A Reexamination of “Neck Reflex” Eye Movements in the Rabbit

Role of the cervico-ocular reflex in the "flying" pigeon: interactions with the optokinetic reflex

We studied the cervico-ocular reflex (COR) alone and in combination with the optokinetic (OKN) reflex in head-fixed pigeons. We analyzed these responses in two behavioral conditions: (1) animals were hung in a harness ("resting" condition); and (2) animals were additionally submitted to a frontal airflow that provoked a flight posture ("flying" condition). In both conditions, cervical stimulation provoked a slow phase of very low gain (around 0.05) in the opposite direction to that of the stimulation and fast phases triggered near the head-body alignment in the same direction as the stimulation. The slow phase showed a phase lag of 20 deg at 0.5 Hz. The gain of the slow phase was not modified by the velocity, amplitude, or frequency of the stimuli. This gain was not changed by the presence of a fixed visual surround. When cervical stimuli (0.05-0.5 Hz) were added to an optokinetic stimulation (30 deg/s) in the "resting" condition, the slow phase velocity (SPV) of the optokinetic reflex was modulated with a time course close to that produced by the cervico-ocular reflex alone. The SPV was alternately increased and decreased round the SPV level corresponding to the steady-state OKN. In the "flying" condition, optokinetic-cervical stimulation provoked an eye beating field and a strong SPV modulation synchronized with the position of the cervical stimulation. The number of nystagmic beats (OKN) and the amplitude and velocity of the fast phases were modulated in correlation with the SPV. Consequently, the optokinetic response was increased or decreased according to whether the cervical stimuli were in the reverse direction or in the same direction as the optokinetic stimulation, respectively. These data are interpreted as an improvement of gaze stabilization by the COR. This mechanism is context dependent, since it is strongly reinforced during the flight.

Signal Processing in the Vestibular System During Active Versus Passive Head Movements

In everyday life, vestibular receptors are activated by both self-generated and externally applied head movements. Traditionally, it has been assumed that the vestibular system reliably encodes head-in-space motion throughout our daily activities and that subsequent processing by upstream cerebellar and cortical pathways is required to transform this information into the reference frames required for voluntary behaviors. However, recent studies have radically changed the way we view the vestibular system. In particular, the results of recent single-unit studies in head-unrestrained monkeys have shown that the vestibular system provides the CNS with more than an estimate of head motion. This review first considers how head-in-space velocity is processed at the level of the vestibular afferents and vestibular nuclei during active versus passive head movements. While vestibular information appears to be similarly processed by vestibular afferents during passive and active motion, it is differentially processed at the level of the vestibular nuclei. For example, one class of neurons in vestibular nuclei, which receives direct inputs from semicircular canal afferents, is substantially less responsive to active head movements than to passively applied head rotations. The projection patterns of these neurons strongly suggest that they are involved in generating head-stabilization responses as well as shaping vestibular information for the computation of spatial orientation. In contrast, a second class of neurons in the vestibular nuclei that mediate the vestibuloocular reflex process vestibular information in a manner that depends principally on the subject's current gaze strategy rather than whether the head movement was self-generated or externally applied. The implications of these results are then discussed in relation to the status of vestibular reflexes (i.e., the vestibuloocular, vestibulocollic, and cervicoocular reflexes) and implications for higher-level processing of vestibular information during active head movements.

Vestibuloocular reflex signal modulation during voluntary and passive head movements

