Identification of the location of vestibular lesions on the basis of vestibulo-ocular reflex measurements

Quantitative analysis of electro-oculographic recordings of eye movement in response to precise visual and vestibular stimuli makes possible the differentiation of three categories of vestibular syndromes due to pathological changes in three different parts of the visual vestibulo-ocular reflex arc: (1) decreased vestibulo-ocular reflex gain (e.g., decrease in slow component velocity), but normal fast components and visual-vestibular interaction (labyrinth and eighth nerve); (2) normal slow component velocity but abnormal fast components to all stimuli (pontine or medullar reticular formation); and (3) normal slow component velocity to vestibulo-ocular stimulaton but abnormal visual-vestibular interaction as well as normal fast components (visual-motor pathways or cerebellum).     

Is visual experience necessary for the maturation of vestibular control of eye movement

The vestibular system contributes to the stabilisation of visual images on the retina by means of vestibulo-ocular compensatory reactions. The development of vestibular control of eye movements has been studied in twelve week old kittens, reared in total darkness, which have been compared with a control group of kittens reared in normal conditions. Post-rotatory nystagmus, nystagmus during sinusoidal oscillations, visual suppression of vestibular nystagmus by fixation, have been used as indicators of the functional state of the vestibulo-ocular control system. The results show that most of the essential features of this control are present in dark-reared kittens. However differences have been noted which possibly concern precise regulation of compensatory movements and head-eye coordination. The frequency of vestibular nystagmus is much smaller in dark-reared animals and the initial deviation of post rotatory nystagmus in the direction of the change of movement is absent in dark-reared kittens. Habituation also seems to operate differently in the two groups of kittens. Visual suppression of vestibular nystagmus is present.     

Gaze stabilization by optokinetic reflex (OKR) and vestibulo-ocular reflex (VOR) during active head rotation in man

Vestibulo-ocular reflex (VOR)-optokinetic reflex (OKR) interaction was studied in normal human subjects during active sine-like head movements in the horizontal plane for a variety of vestibular-optokinetic stimulus combinations (frequency range, 0.05-1.6 Hz). At low to mid frequencies (< 0.2 Hz) the eyes tended to be stabilized on the optokinetic pattern, independently of whether the head, the pattern, or both were rotated. At higher frequencies, the OKR gain was attenuated and, in each of the differing stimulus combinations, the eyes became increasingly stabilized in space. Qualitatively similar results were obtained when, for the same visual-vestibular combinations, the head was passively rotated at 0.05 and 0.8 Hz. The data could be simulated by a model which assumes a linear interaction of vestibular and optokinetic signals. It considers the OKR with its negative feedback loop of primordial importance for image stabilization on the retina and the VOR only as a useful addition which compensates for the limited bandwidth of the OKR during high frequency/velocity head rotations in a stationary visual environment.     

Eye-head coordination in labyrinthine-defective humans

Eye-head coordination during saccadic gaze shifts normally relies on vestibular information. A vestibulo-saccadic reflex (VSR) is thought to reduce the eye-in-head saccade to account for current head movement, and the vestibulo-ocular reflex (VOR) stabilizes postsaccadic gaze while the head movement is still going on. Acute bilateral loss of vestibular function is known to cause overshoot of gaze saccades and postsaccadic instability. We asked how patients suffering from chronic vestibular loss adapt to this situation. Eye and head movements were recorded from six patients and six normal control subjects. Subjects tracked a random sequence of horizontal target steps, with their heads (1) fixed in primary position, (2) free to move, or (3) preadjusted to different head-to-target offsets (to provoke head movements of different amplitudes). Patients made later and smaller head movements than normals and accepted correspondingly larger eye eccentricities. Targeting accuracy, in terms of the mean of the signed gaze error, was better in patients than in normals. However, unlike in normals, the errors of patients exhibited a large scatter and included many overshoots. These overshoots cannot be attributed to the loss of VSR because they also occurred when the head was not moving and were diminished when large head movements were provoked. Patients' postsaccadic stability was, on average, almost as good as that of normals, but the individual responses again showed a large scatter. Also, there were many cases of inappropriate postsaccadic slow eye movements, e.g., in the absence of concurrent head movements, and correction saccades, e.g., although gaze was already on target. Performance in patients was affected only marginally when large head movements were provoked. Except for the larger lag of the head upon the eye, the temporal coupling of eye and head movements in patients was similar to that in normals. Our findings show that patients with chronic vestibular loss regain the ability to make functionally appropriate gaze saccades. We assume, in line with previous work, three main compensatory mechanisms: a head movement efference copy, an active cervico-ocular reflex (COR), and a preprogrammed backsliding of the eyes. However, the large trial-to-trial variability of targeting accuracy and postsaccadic stability indicates that the saccadic gaze system of patients does not regain the high precision that is observed in normals and which appears to require a vestibular head-in-space signal. Moreover, this variability also permeates their gaze performance in the absence of head movements.     

