper-tube heat exchangers. Eye movements and fetal cortical activity were assessed before, during, and after 2 min irrigations with water at 6, 46, or 39.5°C (isothermic). Cold water induced slow-phase eye movements toward the irrigated ear followed by saccades toward the opposite ear after a delay of several seconds. The direction of the response reversed with warm water, and saccades were absent during irrigation with body-temperature water. Cold-water irrigations of the skin over the jaw did not result in nystagmus. Arousal-like responses were elicited with thermal stimulation of the ear or facial skin while the fetus was in either rapid eye movement (REM) or non-REM states. Circulation of 39.5°C water through the ear also produced arousal-like responses, possibly due to turbulence-induced noises in the heat exchanger or slight deviations between the irrigation temperature and the actual fetal inner ear temperature. These results suggest that mechanisms responsible for saccade suppression during depressed levels of consciousness (i.e. sleep) are inactive in utero. Fetal behavioral state responsiveness to vestibular and somatosensory thermal stimulation may be of great significance, especially in the premature neonate.

Optokinetic eye movements elicited by radial optic flow in the macaque monkey.

We recorded spontaneous eye movements elicited by radial optic flow in three macaque monkeys using the scleral search coil technique. Computer-generated stimuli simulated forward or backward motion of the monkey with respect to a number of small illuminated dots arranged on a virtual ground plane. We wanted to see whether optokinetic eye movements are induced by radial optic flow stimuli that simulate self-movement, quantify their parameters, and consider their effects on the processing of optic flow. A regular pattern of interchanging fast and slow eye movements with a frequency of 2 Hz was observed. When we shifted the horizontal position of the focus of expansion (FOE) during simulated forward motion (expansional optic flow), median horizontal eye position also shifted in the same direction but only by a smaller amount; for simulated backward motion (contractional optic flow), median eye position shifted in the opposite direction. We relate this to a change in Schlagfeld typically observed in optokinetic nystagmus. Direction and speed of slow phase eye movements were compared with the local flow field motion in gaze direction (the foveal flow). Eye movement direction matched well the foveal motion. Small systematic deviations could be attributed to an integration of the global motion pattern. Eye speed on average did not match foveal stimulus speed, as the median gain was only approximately 0.5-0.6. The gain was always lower for expanding than for contracting stimuli. We analyzed the time course of the eye movement immediately after each saccade. We found remarkable differences in the initial development of gain and directional following for expansion and contraction. For expansion, directional following and gain were initially poor and strongly influenced by the ongoing eye movement before the saccade. This was not the case for contraction. These differences also can be linked to properties of the optokinetic system. We conclude that optokinetic eye movements can be elicited by radial optic flow fields simulating self-motion. These eye movements are linked to the parafoveal flow field, i.e., the motion in the direction of gaze. In the retinal projection of the optic flow, such eye movements superimpose retinal slip. This results in complex retinal motion patterns, especially because the gain of the eye movement is small and variable. This observation has special relevance for mechanisms that determine self-motion from retinal flow fields. It is necessary to consider the influence of eye movements in optic flow analysis, but our results suggest that direction and speed of an eye movement should be treated differently.

Optokinetic reflex in squirrel monkeys after long-term monocular deprivation.

Horizontal optokinetic nystagmus (OKN) as well as neuronal response properties in the nucleus of the optic tract and the dorsal terminal nucleus of the accessory optic system (NOT-DTN) were investigated in three monocularly deprived squirrel monkeys. In two monkeys occlusion of one eye was performed at birth (early) and in the third after 7 weeks (late). In adulthood, in early deprived monkeys monocular horizontal OKN tested through the non-deprived eye was symmetrical and in no way different from normal, i.e. stimulation in the temporonasal and nasotemporal direction elicited equal and robust responses. OKN through the early occluded eye, however, was grossly abnormal with low gain and great variability in the consistency of nasotemporal and temporonasal slow phase eye movements. When in the late deprived monkey the non-deprived eye was occluded a strong spontaneous nystagmus developed despite the deprived eye viewing a stationary pattern. The slow phases were directed from nasal to temporal for the deprived eye. When tested through the non-deprived eye all neuronal responses of the NOT-DTN were normal. The deprived eye's influence on NOT-DTN neurons was extremely weak. No neuron with a moderate or even dominant input from the deprived eye was found after early deprivation. In the late deprived case the deficit was not as severe but still the non-deprived eye was clearly dominating the responses in all neurons tested. Velocity tuning of neurons tested through the non-deprived eye was normal and qualitatively corresponded well to slow phase eye velocity in response to equivalent retinal slip during OKN. Through the early deprived eye, however, velocity tuning was extremely poor. It was somewhat better through the late deprived eye. We suggest that the dramatic deterioration in the optokinetic reflex found after long-term monocular deprivation for the amblyopic eye is probably caused by the almost complete loss of retinal and cortical input driven by that eye to the NOT-DTN. These results are discussed in relation to our previous results in cats and reports in the literature for humans with occlusion amblyopia.