The vestibuloocular reflex (VOR) effectively stabilizes the visual world on the retina over the wide range of head movements generated during daily activities by producing an eye movement of equal and opposite amplitude to the motion of the head. Although an intact VOR is essential for stabilizing gaze during walking and running, it can be counterproductive during certain voluntary behaviors. For example, primates use rapid coordinated movements of the eyes and head (gaze shifts) to redirect the visual axis from one target of interest to another. During these self-generated head movements, a fully functional VOR would generate an eye-movement command in the direction opposite to that of the intended shift in gaze. Here, we have investigated how the VOR pathways process vestibular information across a wide range of behaviors in which head movements were either externally applied and/or self-generated and in which the gaze goal was systematically varied (i.e., stabilize vs. redirect). VOR interneurons [i.e., type I position-vestibular-pause (PVP) neurons] were characterized during head-restrained passive whole-body rotation, passive head-on-body rotation, active eye-head gaze shifts, active eye-head gaze pursuit, self-generated whole-body motion, and active head-on-body motion made while the monkey was passively rotated. We found that regardless of the stimulation condition, type I PVP neuron responses to head motion were comparable whenever the monkey stabilized its gaze. In contrast, whenever the monkey redirected its gaze, type I PVP neurons were significantly less responsive to head velocity. We also performed a comparable analysis of type II PVP neurons, which are likely to contribute indirectly to the VOR, and found that they generally behaved in a quantitatively similar manner. Thus our findings support the hypothesis that the activity of the VOR pathways is reduced "on-line" whenever the current behavioral goal is to redirect gaze. By characterizing neuronal responses during a variety of experimental conditions, we were also able to determine which inputs contribute to the differential processing of head-velocity information by PVP neurons. We show that neither neck proprioceptive inputs, an efference copy of neck motor commands nor the monkey's knowledge of its self-motion influence the activity of PVP neurons per se. Rather we propose that efference copies of oculomotor/gaze commands are responsible for the behaviorally dependent modulation of PVP neurons (and by extension for modulation of the status of the VOR) during gaze redirection.

Characteristics of cervico-ocular responses in the chameleon

The cervico-ocular reflex (COR) was investigated in the chameleon. Two kinds of responses were observed by oscillating the body (sine-wave stimuli) in the fixed-head animal: a "smooth response" of very low gain (around 0.08) and a saccadic response composed of 1-12 saccades per cycle of stimulation (depending on the stimulation frequency). Both responses were elicited in the compensatory direction (same direction as the stimulation) and exhibited a frequency dependence with low-pass properties. The saccadic response was especially developed and displayed a higher gain (up to 0.4) than the smooth response. In darkness, the saccades were triggered near the zero point (head-body alignment), whereas in the presence of a fixed visual surround they were elicited more regularly throughout the stimulation cycle. The amplitude of saccades was increased in the light. Consequently, the gain and the phase lag of the saccadic response were enhanced by the visual input. No visuo-cervical interaction was observed for the smooth response. Oscillating the body at a constant velocity (seesaw or ramp stimuli) revealed a frequency effect on the number of saccades (during a cycle of stimulation), but not on the gain of the response. Increasing the amplitude of oscillations augmented only very slightly the amplitude of saccades and consequently decreased the gain. Hence, the best working range of the saccadic response corresponds to body or head movements of low amplitude (up to +/- 20 deg) and low frequency (up to 0.25 Hz), and is improved by a visual input. These properties are discussed on a comparative point of view. It is proposed that, in chameleons, the saccadic response could contribute to gaze stabilization and add to the vestibulo-ocular and the optokinetic responses.

Visual and vestibular reflexes that stabilize gaze in the chameleon

Spontaneous eye movements as well as visual, vestibular, and proprioceptive cervical reflexes which contribute to gaze stabilization were investigated in the chameleon using the magnetic search-coil technique. The oculomotor range of each eye was very large (180 deg horizontally x 80 deg vertically). Spontaneous ocular saccades were independent in the two eyes and could have very large amplitudes. The fast phases of nystagmus during the stabilization reflexes were also independent in the eyes. In the head-restrained condition, optokinetic nystagmus (OKN) had a low gain in both horizontal and vertical planes (0.35 at 5 deg/s) and showed little binocular interaction. The vestibulo-ocular reflex (VOR) exhibited a low gain (0.2-0.3 from 0.05-1 Hz) and a high-phase lead at low frequency (140 deg at 0.05 Hz). Rotation of the animal in the presence of a visible surround increased the overall gain of gaze stabilization to 0.4-0.5 (P < 0.01) and considerably reduced the phase lead (38 deg at 0.05 Hz). In the head-free condition, head and eye reflexes were active simultaneously during both optokinetic and vestibular stimulation, but nystagmic head movements appeared only occasionally with a rather loose eye-head coordination. During optokinetic stimulation, eye movements contributed more than head movements to gaze stabilization, whereas, during vestibular or visuo-vestibular stimulation, the relative contribution of eye and head responses varied with stimulus frequency. When the head was freed, overall gain for gaze stabilization increased from 0.35 to 0.45 (P < 0.05) for optokinetic stimulation at 5 deg/s and from 0.2-0.3 to 0.4-0.75 (P < 0.001) for vestibular stimulation at 0.05-1 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of saccades induced by neck torsions: a re-examination in the normal guinea pig