Cooperative formation of the ligand-binding site of the inositol 1,4, 5-trisphosphate receptor by two separable domains

According to Einstein's equivalence principle, inertial accelerations during translational motion are physically indistinguishable from gravitational accelerations experienced during tilting movements. Nevertheless, despite ambiguous sensory representation of motion in primary otolith afferents, primate oculomotor responses are appropriately compensatory for the correct translational component of the head movement. The neural computational strategies used by the brain to discriminate the two and to reliably detect translational motion were investigated in the primate vestibulo-ocular system. The experimental protocols consisted of either lateral translations, roll tilts, or combined translation-tilt paradigms. Results using both steady-state sinusoidal and transient motion profiles in darkness or near target viewing demonstrated that semicircular canal signals are necessary sensory cues for the discrimination between different sources of linear acceleration. When the semicircular canals were inactivated, horizontal eye movements (appropriate for translational motion) could no longer be correlated with head translation. Instead, translational eye movements totally reflected the erroneous primary otolith afferent signals and were correlated with the resultant acceleration, regardless of whether it resulted from translation or tilt. Therefore, at least for frequencies in which the vestibulo-ocular reflex is important for gaze stabilization (>0.1 Hz), the oculomotor system discriminates between head translation and tilt primarily by sensory integration mechanisms rather than frequency segregation of otolith afferent information. Nonlinear neural computational schemes are proposed in which not only linear acceleration information from the otolith receptors but also angular velocity signals from the semicircular canals are simultaneously used by the brain to correctly estimate the source of linear acceleration and to elicit appropriate oculomotor responses.     

Bilateral vestibulopathy revisited

Bilateral vestibular failure (BVF) is an often undetected disorder of the peripheral labyrinths or the eighth nerves. Key symptoms are oscillopsia during locomotion or head movements and unsteadiness, particularly in the dark. Diagnosis is made by a bedside test for defective vestibulo-ocular reflex and the absence of nystagmic reaction to both caloric and rotatory pendular testing. Most frequent etiologies include ototoxicity, cerebellar degeneration, meningitis, neuropathies, sequential vestibular neuritis, autoimmune disorders, tumors, and miscellaneous otological diseases. Idiopathic BVF is found in more than twenty percent of the patients. Recovery is possible but mostly incomplete. Somatosensory and visual input largely substitute the vestibular deficit for spatial orientation, postural balance and ocular motor control.     

Post-ejection thickening as a marker of viable myocardium. An echocardiographic study in patients with chronic coronary artery disease

