Head nodding is compensatory in spasmus nutans.

BACKGROUND: Spasmus nutans is defined as asymmetric nystagmus with associated head nodding in childhood. It is not clear whether head nodding is a compensatory mechanism to control the nystagmus or an involuntary movement of pathologic origin. METHODS: The authors analyzed the relation between head and eye movements by simultaneous eye and head movement recordings of 35 patients with spasmus nutans. RESULTS: In 21 of these patients, the fine, fast, dissociated nystagmus changed during head nodding to larger and slower symmetric eye movements with both eyes oscillating at the same amplitude in phase and 180 degrees out of phase to the head movements, corresponding to a normal compensatory vestibulo-ocular reflex. CONCLUSION: These findings indicate that head nodding is compensatory in spasmus nutans.     

The influence of visual input on the vestibulo-ocular reflex

In order to stabilise a fixation target on the retina, eye movements have to compensate for head movements. During slow head movements visual feedback can control these eye movements. During fast movements of the head, mainly the vestibulo-ocular reflex (VOR) controls eye movements, as visual feedback is too slow. However, visual feedback is an important factor in controlling the VOR; e.g. the gain of the VOR depends on the distance of the target. This study investigates the influence of retinal image position during fast head movements. The experiments were carried out in five human subjects using scleral search coils. The adaptation of each eye individually to a change of retinal position of a target was examined during head shaking. The change in visual input was carried out by placing Fresnel prisms of different strengths in front of both eyes, thus inducing a change in retinal image position without changing the retinal slip. The results show, that both eyes make the appropriate corrections when the visual input changes, even during fast head-movements. These corrections did not influence the gain of the VOR. From these results we conclude, that retinal image position besides retinal slip has a major influence on the monocular eye movements even at high head rotation frequencies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Quality of retinal image stabilization during small natural and artificial body rotations in man

Two-dimensional head rotations recorded from 2 subjects sitting still, without artificial head support, showed appreciable movement over the frequency range d.c. to 7 Hz. Capacity of vestibuloocular reflex and visually guided eye movements to null motion over this dynamic range was examined by simultaneously recording 2-dimensional head and eye rotations while sinusoidally rotating subjects over the frequency range 0.1 to 15 Hz using small amplitudes. At best, oculomotor compensation removed about 90% of head motion from eye motion in space. Representative compensation was poorer. Compensation for natural motions of unsupported heads while sitting and standing was also incomplete resulting in substantially more eye motion in space than was observed with head supported.These observations, coupled with recent demonstrations of plasticity of the vestibulo-ocular reflex, led us to suggest that the degree of compensatory oculomotor response is actively adjusted downwards so as to guarantee sufficient retinal image motion to prevent perceptual fading when the body is relatively stationary and is actively adjusted upwards, so as to guarantee sufficient retinal stability to prevent perceptual blurring when the body moves actively. Seen this way. the goal of oculomotor compensation is not retinal image stabilization, but rather controlled retinal image motion adjusted so as to be optimal for visual processing over the full range of natural motions of the body.     

Visuo-vestibular eye movements: infantile strabismus in 3 dimensions

Infantile strabismus is accompanied by latent nystagmus, primary inferior oblique muscle overaction, and dissociated vertical divergence. If we examine the evolutionary underpinnings of these ocular rotations, we can construct a unifying mechanism for the sensorimotor abnormalities that arise in humans with infantile strabismus. Latent nystagmus, primary inferior oblique muscle overaction, and dissociated vertical divergence correspond to visual balancing reflexes that are operative in lateral-eyed animals in yaw, pitch, and roll, respectively. In humans with infantile strabismus, these subcortical visual reflexes are reactivated by a physiologic imbalance in binocular visual input, which resets central vestibular tone in 3-dimensional space. These visual reflexes reveal the evolutionary role of the eyes as sensory balance organs that can directly modulate central vestibular tone. Latent nystagmus, primary oblique muscle overaction, and dissociated vertical divergence should be reclassified as visuo-vestibular eye movements.     

Study of dynamic counterrolling of the eye--quantitative analysis of effects by visual and vestibular systems

Dynamic counterrolling of the eye is induced when the head is tilted to one side: it includes cyclorotatory nystagmus, which is affected by visual and vestibular systems. Both of these two ocular movements were measured quantitatively through a video recording system, and the role of visual and vestibular systems in visual stabilization was analyzed. The relationship between the optokinetic and vestibulo-ocular systems was variable in relation to the velocity of head tilting. The optokinetic system contributed more in slow head tilting. As the velocity of head tilting increased, the suppression by the optokinetic system was decreased and eventually the vestibulo-ocular system became more dominant. This suggests that the role of visual and vestibular systems in head tilting is similar to that in head rotation. Visual and vestibular systems seem to work together mutually with some individual variation in stabilizing vision with sensory control in the higher level of the visual cortex.     