The Velocity Storage Mechanism of the Optokinetic Nystagmus Under Apparent Stimulus Movements in Squirrel Monkeys

Experiments in two awake untrained squirrel monkeys were performed to study the velocity storage mechanism during fast rise of OKN slow phase velocity. This was done by testing the monkey's capability to perform OKN in response to a stationary-appearing stroboscopically illuminated stripe pattern of a horizontally rotating drum. Nystagmus was initially elicited during constant illumination lasting between 0.6 and 25 s. The periodicity of the stripe pattern was 2.37 degrees. When after the constant light the flash illumination was switched on again, two types of behavior could occur, depending on the length of the constant light interval (CLI): 1) when the CLI was shorter than a threshold value of 6.2 seconds, the OKN ceased under the flash stimulation. Then a "post-OKN" occurred that increased with the length of the CLIs, indicating that the intermittently illuminated pattern did not provoke fixation suppression of OKN aftereffects. 2) when the CLI was above threshold, the OKN continued under the flash light; it will be called "apparent movement OKN." The threshold CLI between the type 1 and the type 2 response did not depend on drum velocities between 21.5 degrees/s and 71.3 degrees/s. The average gain of the apparent movement OKN was 0.83 +/- 0.04; gain and stability of slow phase eye movement velocity did not deviate systematically from the usually elicited OKN. OKAN after apparent movement OKN did not deviate from OKAN after constantly illuminated moving patterns. In response to the OKN initiation by a constantly illuminated pattern up to pattern velocities of 100 degrees/s, the OKN steady state gain was reached within the first 2 or 3 nystagmus beats. We ascribe the increase of the post-OKN with CLI and the existence of a threshold constant light interval to activity-accumulation in the common velocity-to-position integrator (velocity storage) of the brain stem. Loading of the velocity storage takes place after the OKN gain has already reached the steady-state value. Apparent movement OKN could also be elicited in guinea pigs that lack an effective smooth pursuit system. We suggest that apparent movement OKN is produced by mechanisms located in the brain stem.

Open Loop Optokinetic Responses of the Turtle

Turtle eye movements were measured during full-field horizontal optokinetic stimulation under closed and open loop conditions. Because these animals display unyoked slow-phase eye movement behavior, open loop stimulation could not be presented to a paralyzed eye, while monitoring the position of the contralateral eye. The turtle's optokinetic reflex loop was opened electronically by a continuous adjustment of the pattern's position that effectively canceled the effect of the movement of the recorded eye. The highest open loop gains (2–3) were observed at low speeds (<1 deg/sec), demonstrating a more limited speed range and lower gain in turtle than in the mammalian optokinetic system. These results in the intact animal can be correlated with the visual response properties of the turtle's pretectum and accessory optic system recorded in vitro. © 1997 Elsevier Science Ltd. All rights reserved.