Useful medical diagnostic information has been reported from low-frequency rotational testing of the horizontal vestibulo-ocular reflex (VOR) of patients with vestibular disorders. Servocontrolled rotating systems have been used as the only practical method to generate stimuli over lower VOR frequency response ranges, the decade from 0.01 to 0.1 Hz. Active head movements have been used for testing the human VOR at higher frequencies, exceeding 0.5 Hz. We examined whether active head movements could be used also to test the VORs of subjects over lower frequency ranges, extending to 0.02 Hz. We used a swept-frequency, active head movement protocol to generate a broad-band stimulus. Eye position was recorded with electro-oculography. Head velocity was recorded with a rotational sensor attached to a head band. Six individual test epochs from human subjects were concatenated to form complex, periodic waveforms of head and eye velocity, 75 seconds in duration. Broad-band cross-spectral signal processing methods were used to compute horizontal VOR system characteristics from these waveforms extending from 0.02 to 2 Hz. The low-frequency VOR data appeared to originate from amplitude modulation of high-frequency active movements, acting as carrier signals. Control experiments and processing of simulated data from a known system excluded the possibility of signal processing artifacts. Results from six healthy subjects showed low-frequency gains and phase values in ranges similar to those from published rotational chair studies of normal subjects. We conclude that it is feasible to test the human VOR over extended low-frequency ranges using active head movements because of amplitude modulation of the head and eye signals.     

Modeling vestibulo-ocular reflexes in everyday activities

Fundamental data for the establishment of a new clinical testing procedure for patients suffering from equilibrium disorders were developed. A mathematical model that predicts vestibulo-ocular reflex responses (eye velocities) to three-dimensional head movements was investigated. An experimental set-up permitting the simultaneous recording of the head and eye movements was developed to investigate the vestibulo-ocular reflex (VOR) during everyday activities. The test results show that computer simulation of the VOR occurring during complex movements is indeed possible. The model was able to predict the trend of the experimentally determined eye velocities. It was ascertained that for the further development of the model, the influence of the cervico-ocular reflex (COR) on the resulting eye velocities also has to be taken into account. The proposed method for investigating patients with equilibrium disorders appears suitable enough to make further development to the clinical stage worthwhile. An adaptation of the feedback amplification parameters of the model to take account of the nature of the stimulus and the influence of COR is needed to improve the agreement between the amplitudes of measured and predicted eye velocities. To reliably quantify the feedback amplification parameters, tests need to be carried out in a large number of subjects.     

Temporal and spatial pattern of expression of the HDL receptor SR-BI during murine embryogenesis

Cerebellar long-term depression (LTD) is a model system for neuronal information storage that has an absolute requirement for activation of protein kinase C (PKC). It has been claimed to underlie several forms of cerebellar motor learning. Previous studies using various knockout mice (mGluR1, GluRdelta2, glial fibrillary acidic protein) have supported this claim; however, this work has suffered from the limitations that the knockout technique lacks anatomical specificity and that functional compensation can occur via similar gene family members. To overcome these limitations, a transgenic mouse (called L7-PKCI) has been produced in which the pseudosubstrate PKC inhibitor, PKC[19-31], was selectively expressed in Purkinje cells under the control of the pcp-2(L7) gene promoter. Cultured Purkinje cells prepared from heterozygous or homozygous L7-PKCI embryos showed a complete blockade of LTD induction. In addition, the compensatory eye movements of L7-PKCI mice were recorded during vestibular and visual stimulation. Whereas the absolute gain, phase, and latency values of the vestibulo-ocular reflex and optokinetic reflex of the L7-PKCI mice were normal, their ability to adapt their vestibulo-ocular reflex gain during visuo-vestibular training was absent. These data strongly support the hypothesis that activation of PKC in the Purkinje cell is necessary for cerebellar LTD induction, and that cerebellar LTD is required for a particular form of motor learning, adaptation of the vestibulo-ocular reflex.     

Phenotype, cytokine production and cytolytic capacity of fresh (uncultured) tumour-infiltrating T lymphocytes in human renal cell carcinoma

Abnormalities in the vestibulo-ocular reflex (VOR) after unilateral vestibular injury may cause symptomatic gaze instability. We compared five subjects who had unilateral vestibular lesions with normal control subjects. Gaze stability and VOR gain were measured in three axes using scleral magnetic search coils, in light and darkness, testing different planes of rotation (yaw and pitch), types of stimulus (sinusoids from 0.8 to 2.4 Hz, and transient accelerations) and methods of rotation (active and passive). Eye velocity during horizontal tests reached saturation during high-velocity/acceleration ipsilesional rotation. Rapid vertical head movements triggered anomalous torsional rotation of the eyes. Gaze instability was present even during active rotation in the light, resulting in oscillopsia. These abnormal VOR responses are a consequence of saturating nonlinearities, which limit the usefulness of frequency-domain analysis of rotational test data in describing these lesions.     

