Passive Head Movements Research Articles

Intrinsic membrane properties and dynamics of medial vestibular neurons: a simulation.

The vestibulo-ocular and vestibulo-spinal network provides the ability to hold gaze fixed on an object during passive head movement. Within that network, most of the second-order neurons of the medial vestibular nucleus (MVNn) compute internal representations of head movement velocity in the horizontal plane. Our previous in vitro studies of the MVNn membrane properties indicated that they may play a major role in determining the dynamic properties of these neurons independently of their connectivity. The present study investigated that hypothesis at a theoretical level. Biophysical models of type A and B MVNn were developed. Two factors were found to be important in modeling tonic and phasic firing activity: the activation of the delayed potassium current and the rate of calcium flux. In addition, the model showed that the strength of the delayed potassium current may determine the different forms of action potentials observed experimentally. These two models (type A and B cells) were examined using depolarizing stimulation, random noise, step, ramp and sinusoidal inputs. For random noise, type A cells showed stable (regular) firing frequencies, while type B cells exhibited irregular activity. With step stimulation, the models exhibited tonic and phasic firing responses, respectively. Using ramp stimulations, frequency versus current curves showed a linear response for the type B neuron model. Finally, with sinusoidal stimulation of increasing frequencies, the type A model demonstrated a decrease in sensitivity, while the type B model exhibited an increase in sensitivity. These theoretical results support the hypothesis that MVNn intrinsic membrane properties specify various types of dynamic properties amongst these cells and therefore contribute to the wide range of dynamic responses which characterize the vestibulo-ocular and vestibulo-spinal network.

Head-Only Rotational Testing in the Elderly

Head-only rotational testing (HORT) is a noninvasive technique to assess the vestibuloocular reflex (VOR) that can be performed in a nonlaboratory environment with inexpensive, portable equipment. Little data are available regarding the applicability of this technique to the elderly in whom limitations of cervical range of motion might reduce the usefulness of the technique. This study was designed to apply HORT to a small group of normal elderly subjects to determine whether the technique could be used successfully in this population. Also, the VOR parameters derived from HORT were compared with the results obtained from a group of healthy young control subjects. HORT was performed both with and without visual fixation of an earth-fixed target, using both active head movement and passive head movement. Results indicated that the responses of the elderly were indistinguishable from those of the young except for a slightly higher gain at 1 Hz in the young subjects. Other findings, consistent across young and elderly, were that visual fixation increased gain and decreased phase lead at lower frequencies and that VOR parameters were unaffected by volition, that is, active and passive head rotation produced similar responses. These findings suggest that HORT can be applied to the elderly without difficulty. Future studies will assess the usefulness of HORT in elderly patients with VOR disorders.

Rostral Fastigial Nucleus Activity in the Alert Monkey During Three-Dimensional Passive Head Movements