Torsion of the neck relative to the fixed head results in several reflexes involving the eyes, the neck and the body. One of these reflexes, the cervico-ocular reflex, has been described as having a small gain in the normal animal. However, when the body of a guinea pig is moved relative to the fixed head with a ramp-like velocity profile, saccades are systematically elicited in the direction of body movement. We re-examined the characteristics of this reflex in the normal guinea pig and demonstrated that: (1) it occurs mainly in the range of high velocity body movements; (2) the latency of the saccades is shorter than previously suspected; (3) the saccades are triggered at specific positions relative to the starting and ending positions of rotation, revealing some degree of flexibility in the reflex. We hypothesize that these saccades of nuchal origin are under the control of the same neuronal circuit as visually triggered saccades and quick phases of vestibular nystagmus. Thus, this nuchal reflex may fundamentally subserve orienting behaviour in normal animal.

The cervico-ocular reflex in intact and chronically labyrinthectomized frogs

Eye movements in response to horizontal oscillation of the body against the stationary head (cervico-ocular reflex) were measured with search coils in the frequency range between 0.02 and 1.0 Hz in intact and chronica bilaterally labyrinthectomized frogs. The evoked eye movements were compensatory in direction but only about 1-2% of the amplitude of the stimulus (+/- 5-20 degrees). In chronic bilaterally labyrinthectomized frogs (n = 5) very similar response characteristics were measured. The very small amplitudes of these responses in controls and in frogs with removed labyrinthine organs render this reflex functionally irrelevant for both gaze stabilization and recovery from dynamic vestibular deficits after a loss of labyrinthine function.

Neck muscle afferent projections to the brainstem of the monkey: implications for the neural control of gaze

Brainstem projections of first-order afferent neurons that innervate the suboccipital muscles of the monkey have been determined by using the technique of transganglionic transport of wheat-germ-agglutinin-conjugated horseradish peroxidase (WGA/HRP) and HRP. Neck muscle afferents distribute to several distinct regions located within the caudal brainstem and rostral spinal cord. Terminal labeling was heaviest within the ventral portion of the ipsilateral lateral cuneate nucleus. Muscle afferent terminals also distributed to ventrolateral portions of the pars triangularis division of the cuneate nucleus. Projections were consistent with the known somatotopic (i.e., both place and modality) organization of the cuneate nucleus. Moreover, neck muscle projections to the cuneate nucleus were, in part, coincident with those previously demonstrated for the extraocular muscles (Porter: J. Comp. Neurol. 247:133-143, '86). Sparse terminal projections were noted in the central cervical nucleus. In addition, light terminal labeling was present in group x of the vestibular complex and in an ill-defined region along the lateral margin of the brainstem. Present observations, which provide the first complete description of the central distribution of neck muscle afferent neurons in the primate, may contribute to the known substrate for eye/head coordination.

Active head turning and correlated cerebral potentials. Experimental and clinical aspects

Fast voluntary horizontal head movements (n = 400, maximum acceleration 8,500 deg/sec2) were recorded together with surface EEG over pre- and post-central regions in 30 healthy volunteers while they were gazing at a head-fixed target in darkness. Artifacts of mechanical, myogenic and oculomotor origin could be precluded. Selective averaging of the fastest movements revealed a biphasic cortical potential. Median latency of its onset was 123.5 msec, of its first peak 189 msec, of the second peak 373 msec, with amplitudes ranging around 5.7 microV. Similar recordings in patients with uni- or bilateral vestibular nerve lesions demonstrated that strong stimulation of the joint vestibular and somatosensory (neck afferent) input facilitated characteristically related EEG activity: side differences of vestibular input could be measured on this level of the CNS, i.e. measured on the cortical level as additional information to the commonly used vestibulo-oculomotor response; patients with early, bilateral loss of vestibular functions showed a specific long-term adaptation of their related EEC activity. We conclude that clinical studies of this kind might aid in diagnosis of vertigo and related phenomena.