Oculomotor control of primary eye position discriminates between translation and tilt

We have previously shown that fast phase axis orientation and primary eye position in rhesus monkeys are dynamically controlled by otolith signals during head rotations that involve a reorientation of the head relative to gravity. Because of the inherent ambiguity associated with primary otolith afferent coding of linear accelerations during head translation and tilts, a similar organization might also underlie the vestibulo-ocular reflex (VOR) during translation. The ability of the oculomotor system to correctly distinguish translational accelerations from gravity in the dynamic control of primary eye position has been investigated here by comparing the eye movements elicited by sinusoidal lateral and fore-aft oscillations (0.5 Hz +/- 40 cm, equivalent to +/- 0.4 g) with those during yaw rotations (180 degrees/s) about a vertically tilted axis (23.6 degrees). We found a significant modulation of primary eye position as a function of linear acceleration (gravity) during rotation but not during lateral and fore-aft translation. This modulation was enhanced during the initial phase of rotation when there was concomitant semicircular canal input. These findings suggest that control of primary eye position and fast phase axis orientation in the VOR are based on central vestibular mechanisms that discriminate between gravity and translational head acceleration.     

Optokinetic-vestibular interaction in patients with increased gain of the vestibulo-ocular reflex

We studied optokinetic nystagmus (OKN), optokinetic afternystagmus (OKAN) and visual-vestibular interaction in five patients with markedly elevated vestibulo-ocular reflex (VOR) gain due to cerebellar atrophy. All had impaired smooth pursuit, decreased initial slow phase velocity of OKN, and impaired ability to suppress the VOR with real or imagined targets. OKN slow phase velocity gradually built up over 25-45 s, reaching normal values for low stimulus velocities (< or = 30 deg/s). Initial velocity of OKAN was increased, but the rate of decay of OKAN was normal. These findings can be explained by models that include separate velocity storage and variable gain elements shared by the vestibular and optokinetic systems.     

Human gaze stabilization during natural activities: translation, rotation, magnification, and target distance effects

Stability of images on the retina was determined in 14 normal humans in response to rotational and translational perturbations during self-generated pitch and yaw, standing, walking, and running on a treadmill. The effects on image stability of target distance, vision, and spectacle magnification were examined. During locomotion the horizontal and vertical velocity of images on the retina was <4 degrees /s for a visible target located beyond 4 m. Image velocity significantly increased to >4 degrees /s during self-generated motion. For all conditions of standing and locomotion, angular vestibulo-ocular reflex (AVOR) gain was less than unity and varied significantly by activity, by target distance, and among subjects. There was no significant correlation(P > 0.05) between AVOR gain and image stability during standing and walking despite significant variation among subjects. This lack of correlation is likely due to translation of the orbit. The degree of orbital translation and rotation varied significantly with activity and viewing condition in a manner suggesting an active role in gaze stabilization. Orbital translation was consistently antiphase with rotation at predominant frequencies <4 Hz. When orbital translation was neglected in computing gaze, computed image velocities increased. The compensatory effect of orbital translation allows gaze stabilization despite subunity AVOR gain during natural activities. Orbital translation decreased during close target viewing, whereas orbital rotation decreased while wearing telescopic spectacles. As the earth fixed target was moved closer, image velocity on the retina significantly increased (P < 0.05) for all activities except standing. Latency of the AVOR increased slightly with decreasing target distance but remained <10 ms for even the closest target. This latency was similar in darkness or light, indicating that the visual pursuit tracking is probably not important in gaze stabilization. Trials with a distant target were repeated while subjects wore telescopic spectacles that magnified vision by 1.9 or 4 times. Gain of the AVOR was enhanced by magnified vision during all activities, but always to a value less than spectacle magnification. Gain enhancement was greatest during self-generated sinusoidal motion at 0.8 Hz and was less during standing, walking, and running. Image slip velocity on the retina increased with increasing magnification. During natural activities, slip velocity with telescopes increased most during running and least during standing. Latency of the visually enhanced AVOR significantly increased with magnification (P < 0.05), probably reflecting a contribution of the visual pursuit system. The oculomotor estimate of target distance was inferred by measuring binocular convergence, as well as from monocular parallax during head translation. In darkness, target distance estimates obtained by both techniques were less accurate than in light, consistently overestimating for near and underestimating for far targets.     