The fastigial nucleus (FN) receives vestibular information predominantly from Purkinje cells of the vermis. FN in the monkey can be divided in a rostral part, related to spinal mechanisms, and a caudal part with oculomotor functions. To understand the role of FN during movements in space, single-unit activity in alert monkeys was recorded during passive three-dimensional head movements from rostral FN. Seated monkeys were rotated sinusoidally around a horizontal earth-fixed axis (vertical stimulation) at different orientations 15 degrees apart (including roll, pitch, vertical canal plane and intermediate planes). In addition, sinusoidal rotations around an earth-vertical axis (yaw stimulus) included different roll and pitch positions (+/-10 degrees, +/-20 degrees). The latter positions were also used for static stimulation. One hundred fifty-eight neurons in two monkeys were modulated during the sinusoidal vertical search stimulation. The vast majority showed a uniform response pattern: a maximum at a specific head orientation (response vector orientation) and a null response 90 degrees apart. Detailed analysis was obtained from 111 neurons. On the basis of their phase relation during dynamic stimulation and their response to static tilt, these neurons were classified as vertical semicircular canal related (n = 79, 71.2%) or otolith related (n = 25; 22.5%). Only seven neurons did not follow the usual response pattern and were classified as complex neurons. For the vertical canal-related neurons (n = 79) all eight major response vector orientations (ipsilateral or contralateral anterior canal, posterior canal, roll, and nose-down and nose-up pitch) were found in Fn on one side. Neurons with ipsilateral orientations were more numerous and on average more sensitive than those with contralateral orientations. Twenty-eight percent of the vertical canal-related neurons also responded to horizontal canal stimulation. None of the vertical canal-related neurons responded to static tilt. Otolith-related neurons (n = 25) had a phase relation close to head position and were considerably less numerous than canal-related neurons. Except for pitch, all other response vector orientations were found. Seventy percent of these neurons responding during dynamic stimulation also responded during static tilt. The sensitivity during dynamic stimulation was always higher than during static stimulation. Sixty-one percent of the otolith-related neurons responded also to horizontal canal stimulation. These results show that in FN, robust vestibular signals are abundant. Canal-related responses are much more common than otolith-related responses. Although for many canal neurons the responses can be related to single canal planes, convergence between vertical canals but also with horizontal canals is common.

Typical and atypical fractures of the odontoid process in young children

The most common injury to the odontoid process in children under the age of seven years is a fracture through the synchondrosis with or without anterior displacement of the odontoid process, but this is not the only type of fracture of the odontoid process in this age-group. Fractures above and below the synchondrosis and fractures with posterior displacement were described. Typical clinical features of these fractures are: (1) major and blunt trauma, (2) neck pain and resistance to active and passive head movements; and (3) no or only slight neurological deficits. Conservative treatment had excellent results in the majority of cases. Nevertheless, there are a few specific indications for surgery.

Active and passive head and body movements

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Visual-Vestibular Interaction in Humans During Active and Passive, Vertical Head Movement1

We studied visual-vestibular interaction (VVI) in 9 normal human subjects using active and passive vertical head rotations. Gain and phase of the vertical vestibulo-ocular reflex (VOR) and visually enhanced vestibulo-ocular reflex (VVOR) were measured for single frequency sinusoidal motion, as well as for sinusoidal motion of continuously increasing frequency, over the range of 0.4 to 4.0 Hz. In addition to measurement of VVOR during normal vision, telescopic spectacles having a magnification of 1.9× were used to challenge VVI to facilitate measurement of visual enhancement of VOR gain. In the mid-frequency range (1.6 to 2.4 Hz), the active VOR exhibited gain closer to compensatory than did the passive VOR; at other frequencies, active and passive VOR gains were similar. VVOR gain during normal vision was compensatory for both active and passive motion throughout the frequency range tested. VVOR gain with 1.9× telescopic spectacles was greater than VOR gain at all frequencies tested, including up to 3.2 Hz for passive bead movements, and up to 4.0 Hz for active head movement. However, gain enhancement with telescopic spectacles was consistently greater during active than during passive head movement. Phase errors for the VOR and VVOR were small under all testing conditions. Although active VOR and VVOR were directionally symmetrical, gain of upward slow phases differed from that of downward slow phases for passive VOR and VVOR in a manner depending on rotational frequency. For both active and passive testing, gain and phase values obtained during swept frequency rotations were similar to those obtained during single frequency sinusoidal testing. These data indicate that VVI can enhance gain of the passive vertical VOR even at frequencies above what is usually considered to be the upper limit of visual pursuit tracking. The additional enhancement observed during active bead movements at these high frequences is attributable to use of efference copy of the skeletal motor command to neck musculature.