The horizontal and vertical cervico-ocular reflexes of the rabbit

Horizontal and vertical cervico-ocular reflexes of the rabbit (HCOR, VCOR) were evoked by sinusoidal oscillation of the body about the vertical and longitudinal axes while the head was fixed. These reflexes were studied over a frequency range of 0.005-0.800 Hz and at stimulus amplitudes of +/- 10 degrees. When the body of the rabbit was rotated horizontally clockwise around the fixed head, clockwise conjugate eye movements were evoked. When the body was rotated about the longitudinal axis onto the right side, the right eye rotated down and the left eye rotated up. The mean gain of the HCOR (eye velocity/body velocity) rose from 0.21 and 0.005 Hz to 0.27 at 0.020 Hz and then declined to 0.06 at 0.3Hz. The gain of the VCOR was less than the gain of the HCOR by a factor of 2-3. The HCOR was measured separately and in combination with the horizontal vestibulo-ocular reflex (HVOR). These reflexes combine linearly. The relative movements of the first 3 cervical vertebrae during stimulation of the HCOR and VCOR were measured. For the HCOR, the largest angular displacement (74%) occurs between C1 and C2. For the VCOR, the largest relative angular displacement (45%) occurs between C2 and C3. Step horizontal clockwise rotation of the head and body (HVOR) evoked low velocity counterclockwise eye movements followed by fast clockwise (resetting) eye movements. Step horizontal clockwise rotation of the body about the fixed head (HCOR) evoked low velocity clockwise eye movements which were followed by fast clockwise eye movements. Step horizontal clockwise rotation of the head about the fixed body (HCOR + HVOR) evoked low velocity counterclockwise eye movements which were not interrupted by fast clockwise eye movements. These data provide further evidence for a linear combination of independent HCOR and HVOR signals.

Eye and head movements during vestibular stimulation in the alert rabbit

Rabbits passively oscillated in the horizontal plane with a free hand tended to stabilize their head in space (re: earth-fixed surroundings) by moving the head on the trunk (neck angular deviation, NAD) opposite the passively imposed body rotation. The gain (NAD/body rotation) of head stabilization varied from 0.0 to 0.95 (nearly perfect stability) and was most commonly above 0.5. Horizontal eye movement (HEM) was inversely proportional to head-in-space stability, i.e. the gaze (sum of HEM, NAD, and body rotation) was stable in space (regardless of the gain of head stabilization). When the head was fixed to the rotating platform, attempted head movements (head torque) mimicked eye movements in both the slow and fast phases of vestibular nystagmus; tonic eye position was also accompanied by conjugate shifts in tonic head torque. Thus, while eye and head movements may at times be linked, that the slow eye and head movements vary inversely during vestibular stimulation with a free head indicates that the linkage is not rigid. Absence of a textured stationary visual field consistently produced a response termed 'visual inattentiveness,' which was characterized by, among other things, a reduction of head and gaze stability in space. This behavioral response could also be reproduced in a subject allowed vision during prolonged vestibular stimulation in the absence of other environmental stimuli. It is suggested that rabbits optimize gaze stability (re: stationary surroundings), with the head contributing variably, as long as the animal is attending to its surroundings.

Cervico-ocular reflex in the normal adult

(1) The cervico-ocular reflex (COR) in humans was measured while the subjects (n = 10) stood on a rotatable platform in a dark room with the head fixed by a stationary biteplate. Eye movements were measured in response to active and passive rotations about a vertical axis. (2) The COR gain (i.e., horizontal eye movement/amplitude of body rotation) was as great as 22% at low frequency of body rotation (0.025 Hz). With increasing frequency (e.g., at 0.4 Hz) the gain decreased to about 2%. (3) The phase angle of the eye movement ranged generally between -80 and -240 degrees with an average response around -180 degrees. (4) During active rotation, the COR response was similar to the responses measured during passive body rotation. (5) The principal conclusion drawn is that in normal adult humans the COR does not aid in stabilization of the image on the retina during passive or active body rotations. A theoretical function for the COR is presented and discussed.

Vestibular and somatosensory interaction in the cat vestibular nuclei

The vestibular nuclei of cats were explored extracellulary with micropipettes to locate units with a resting discharge rate which responded to rotation in the horizontal plane. These units were examined for somatosensory input from neck and limbs. Fewer than half responded to somatosensory stimulation. The neck region was the body area most effective in influencing unitary activity. The response pattern most often noted was an increase and decrease in discharge frequency when the body was moved towards and away from the recording electrode respectively. Change in discharge rate was observed to be primarily dependant upon neck velocity and not upon absolute neck position. Half of the somatosensory units received input from either the forelimbs or the hindlimbs, while the remaining half responded to both.