Eye movements evoked by proprioceptive stimulation along the body axis in humans

Proprioceptive input arising from torsional body movements elicits small reflexive eye movements. The functional relevance of these eye movements is still unknown so far. We evaluated their slow components as a function of stimulus frequency and velocity. The horizontal eye movements of seven adult subjects were recorded using an infrared device, while horizontal rotations were applied at three segmental levels of the body [i.e., between head and shoulders (neck stimulus), shoulders and pelvis (trunk stimulus), and pelvis and feet (leg stimulus)]. The following results were obtained: (1) Sinusoidal leg stimulation evoked an eye response with the slow component in the direction of the movement of the feet, while the response to trunk and neck stimulation was oriented in the opposite direction (i.e., in that of the head). (2) In contrast, the gain behavior of all three responses was similar, with very low gain at mid- to high frequencies (tested up to 0.4 Hz) but increasing gain at low frequencies (down to 0.0125 Hz). We show that this gain behavior is mainly due to a gain nonlinearity for low angular velocities. (3) The responses were compatible with linear summation when an interaction series was tested in which the leg stimulus was combined with a vestibular stimulus. (4) There was good correspondence of the median gain curves when eye responses were compared with psychophysical responses (perceived body rotation in space; additionally recorded in the interaction series). However, correlation of gain values on a single-trial basis was poor. (5) During transient neck stimulation (smoothed position ramp), the neck response noticeably consisted of two components -- an initial head-directed eye shift (phasic component) followed by a shift in the opposite direction (compensatory tonic component). Both leg and neck responses can be described by one simple, dynamic model. In the model the proprioceptive input is fed into the gaze network via two pathways which differ in their dynamics and directional sign. The model simulates either leg or neck responses by selecting an appropriate weight for the gain of one of the pathways (phasic component). The interaction results can also be simulated when a vestibular path is added. This model has similarities to one we recently proposed for human self-motion perception and postural control. A major difference, though, is that the proprioceptive input to the gaze-stabilizing network is weak (restricted to low velocities), unlike that used for perception and postural control. We hold that the former undergoes involution during ontogenesis, as subjects depend on the functionally more appropriate vestibulo-ocular reflex. Yet, the weak proprioceptive eye responses that remain may have some functional relevance. Their tonic component tends to stabilize the eyes by slowly shifting them toward the primary head position relative to the body support. This applies solely to the earth-horizontal plane in which the vestibular signal has no static sensitivity.     

Mechanisms of action and targets of nitric oxide in the oculomotor system

Nitric oxide (NO) production by neurons in the prepositus hypoglossi (PH) nucleus is necessary for the normal performance of eye movements in alert animals. In this study, the mechanism(s) of action of NO in the oculomotor system has been investigated. Spontaneous and vestibularly induced eye movements were recorded in alert cats before and after microinjections in the PH nucleus of drugs affecting the NO-cGMP pathway. The cellular sources and targets of NO were also studied by immunohistochemical detection of neuronal NO synthase (NOS) and NO-sensitive guanylyl cyclase, respectively. Injections of NOS inhibitors produced alterations of eye velocity, but not of eye position, for both spontaneous and vestibularly induced eye movements, suggesting that NO produced by PH neurons is involved in the processing of velocity signals but not in the eye position generation. The effect of neuronal NO is probably exerted on a rich cGMP-producing neuropil dorsal to the nitrergic somas in the PH nucleus. On the other hand, local injections of NO donors or 8-Br-cGMP produced alterations of eye velocity during both spontaneous eye movements and vestibulo-ocular reflex (VOR), as well as changes in eye position generation exclusively during spontaneous eye movements. The target of this additional effect of exogenous NO is probably a well defined group of NO-sensitive cGMP-producing neurons located between the PH and the medial vestibular nuclei. These cells could be involved in the generation of eye position signals during spontaneous eye movements but not during the VOR.     