An Evaluation of the Head‐Shaking Nystagmus Test

Head-shaking nystagmus can be observed in some patients with vestibular lesions. This nystagmus is transient and appears after 15 to 20 cycles of rapid head-shaking. Horizontal head-shaking may be associated with a low-amplitude reversal phase, which follows the primary response. Several previous studies using active head movement have indicated that head-shaking nystagmus is a sensitive indicator of vestibular loss. In this study, the head-shaking nystagmus test using passive head movement was evaluated in 115 patients and 17 control subjects, using three examiners. The data indicate that using passive head movement, the head-shaking nystagmus test is neither sensitive nor specific enough for use as a screening test for vestibular loss.

Deafferentation of the vestibular organ: Effects on atropine-resistant EEG in rats

Cortical and hippocampal EEG patterns that persist after atropine sulfate administration are defined as atropine-resistant. In the rat, such patterns are movement-related, cortical low-voltage fast activity (LVFA) and movement-related, hippocampal rhythmic slow activity (RSA or “theta”). A close relationship exists between head movement and activation of atropine-resistant EEG. In the present study, we asked whether manipulation of vestibular sensory input would affect atropine-resistant EEG. To allow experimenter control, 6 vestibular-intact rats were made cataleptic with haloperidol and were then injected with atropine sulfate. Movement of the rats* heads by the experimenter (passive head movement) failed to activate atropine-resistant EEG. These animals were then rotated on a turntable. Such vestibular stimulation, the effectiveness of which was reflected by postrotatory ocular nystagmus, &lso failed to activate atropine-resistant EEG. These and 5 other rats were then subjected to surgical or chemical deafferentation of the labyrinth. They were tested for EEG atropine resistance before and after this surgery. Although atropine-resistant EEG patterns in response to active bodily movements still appeared after vestibular deafferentation, the frequency of hippocampal RSA during active movement was reduced with or without atropine. In contrast, &n atropine-sensitive component in the hippocampal EEG during immobility was not changed by vestibular deafferentation. We conclude that vestibular reafferent feedback is neither necessary nor sufficient for the atropine-resistant EEG, but that such feedback from active movement has a modulatory effect on hippocampal RSA.

受動的頸部捻転による眼運動の観察

Passive head movement was tested in 15 volunteers and four patients with uilateral vestibular dysfunction and compared with the results of pendular rotation test. The head of test subjects, sitting on a rotating chair with eyes closed, was fixed with a helmet which sinusoidally rotated with 30 peak to peak amplitude at 0.1,0.3, and 0.5 Hz.The gain of slow phase velocity elicited by both stimuli was analyzed by a mini-computer (PDP11/34).In normal subjects, the gain of passive head movement was slightly higher than that of pendular rotation at every test frequency. However, in the patients the gain of pendular rotation was higher than that of passive head movement rotation at every test frequency.

Goal-directed vestibulo-ocular function in man: gaze stabilization by slow-phase and saccadic eye movements.

Vestibular function was examined during passive head movements having profiles that approximated low-to-intermediate range of natural self-generated movements (10-220 degrees/s peak velocity, about 0.5 s duration). A seated subject looked at a point target on the wall, the lights were extinguished and the chair was briefly turned while the subject tried to "look" at the just-viewed point. The chair was stopped, the lights were turned on again and the target was re-fixated, if necessary. Ocular stabilization was characterized (1) by "net stabilization" that was due to the combined effects of both slow-phase and rapid (saccadic or quick-phase) eye movements, (2) by "cumulative-slow-phase stabilization" that was due to slow-phase eye movements, and (3) by "cumulative-saccadic stabilization" that was due to effects of all rapid eye movements. It was found that both slow-phase and saccadic eye movements tended to keep the eyes on the actual unseen target. During repeatedly applied head movements, net and cumulative-slow-phase stabilization tended to be almost perfect. However, the average magnitude of the error in net stabilization (i.e., deviation from perfection) was always less than the corresponding error in slow-phase stabilization. This occurred because in a given turn, saccadic movements tended to supplement deficient slow-phase movements and to decrement excessive slow-phases. In 4 of 5 subjects, cumulative-saccadic stabilization tended to equal the error in cumulative-slow-phase stabilization. All results were unaffected by head velocities up to +/- 220 degrees/s. It was concluded that these saccades tended to stabilize gaze (eye + head) in space during head movements in total darkness.