On the Optokinetic Response During Step-Wise Changes in Stimulus Velocity in Squirrel Monkeys

In two awake untrained squirrel monkeys the horizontal optokinetic nystagmus (OKN) was studied. The goal was to quantify the buildup of the slow-phase eye movement velocity during the first two seconds and the eye movements after OKN interruption by a stationary surround. We intended to uncover possible effects of a "charged' velocity storage on eye movements at a stationary surround. Using an optokinetic drum, a paradigm was designed to create sudden changes of the pattern (within 5 ms) between appearing to be stationary or rotating. Velocity steps from zero to 14 to 73 degrees/s and back to zero could be achieved. OKN onset: 201 velocity trajectories were analyzed. The mean latency between the onset of pattern movement and the onset of slow-phase eye movements was 82.8 +/- 16.5 ms. Over a limited period the initial increase in slow-phase velocity could be approximated by a straight line. The slope was on average 103 +/- 67 degrees/s2 and did not show a significant dependency on pattern movement velocity. Eye movement velocity at the end of the linear part increased linearly with drum velocity; the slope was 0.59. After the linear range, the slow-phase velocity increased further but at slower accelerations and usually reached the final gain within the two seconds. The initial linear acceleration component is an open-loop reflex response and we conclude that closing the loop happens when about 60% of the stimulus velocity is reached. OKN-offset: The influence of a fully charged OKN velocity storage mechanism on eye movements after a sudden exposure of a stationary surround was studied in 23 trials. After OKN interruption the velocity decay commenced after an interval of 83.5 +/- 16.6 ms. On average the slope of the consecutive linear velocity decay was -195.4 +/- 83.6 degrees/s2. During 5.8 +/- 0.98 s the OKN still had some impact on the fixating eye movements. We conclude that this time represents the time for velocity storage discharging. An active process seems to control the impact of velocity storage on eye movements.

Chapter 22 Role of the Y-Group of the vestibular nuclei and flocculus of the cerebellum in motor learning of the vertical vestibulo-ocular reflex

Publisher Summary The vestibulo-ocular reflex (VOR) stabilizes ocular gaze during head movement. The gain of this reflex, eye velocity/head velocity can be plastically adapted by manipulating the animal's vision so that the gain remains compensatory. The VOR and particularly a vertical slow phase eye movement of vestibular origin are among the simplest of motor acts. This chapter illustrates the putative neuronal circuit underlying the vertical VOR. Because the illustrated elements are inclusive, motor learning should take place within or among them. There are three sites, which satisfy the criterion of receiving both a head and eye velocity signal. They are the Y group of the vestibular nuclei, the flocculus target neurons (FTN) of the superior vestibular nucleus, and the floccular Purkinje cells. Additionally, for the case where the learning is precerebellar and transmitted through the flocculus, it is considered that the eye velocity circuit might also undergo the modification of its output to command a gain change in the system. The Y group receives both a head velocity signal via a disynaptic pathway through the vestibular nuclei and an eye velocity signal via floccular vertical zone Purkinje cells that innervate Y.

Responses of Neurons of the Nucleus of the Optic Tract and the Dorsal Terminal Nucleus of the Accessory Optic Tract in the Awake Monkey

The nucleus of the optic tract (NOT) and the dorsal terminal nucleus of the accessory optic tract (DTN) are essential nuclei for the generation of slow-phase eye movements during horizontal optokinetic nystagmus. We recorded from 101 neurons (all directionally selective) in four NOT/DTN of three trained and behaving rhesus monkeys. Neuronal activity increased when stimuli moved ipsiversively with respect to the recording site and decreased below spontaneous activity when stimuli moved contraversively. While the monkey fixated a small spot, some NOT/DTN neurons did not respond at all to the retinal image slip of a whole-field random dot pattern; others showed a monotonic increase of activity to increasing velocities of that stimulus. The velocity range tested was up to 100 degrees/s. During the execution of optokinetic nystagmus, 39 of 73 cells tested showed a velocity-tuned response with an average optimum at 21 degrees/s retinal image slip. Following saccades during optokinetic nystagmus (quick phases), the NOT/DTN neuronal activity briefly attained the level of spontaneous activity, as predicted from the velocity selectivity during optokinetic nystagmus. Immediately upon cessation of optokinetic stimulation in the preferred direction, NOT/DTN activity returned to the spontaneous level and did not reflect the ongoing optokinetic afternystagmus in darkness. Most NOT/DTN neurons displayed direction selectivity also during smooth pursuit. Twenty-one of 50 cells tested (42%) always responded to the retinal slip of the target (target velocity cells), 16 cells (32%) responded to the retinal slip of the background (background velocity cells), and 13 cells (26%) did not respond at all during smooth pursuit. We conclude from our results that the NOT/DTN is an essential structure for the processing of the direction and speed of retinal image slip. This information is then used for the generation and maintenance of slow eye movements, preferentially during horizontal optokinetic nystagmus but also during pursuit eye movements.