Changes in dorsal neck muscle activity related to imposed eye movement in the decerebrate pigeon

Movements of the head and eyes are known to be intimately related. Eye position has also been shown to be closely related to the electromyographic activity of dorsal neck muscles; however, extraocular muscle proprioception has not generally been considered to play a part in the control of such movements. We have previously shown that, in the pigeon, imposed movements of one eye modify the vestibular responses of several dorsal neck muscles in ways that are dependent on stimulus parameters such as the amplitude and velocity of imposed eye movement. The present study examines more closely the interactions between imposed eye movements and different muscle pairs. The three neck muscle pairs studied each responded to afferent signals from the extraocular muscles in discrete and specific ways which appeared to be correlated with their different actions. Complementary effects of imposed eye movements in the horizontal plane were seen for both the complexus and splenius muscle pairs, with imposed eye movements in one direction producing the largest inhibition of the ipsilateral muscle's vestibular response and imposed eye movements in the opposite direction the largest inhibition of the contralateral muscle's vestibular response. During roll tilt oscillation (ear-up/ear-down) in the frontal plane, similar complementary effects of imposed eye movement were seen in the complexus muscle pair, but the splenius muscle pair showed little tuning, with similar inhibition for imposed eye movement directed either upwards or downwards. In contrast to these complementary effects, the biventer cervicis muscle pair showed no vestibular modulation during vestibular stimulation in the horizontal plane and their spontaneous activity was not altered by imposed eye movement. During roll-tilt oscillation (ear-up/ear-down) in the frontal plane imposed eye movement directed vertically upwards increased both muscles' vestibular responses and imposed eye movement directed vertically downwards inhibited both muscles' vestibular responses. Section of the ophthalmic branch of the trigeminal nerve (deafferenting the eye muscles) abolished the effects of imposed eye movement on the neck muscle pairs. In conjunction with further control experiments these results provide compelling evidence that proprioceptive signals from the extraocular muscles reach the neck muscles and provide them with a functionally significant signal. We have previously shown that signals from the extraocular muscles appear to be involved in the control of the vestibulo-ocular reflex. It follows from the experiments reported here that proprioceptive signals from the extraocular muscles are also likely to be involved in the control of gaze.     

Directional organization of eye movement and visual signals in the floccular lobe of the monkey cerebellum

The floccular lobe of the monkey is critical for the generation of visually-guided smooth eye movements. The present experiments reveal physiological correlates of the directional organization in the primate floccular lobe by examining the selectivity for direction of eye motion and visual stimulation in the firing of individual Purkinje cells (PCs) and mossy fibers. During tracking of sinusoidal target motion along different axes in the frontoparallel plane, PCs fell into two classes based on the axis that caused the largest modulation of simple-spike firing rate. For "horizontal" PCs, the response was maximal during horizontal eye movements, with increases in firing rate during pursuit toward the side of recording (ipsiversive). For "vertical" PCs, the response was maximal during eye movement along an axis just off pure vertical, with increases in firing rate during pursuit directed downward and slightly contraversive. During pursuit of target motion at constant velocity, PCs again fell into horizontal and vertical classes that matched the results from sinusoidal tracking. In addition, the directional tuning of the sustained "eye velocity" and transient "visual" components of the neural responses obtained during constant velocity tracking were very similar. PCs displayed very broad tuning approximating a cosine tuning curve; the mean half-maximum bandwidth of their tuning curves was 170-180 degrees. Other cerebellar elements, related purely to eye movement and presumed to be mossy fibers, exhibited tuning approximately 40 degrees narrower than PCs and had best directions that clustered around the four cardinal directions. Our data indicate that the motion signals encoded by PCs in the monkey floccular lobe are segregated into channels that are consistent with a coordinate system defined by the vestibular apparatus and eye muscles. The differences between the tuning properties exhibited by PCs compared with mossy fibers indicate that a spatial transformation occurs within the floccular lobe.     