Head movement co-ordination in the African chameleon

Chameleon head movement was studied to learn how information from more than one sensory system can be co-ordinated to produce a single motor behavior. In this study, the co-ordination of visual, vestibular and kinesthetic senses was examined by: (1) moving a cricket back and forth in front of a hungry chamelon, (2) moving the body of an alert chameleon or the body of an anesthetized chameleon, and (3) moving both a cricket and the body of an alert chameleon. During cricket movement, the chameleon locked both eyes straight forward in their orbits and followed the cricket movement with a visually guided head movement. During movement of the body of an alert chameleon, the vestibulo-collic reflex kept the chameleon's head relatively stationary in space. During movement of the body of an anesthetized chameleon, there was no measurable movement of the head relative to the body. Thus, during the body movements used in this study, passively induced kinesthetic stimulation was negligible. It was hypothesized that during movement of both a cricket and the body of an alert chameleon, the visually guided head movement and the vestibulo-collic reflex were additive. This hypothesis was supported by demonstrating that in this situation, the vestibulo-collic reflex was not suppressed. It is suggested that the vestibulo-collic reflex compensated for the passive head movement produced by body movement without canceling active, visually guided head movement. This may have been accomplished by the use of an efference copy system associated with the motor commands that produce visually guided head movement.

Secondary vestibular and neck position signals in the vestibular nuclei of alert rhesus monkeys performing active head movements

Rhesus monkeys were trained to track a visual target with head and eye movements in order to study central vestibular neurons under natural conditions. Single unit recordings of cells in the vestibular nuclei were obtained during active head rotations in the horizontal plane, and also during passive copies of these self-induced movements. Most cells exhibited secondary responses immediately following the primary vestibular responses to active or passive rapid head movements. They were of opposite polarity to the primary responses, and generally rate enhancements of secondary responses were of greater amplitude than rate suppressions. In addition, vestibular nuclei cells also encoded tonic neck position. The corresponding signal consisted of a variation in the basal discharge rate as a function of neck, and thus head, position. These observations prove for the first time that dynamic and static response characteristics recorded earlier from lesioned brains and under anaesthesia are, at least qualitatively, representative for the normally behaving vestibulomotor system in primates.

The role of the posterior vermis of monkey cerebellum in smooth-pursuit eye movement control. I. Eye and head movement-related activity.

1. The observation of smooth-pursuit eye and retinal image velocity signals in lobules VI and VII of the vermis has given rise to the hypothesis that a neural correlate of a target velocity signal exists in this region of the cerebellum (29). However, activity signaling head velocity is also required to regenerate a target velocity signal. Vermal Purkinje cell activity was, therefore, recorded during the performance of paradigms designed to dissociate head movement-related responses. 2. The activity of 107 Purkinje cells was found to be related to horizontal head velocity. Of these, 52% increased their discharge rate for ipsilaterally directed passive head movement (type I), and 48% were excited by contralateral head movements (type II). 3. In five Purkinje cells in which sufficient data were obtained, cell discharge rate increased monotonically with head velocity over the range of 5-40 deg/s. The sensitivity to head velocity at 0.4 Hz +/- 25 deg/s averaged approximately 0.5 spikes.s-1/deg.s-1 in a larger sample of cells (n = 39). The sensitivities to head velocity, at this same frequency and velocity, of type I and type II Purkinje cells were comparable at 0.44 and 0.51 spikes.s-1/deg.s-1, respectively. 4. The Purkinje cell responses led head velocity by an average of 12 degrees at 0.4 Hz +/- 25 deg/s of passive head rotation. The phase shifts associated with type I and II responses were similar with phase leads of 13 and 9 degrees with respect to head velocity, respectively. 5. A linear interaction of smooth-pursuit eye and head velocity signals was observed during the performance of a variety of antiphase and inphase eye and head movement paradigms. The results support the conclusion that some Purkinje cells in lobules VI and VII of the cerebellar vermis encode a gaze velocity signal. Contributions of the head velocity signal to the regeneration of target velocity are considered in a companion paper (32).