Adaptive control of pursuit, vergence and eye torsion in humans: basic and clinical implications.

Recent research from our laboratory has been directed at understanding the range of capabilities for adaptive control of eye movements in normal human subjects. For smooth pursuit, different motor responses to the same sensory stimulus (horizontal target motion) can be learned, stored and gated in or out, according to context (vertical eye position). The dynamic properties of the 'open-loop' portion of horizontal, disparity-driven vergence eye movements are under adaptive control. Eye torsion is also subject to adaptive control, including torsional 'phoria adaptation' and cross-coupling of torsion into the horizontal vestibulo-ocular reflex (VOR). Finally, lesions of the oculomotor vermis in monkeys produce disordered binocular ocular motor function: 'esodeviations' in the absence of disparity cues, and decreased adaptation of the horizontal phoria to a sustained disparity induced by wearing a horizontal prism in front of one eye.     

Accuracy of eye position information for motor control

Two dimensional eye movements were recorded to show that subjects could return their eyes to a reference position following both saccades and slow phase of vestibular nystagmus suggesting eye position was signalled accurately during both movement types and this signal was closely timelocked to saccades. In addition, this signal could be used by naive subjects for oculomotor control without special training. Finally, subjects could strike blows within a few minutes of arc of a target localized only by eye position information suggesting both that motor systems detect eye position to better than 0.5° arc and that large errors in controlling eye position in the dark could be attributed to poor spatial memory.     

Amplitude changes in response to target displacements during human eye-head movements

Sensorimotor adaptation, the ability to adjust motor output in response to persistent changes in sensory input, is a key function of the central nervous system. Although a great deal is known about vestibulo-ocular reflex and saccadic adaptation, relatively little is known about the behavior and neural mechanisms underlying gaze adaptation when the head is free to move. In an attempt to understand the mechanisms of gaze adaptation, and constrain hypotheses concerning the locus at which changes in gaze control may be implemented, we altered the size of large, head-unrestrained gaze shifts made to visual targets by surrepetitiously moving the visual target forward (30 degrees -->60 degrees ) or backwards (60 degrees -->30 degrees ) during gaze shifts. In our 10 human subjects, after a few hundred back-step trials, gaze amplitudes were reduced by between 6 degrees and 27 degrees. Similarly, after a few hundred forward adaptation trials, our subjects increased gaze amplitude by between 0 degrees and 26 degrees. Changes in the amplitude of primary gaze shifts occurred regardless of the particular combinations of eye and head movements that made up the amplitude-altered gaze shifts. When gaze shifts were initiated with the eyes in systematically different positions relative to the head, the resulting changes in gaze, eye and head movement amplitudes were consistent with the hypothesis that gaze adaptation occurs at the level of a gaze shift command and not by altering separately the signals that produce eye and head movements.     

Congenital oculomotor apraxia. Presentation--developmental problems--differential diagnosis]

Congenital oculomotor apraxia (COMA) was initially defined by Cogan in 1952. In this condition voluntary horizontal saccades cannot be generated, while slow horizontal pursuit movements and vertical eye movements are intact. Affected infants usually present with delayed visual and/or psychomotor development or may even appear to be blind. In the second half of the first year "compensatory" head thrust movements become apparent. While the oculomotor abnormalities tend to improve with increasing age most affected children have delayed motor and speech development. The cognitive development is commonly impaired and many children require a special scholastic education. In our personal series of 9 children we have found variable and nonspecific neuroradiological findings, including cerebellar hypoplasia, hypoplasia of corpus callosum and grey matter heterotopias. COMA has to be differentiated from acquired forms of ocular apraxia as seen in Morbus Gaucher type 3, ataxia teleangiectasia and Morbus Leigh.     

Three-dimensional organization of vestibular related eye movements to rotational motion in pigeons

During rotational motions, compensatory eye movement adjustments must continually occur in order to maintain objects of visual interest as stable images on the retina. In the present study, the three-dimensional organization of the vestibulo-ocular reflex in pigeons was quantitatively examined. Rotations about different head axes produced horizontal, vertical, and torsional eye movements, whose component magnitude was dependent upon the cosine of the stimulus axis relative to the animal's visual axis. Thus, the three-dimensional organization of the VOR in pigeons appears to be compensatory for any direction of head rotation. Frequency responses of the horizontal, vertical, and torsional slow phase components exhibited high pass filter properties with dominant time constants of approximately 3 s.     

In what ways do eye movements contribute to everyday activities?