"Dumping" of rebound nystagmus and optokinetic afternystagmus in humans.

Rebound nystagmus (RN) is an involuntary movement of the eyes, characterized by slow-phase eye velocity in the direction of previously maintained eccentric gaze. The purpose of this study was to clarify the neural or neuromuscular events that are responsible for the generation of RN. To do so, we examined whether a briefly presented visual stimulus during RN reduces (i.e., "dumps") subsequent eye velocity, compared with the velocity of slow-phase eye movements when no visual stimulus was presented. For comparison, "dumping" was examined also for optokinetic afternystagmus (OKAN), which is generally believed to result from eye-velocity signals stored in a central neural integrator as a consequence of optokinetic stimulation. Results obtained from ten normal observers showed that RN and OKAN both exhibit "dumping": average slow-phase eye velocities were reliably slower after fixation of a 0.6 deg stationary target than on trials when no fixation target was presented. Although RN decayed faster than OKAN in darkness, the magnitude of "dumping" increased similarly with the duration of the visual stimulus (25 ms to 4 s) for both types of eye movement. The results imply that signals from a central velocity-storage mechanism contribute to the generation of RN.

Investigation of the Horizontal, Vertical, and Oblique Optokinetic Nystagmus and Afternystagmus in Squirrel Monkeys

A moving random dot pattern was projected onto a tangent screen in front of awake untrained monkeys that were always placed in upright position. Eye movements were recorded in two dimensions to study the oblique optokinetic nystagmus (OKN) and compare it to the horizontal and vertical OKN. Any direction of pattern movement across the screen could be achieved. The angular velocity of pattern movement was varied between 6 and 180 degrees/s. To display off-horizontal and off-vertical eye movements, the instantaneous direction and velocity of the eye movements were computed from the horizontal and vertical search coil voltages. At pattern velocities below 90 degrees/s, stimulus-direction and direction of the OKN slow phase matched very precisely. Above 90 degrees/s the slow-phase eye movement direction was systematically shifted toward the horizontal except for pure vertical stimulation. The slow-phase eye velocity at off-horizontal stimulation was inconstant, however; stable periods occurred repeatedly that were used to define the gain of OKN. Up to stimulus speeds of about 90 degrees/s the OKN gain did not depend on the direction of stimulation and of OKN. At higher velocities the gain decreased with the increasing angle between stimulus direction and horizontal. Practically no vertical optokinetic afternystagmus (OKAN) could be observed, in either the up or down direction. At the onset of afternystagmus after oblique stimulation the direction of the OKAN slow phase immediately shifted over to the horizontal. The data indicate that the slow-phase direction and gain of oblique OKN with the monkey's head upright can be described by the sum of a horizontal and a vertical velocity vector obtained during stimulation in these cardinal directions.

The cerebellar nodulus and ventral uvula control the torsional vestibulo-ocular reflex

1. The vestibulo-ocular reflex (VOR) was investigated in rhesus monkeys before and after surgical ablation of the cerebellar nodulus and ventral uvula. The lesion resulted in an alteration of the torsional VOR: compensatory eye movements were poor in the low frequency range and the time constant was reduced to values comparable to those of primary semicircular canal afferents. In addition, animals permanently lost their ability to generate torsional optokinetic nystagmus (OKN). 2. The effects of the lesion on the torsional VOR differed from those observed in the horizontal and vertical vestibulo-ocular systems. While the vertical VOR and OKN were unaltered, the horizontal VOR and OKN were characterized by increased time constants and smaller phase leads during low frequency head oscillations. 3. These results suggest that the cerebellar nodulus and/or ventral uvula exert a distinct and specific dynamic control on the torsional vestibulo-ocular and optokinetic reflexes. Such specific effects on the torsional system could reflect a functional segregation of the vestibulo-cerebellum in terms of the controls of torsional versus horizontal and vertical slow phase eye movements.