Vestibular nuclear neuron activity during active and passive head movement in the alert rhesus monkey.

Responses of single neurons were recorded in the medial and descending vestibular nuclei (MVN and DVN) and in the deep cerebellar nuclei of three juvenile rhesus monkeys (Macaca mulatta). Neuronal activity was measured during both passive sinusoidal and nonsinusoidal whole body rotation (peak velocities were under 90 degrees/s) and during active head movements. Although the active head movements occasionally exceeded 300 degrees/s, most exhibited peak velocities of less than 200 degrees/s. A total of 133 units sensitive to horizontal head rotation were recorded, and of these, 38 were held for sufficient time to obtain both passive and active head movement data. Comparison of the neuronal firing patterns obtained during active and passive head movements revealed no apparent differences. Thus neurons that were observed to burst or pause during saccades with the head fixed continued to do so when the head was free. Both the sensitivity to head velocity and the "inferred" spontaneous firing rate were compared during active and passive head movements by plotting rate-velocity curves for both conditions. When the data points were fitted with linear regression lines, no statistically significant differences in either sensitivity or spontaneous rate were found. The present study provides no evidence that efferent vestibular activity alters the properties of afferent vestibular neurons during active head movements, as has previously been suggested (21). Furthermore, neurons in the rostral portions of the vestibular nuclei in primates encode head velocity based entirely on labyrinthine information. Neither neck proprioceptors nor an efference copy of the head movement motor program seem to contribute significantly to the firing patterns observed.

Comparative neurobiology of the organization of gaze-stabilizing reflex systems in vertebrates

During locomotion gaze is stabilized against passive head movements by compensatory eye movements. The efficacy and the neuronal organization of optokinetic and vestibular reflexes of different vertebrate species is compared. Besides many similarities between species a number of differences can be found as well. Increase in the efficacy of compensatory reflexes is not correlated with an increase in the efficacy of basic neuronal circuits but with the appearance of functionally new connections and of new network properties. This increasingly higher complexity allows to maintain gaze stability at increasingly higher speeds of locomotion or to suppress these reflexes during visual pursuit of a moving object.

Behavioral organization of reticular formation: studies in the unrestrained cat. I. Cells related to axial, limb, eye, and other movements.

Veterans Administration Medical Center, Sepulveda 91343, and Department of Psychiatry, School of Medicine, University of California, Los Angeles, California 90024 SUMMARY AND CONCLUSIONS 1. We have recorded single-unit activity in medial reticular formation (RF) units in unrestrained, behaving cats. A total of 306 cells have been analyzed. Neuronal activity was observed during a variety of natural, spontaneously occurring behaviors, during rapid eye movement (REM) and non-REM sleep states, after sensory stimulation, and during elicited reflexes. 2. Most RF units discharged maximally in conjunction with a specific movement or group of movements. The companion paper (41) deals with cells related to movements of the facial musculature, while the present paper deals with all other cell types. 3. The most common RF cell types discharged during specific movements of the axial skeleton. Cells related to limb, respiratory, pharyngeal and laryngeal, jaw, and tongue movements were also observed. Reticular eye movement-related cells, previously investigated by others, were also seen in our unrestrained cats. A small percentage of cells were maximally activated by applied auditory, visual, vestibular, somatosensory, or proprioceptive stimuli. 4. Cells related to axial movement, which as a group constituted 38.2% of all RF cells, could be subdivided into cells related to neck extension, neck dorsiflexion, and ipsilateral or contralateral movement of the spine. 5. Cells related to active ipsilateral movement, constituting 19.3% of all medial RF cells, were the single most common cell type in the RF. Thirty-four percent of these cells did not respond to passive head movement, while 48% responded to passive head movement to the contralateral side and 18% to passive movement to the ipsilateral side. Neck proprioceptors contribute to the passive movement response in certain of these cells. 6. Cells related to limb movement constituted 6.9% of the cells encountered. Most were related to movement of the proximal portion of one limb. Cells related to movement of the distal portion of the limb were quite rare, constituting only 1.0% of RF cells. 7. While most RF cells were active only in relation to a single, directionally specific movement, we found a cluster of pontine RF cells, which discharged in relation to several limb and neck movements. 8. Cells related to several movements, eye movement, vestibular stimulation, and cells without spontaneous activity in sleep or waking were localized to restricted portions of the medial RF fields. Cells related to axial, proximal limb movements or somatic stimulation were intermingled throughout the entire region explored. 9. The unrestrained preparation allows the direct observation of the behavioral correlates of increased RF unit discharge. Most RF cells discharge in relation to a specific movement or group of movements of the axial musculature. Each movement-defined cell type has a different pattern of sleep, sensory and reflex activity, and anatomical localization. Anatomical intermingling of certain cell types may facilitate the synthesis of complex movement sequences from simpler elements commanded by individual RF cells.