Two recent studies have investigated the relations of eye and hand movements in extended food preparation tasks, and here the results are compared. The tasks could be divided into a series of actions performed on objects. The eyes usually reached the next object in the sequence before any sign of manipulative action, indicating that eye movements are planned into the motor pattern and lead each action. The eyes usually fixated the same object throughout the action upon it, although they often moved on to the next object in the sequence before completion of the preceding action. The specific roles of individual fixations could be identified as locating (establishing the locations of objects for future use), directing (establishing target direction prior to contact), guiding (supervising the relative movements of two or three objects) and checking (establishing whether some particular condition is met, prior to the termination of an action). It is argued that, at the beginning of each action, the oculomotor system is supplied with the identity of the required object, information about its location, and instructions about the nature of the monitoring required during the action. The eye movements during this kind of task are nearly all to task-relevant objects, and thus their control is seen as primarily 'top-down', and influenced very little by the 'intrinsic salience' of objects.     

Eye movements and the control of actions in everyday life

The patterns of eye movement that accompany static activities such as reading have been studied since the early 1900s, but it is only since head-mounted eye trackers became available in the 1980s that it has been possible to study active tasks such as walking, driving, playing ball games and ordinary everyday activities like food preparation. This review examines the ways that vision contributes to the organization of such activities, and in particular how eye movements are used to locate the information needed by the motor system in the execution of each act. Major conclusions are that the eyes are proactive, typically seeking out the information required in the second before each act commences, although occasional 'look ahead' fixations are made to establish the locations of objects for use further into the future. Gaze often moves on before the last act is complete, indicating the presence of an information buffer. Each task has a characteristic but flexible pattern of eye movements that accompanies it, and this pattern is similar between individuals. The eyes rarely visit objects that are irrelevant to the action, and the conspicuity of objects (in terms of low-level image statistics) is much less important than their role in the task. Gaze control may involve movements of eyes, head and trunk, and these are coordinated in a way that allows for both flexibility of movement and stability of gaze. During the learning of a new activity, the eyes first provide feedback on the motor performance, but as this is perfected they provide feed-forward direction, seeking out the next object to be acted upon.     

Rebound nystagmus: EOG analysis of a case with a floccular tumour.

Eye movements were recorded and quantitatively analysed in a patient with a tumour initially involving the cerebellar flocculus. Ocular motor abnormalities included (1) impaired smooth pursuit, (2) impaired cancellation of the vestibulo-ocular reflex when fixating an object rotating with the head, and (3) gaze paretic and rebound nystagmus. Comparable findings have been reported in monkeys with experimental floccular lesions. The rebound nystagmus (but not the other ocular motor abnormalities) disappeared when the tumour appeared to invade the brain stem in the region near the vestibular nuclei. This finding suggests that the floccular lesion unmasked a bias which created rebound nystagmus and that the bias probably arose in the vestibular nuclei.     

Ocular torsion: rotations around the "WHY" axis.

BACKGROUND: Traditional teaching holds that there is a partial compensatory countertorsion after head tilt because the intorters in the eye on the side of the head tilt and the extorters in the contralateral eye are stimulated. This teaching is inconsistent with a number of clinical observations. METHODS: Review of existing literature, reanalysis of data from the investigator's previous experiments, and inductive and deductive reasoning were used to reconcile inconsistencies and present a theory on why torsional movements occur. RESULTS: The inconsistencies can be reconciled if one considers that during the dynamic phase of head tilt, there is an alternating series of intorsional and extorsional movements of both eyes. Each eye has slow dynamic compensatory counterrolling phases that serve as torsional "doll's-head" movements to stabilize the image during head tilt. This counterrolling is partially eliminated by a series of anticompensatory torsional saccades in the direction of head tilt, which is in contrast to traditional teaching. CONCLUSION: Dynamic compensatory counterrolling occurs during head tilt. It is largely eliminated by anticompensatory torsional saccades in the opposite direction so that by the end of head tilt only minimal static countertorsion remains. The dynamic compensatory counterrolling motion is necessary to minimize peripheral visual movement during head tilt. The elimination of most of the counterrolling by the end of head tilt is necessary to preserve convergence and stereopsis.     

Effects of age, viewing distance and target complexity on static ocular counterroll

The ocular counterroll (OCR) reflex generates partially compensatory torsional eye movements during static head roll tilt. We assessed the influence of age, viewing distance and target complexity on the OCR across the age span (13-63 years; n = 47), by recording eye movements during head-on-body roll tilt (0 ± 40° in 5° steps) while subjects viewed simple vs. complex targets at 0.33 and 1 m. We found that subjects ?31 years had lower gains than those ?30 years, but only for far targets. Consistent with prior reports, far targets elicited higher OCR gains than near targets, and target complexity had no effect on gains, suggesting that visual input is primarily used to maintain vergence during OCR.     