Afferent signals from the extraocular muscles of the pigeon modify the vestibulo-ocular reflex

Although the extraocular muscles (EOM) contain stretch receptors it is generally thought that the afferent signals which they provide play no role in the control of eye movement. We have previously shown that these afferent signals do modify both the vestibular responses of single units in the oculomotor control system and the electromyographic responses of the EOM during the vestibulo-ocular reflex (VOR). We have now investigated the effect of EOM afferent signals on the VOR itself, by recording the electro-oculogram of one eye while imposing movements on the other eye during the VOR. Moving the eye in a manner which mimics the slow phase of the VOR, we have found that, as the peak velocity of the imposed eye movement increases, the amplitude of eye movement of the other eye decreases. These results confirm that the output of the VOR itself, expressed as movement of the globe, and not merely some of its component parts, is modified by EOM afferent signals.     

Eye- and head movements in freely moving rabbits

1. Eye- and head movements were recorded in unrestrained, spontaneously behaving rabbits with a new technique, based upon phase detection of signals induced in implanted coils by a rotating magnetic field. 2. Movements of the eye in space were exclusively saccadic. In the intersaccadic intervals the eyes were stabilized in space, even during vigorous head movements. Most of this stability was maintained in darkness, except for the occurrence of slow drift. 3. Many saccades were initiated while the head was stationary. They were accompanied by a similar, but slower head rotation with approximately the same amplitude. The displacement of the eye in space was a pure step without appreciable under- or over-shoot. The deviation of the eye in the head was mostly transient. 4. Other saccades were started while the head was moving and were possibly fast phases of a vestibulo-ocular reflex. The time course of the eye movement in space was identical for all saccades, whether the head was moving prior to the saccade or not. Eye movements without any head movement were not observed. 5. Saccades were mostly large (average 20-6 +/- 12-4 degrees S.D.) and never smaller than 1 degree. The relations of maximal velocity and duration to amplitude were similar to those reported for man. 6. Visual pursuit of moving objects, when elicited, was only saccadic and never smooth. 7. It is concluded that the co-ordination and dynamics of the rabbit's head- and eye movements are similar to those of primates. In the absence of foveal specilization, the eye movements are restricted to a rather global redirection of the visual field, possibly in particular of the binocular area.     

Ocular responses to motion parallax stimuli: the role of perceptual and attentional factors

When human subjects are presented with visual displays consisting of random dots moving sideways at different velocities, they perceive transparent surfaces, moving in the same direction but located at different distances from themselves. They perceive depth from motion parallax, without any additional cues to depth, such as relative size, occlusion or binocular disparity. Simultaneously, large-field visual motion triggers compensatory eye movements which tend to offset such motion, in order to stabilize the visual image of the environment. In a series of experiments, we investigated how such reflexive eye movements are controlled by motion parallax displays, that is, in a situation where a complete stabilization of the visual image is never possible. Results show that optokinetic nystagmus, and not merely active visual pursuit of singular elements, is triggered by such displays. Prior to the detection of depth from motion parallax, eye tracking velocity is equal to the average velocity of the visual image. After detection, eye tracking velocity spontaneously matches the slowest velocity in the visual field, but can be controlled by attentional factors. Finally, for a visual stimulation containing more than three velocities, subjects are no longer able to perceptually dissociate between different surfaces in depth, and eye tracking velocity remains equal to the average velocity of the visual image. These data suggest that, in the presence of flow fields containing motion parallax, optokinetic eye movements are modulated by perceptual and attentional factors.