Compensatory eye movements during active and passive head movements: fast adaptation to changes in visual magnification.

Rotational eye and head movements were recorded with great precision with scleral and cranial search coils in a rotating magnetic field. Compensatory eye movements were recorded in light and darkness during active as well as passive head movements in the frequency range 0.33-1.33 Hz. From the recorded, nominal gaze movements the effective gaze was reconstructed taking into account magnification or reduction factors of corrective spectacles. Effective gain was calculated as the ratio between the velocities of the effective corrective eye movements and the head movements. In the light, effective gain of compensatory eye movements during active head motion was mostly between 0.97 and 1.03. It was never precisely unity and differed systematically between subjects and between the two eyes of each subject. During passive head motion in the light, gain was lower by about 3% than during active motion. During active head movement in the dark, gain was mostly between 0.92 and 1.00; values were about 5% lower than during active motion in the light. During passive head movement in the dark, gain was about 13% lower than during active motion, and the variability of the oculomotor response increased. Adaptation of these base-line conditions was induced by fitting the subjects with magnifying or reducing spectacles for periods of 40 min to 24 h. The largest required change in amplitude of eye movements was 36%. When active head movements were made, the amplitude of compensatory eye movements in the light as well as in the dark adjusted rapidly. Most of the adaptation of the vestibulo-ocular reflex in the dark was completed in about 30 min. This rate is much faster than that found in previous experiments requiring larger adaptive changes. Differential adaptation to unequal demands for the two eyes proved to be very hard or impossible. In a mild conflict situation the system adjusted to an intermediate level, distributing the error symmetrically between the eyes. When the discrepancy was large, the adaptive process of both eyes was controlled by the one eye which provided the most meaningful information. It is concluded that the system generating compensatory eye movements performs best during active rather than passive head movements, and that adaptation to moderate changes in optimal gain are made very rapidly.

Ocular motor deficits in Parkinson's disease. I. The horizontal vestibulo-ocular reflex and its regulation.