The primary position of the eyes, the resetting saccade, and the transverse visual head plane. Head movements around the cervical joints.

Photographic and video analyses show that the primary position of the eyes is a natural constant position in alert normal humans, and the eyes are automatically saccadically reset to this position from any displacement of the visual line. The primary position is not dependent on fixation, the fusion reflex, gravity, or the head position. The primary position is defined anatomically by head and eye planes and lines that are localized by photography, magnetic resonance imaging, and x-rays of the head and neck. The eyes are in the primary position when the principal (horizontal) retinal plane is coplanar with the transverse visual head (brain) plane (TVHP), and the equatorial plane of the eye is coplanar with a fixed orbital plane (Listing's plane). Evidence is presented to indicate an active neurologic basis for the primary position instead of passive mechanical forces. A different understanding of the primary position and the conception of the TVHP may be valuable in analyzing oculomotor defects.     

Fundamentals of the anatomy of oculomotor behavior

The oculomotricity rests on complex anatomo-physiological bases and allows the binocular vision, which is a sensory function very completed in the human. After a short embryologic recall, the binocular vision is defined. The anatomical structures, which take part in the binocularity, are reviewed within three functional stages: an effector peripheral stage (eye ball, extrinsic muscles and their nerves), a sub cortical central stage, generator of the movements (oculomotor nuclei, reticularis formation...), and a central, cortical and subcortical stage, incitator of the movements (cortical areas, vestibular nuclei, cerebellum...). These anatomical bases make it possible to understand the many disorders observed in pathology.     

Asymmetry of perceived motion smear during head and eye movements: evidence for a dichotomous neural categorization of retinal image motion

We measured perceived motion smear when retinal image motion was created either by a physically moving object or by movement of the eyes or head. Consistent with previous reports, the extent of perceived motion smear during an eye or head movement is less than that produced by physical object motion when the eyes are stationary. Moreover, perceived smear is substantially smaller when the motion of the retinal image is in the same direction as the eye or head movement compared to when image motion is in the opposite direction. These results imply that extra-retinal signals associated with eye and head movements contribute to a reduction of perceived motion smear, thereby fostering perceptual clarity. We hypothesize that the visual system uses a simple dichotomous strategy in applying these extra-retinal signals, based only on the direction of retinal image motion with respect to the ongoing eye or head movement.     

Eye and head movements in the pigmented rat

Vestibular, optokinetic, and spontaneous eye and head movements have been examined in the hooded rat. Eye movement range was 18-20 degrees, and frequency of ocular saccades was 5-20/min; there was a weak linkage of eye and head movements and a weak vestibulocollic reflex. Response to optokinetic stimulation with unity gain (eye velocity matches drum velocity) was seen only at velocities below 1 deg/sec; maximal eye velocity evoked by drum velocities of over 20 deg/sec never exceeded 4-6 deg/sec. These motor responses were not altered by head movements: thus, gaze velocity is not improved by optocollic (head movement) responses, and such optocollic activity occurs only when substantial retinal image motion is present.     

Temporal dynamics of ocular position dependence of the initial human vestibulo-ocular reflex

PURPOSE: While an ideal vestibulo-ocular reflex (VOR) generates ocular rotations compensatory for head motion, during visually guided movements, Listing's Law (LL) constrains the eye to rotational axes lying in Listing's Plane (LP). The present study was conducted to explore the recent proposal that the VOR's rotational axis is not collinear with the head's, but rather follows a time-dependent strategy intermediate between LL and an ideal VOR. METHODS: Binocular LPs were defined during visual fixation in eight normal humans. The VOR was evoked by a highly repeatable transient whole-body yaw rotation in darkness at a peak acceleration of 2800 deg/s2. Immediately before rotation, subjects regarded targets 15 or 500 cm distant located at eye level, 20 degrees up, or 20 degrees down. Eye and head responses were compared with LL predictions in the position and velocity domains. RESULTS: LP orientation varied both among subjects and between individual subject's eyes, and rotated temporally with convergence by 5 +/- 5 degrees (+/-SEM). In the position domain, the eye compensated for head displacement even when the head rotated out of LP. Even within the first 20 ms from onset of head rotation, the ocular velocity axis tilted relative to the head axis by 30% +/- 8% of vertical gaze position. Saccades increased this tilt. Regardless of vertical gaze position, the ocular rotation axis tilted backward 4 degrees farther in abduction than in adduction. There was also a binocular vertical eye velocity transient and lateral tilt of the ocular axis. CONCLUSIONS: These disconjugate, short-latency axis perturbations appear intrinsic to the VOR and may have neural or mechanical origins.