Horizontal vestibulo-ocular reflex (VOR) function was quantified in 14 parkinsonian patients and compared with that of 10 age-matched normals. Eight patients had mild and 6 had advanced rigidity and akinesia. In advanced disease VOR gains were subnormal in darkness during sinusoidal wholebody rotation at frequencies from 0.3 to 3.0 Hz. When fixating a stationary target, patients enhanced VOR gains to near unity, but gains of patients with advanced disease were significantly lower than controls. Visual suppression of the VOR by fixating a target moving with the head was defective in all stages of the disease. Advanced patients also exhibited defective voluntary control of the reflex in darkness: both voluntary enhancement, by imagining a stationary target, and voluntary suppression, by imagining a target moving with the head, were abnormal. This disordered voluntary control of the VOR was independent of defective visually mediated smooth pursuit. Attempted visual and voluntary suppression of the reflex produced a rise in VOR gain, above the hypoactive values recorded in darkness in advanced disease. This inappropriate activation of the VOR implied structural integrity of brainstem smooth eye movement pathways, despite vestibulo-ocular hyporeflexia. Unrecognized degeneration in brainstem VOR circuits, or defective cerebral hemisphere control of the reflex caused by the nigrostriatal or cerebral cortical dysfunction of Parkinson's disease, may explain subnormal ocular responses to passive head motion. We attribute the abnormal visual and voluntary modulation of smooth eye movements to disordered long-loop cerebral control of the VOR and suggest that it is analogous to the defective long-loop cerebral regulation of spinal reflexes that occurs in Parkinson's disease.

Vestibulo-ocular reflex (VOR), cervico-ocular reflex (COR) and its interaction in active head movements.

In normal adults the vestibulo-ocular reflex (VOR) and the cervico-ocular reflex (COR) were investigated during passive and active head or body movements, respectively. Sinusoidal rotations around the vertical axis of the body at frequencies of 0.05, 0.1, and 0.2s-1 and total amplitudes of 20 degrees, 40 degrees, 60 degrees, or 80 degrees were employed. The average eye deviations (Schlagfeld) during VOR were directed opposite to the direction of the head turning. During COR, however, slow eye deviations of higher amplitude were anticompensatory relative to the fixed head. During active head turnings the average eye deviations showed the same anticompensatory direction as in COR, but were still larger. The increased with stimulus amplitudes up to 60 degrees. At least a weak cervical nystagmus was elicited in all subjects, with its fast phases beating in the direction of the relative head movement. Its gain reached marked values up to 0.5, but only for peak stimulus velocities below 25%. The nystagmus gain during active head turnings was only slightly higher than during VOR. With higher stimulus velocities, large anticompensatory saccades appeared just before the change of stimulus direction; these are typical for active head movements, but were also found during COR.

Co‐ordination of head and eyes in the gaze changing behaviour of cats

1. Saccadic changes of gaze have been studied in conscious cats implanted with electro-oculographic electrodes to register eye position and held in a device for measuring head position.2. When the head is free to move, almost all saccades larger than 4 deg in amplitude are accompanied by head rotation. This head movement starts at a variable delay after the beginning of the eye movement. Most frequently, the latency is 30 msec.3. After finishing its saccade, the eye rotates retrogressively in the orbit so as to maintain a constant direction of gaze during the remaining portion of the head movement. This compensatory eye movement has an average gain of 0.96 in the absence of visual feed-back.4. The duration and peak velocity characteristics of head-fixed saccades were measured. A typical horizontal saccade of 20 deg in amplitude attains a peak velocity of 250 deg/sec and lasts 110 msec. The duration-amplitude relationship increases by 2.1 msec/deg.5. Saccades aimed obliquely within the orbit possess the same duration/peak-velocity/amplitude relationships as purely horizontal saccades. Consequently, the horizontal component of an oblique saccade lasts longer, and has a lower peak velocity, than a purely horizontal saccade of the same amplitude as this component. This finding is discussed in relation to brain-stem control of oblique saccades.6. A saccade is initially programmed to achieve the desired gaze change without aid from the head. Consequently, the saccade is slowed down so as to prevent gaze overshoot when the head moves too. Since this saccadic attenuation occurs even during passive head movement, it must be due to feed-back from peripheral sensors of head rotation, and probably represents the initial stage of the compensatory eye movement.7. Since the axes of horizontal head and eye rotation are horizontally separated, it may be predicted that the gain of compensatory eye movements must rise as the viewing distance decreases if they are to maintain fixation during head movements. This prediction was confirmed.