The role of compensatory eye and head movements in the rat for image stabilization and gaze orientation

Excitatory Projections of Wide Field Collicular Neurons to the Nucleus of the Optic Tract in the Rat

The superficial layers of the mammalian superior colliculus (SC) contain neurons that are generally responsive to visual stimuli but can differ considerably in morphology and response properties. To elucidate the structure and function of these neurons, we combined extracellular recording and juxtacellular labeling, detailed anatomical reconstruction, and ultrastructural analysis of the synaptic contacts of labeled neurons, using transmission electron microscopy. Our labeled neurons project to different brainstem nuclei. Of particular importance are neurons that fit the morphological criteria of the wide field (WF) neurons and whose dendrites are horizontally oriented. They display a rather characteristic axonal projection pattern to the nucleus of optic tract (NOT); thus, we call them superior collicular WF projecting to the NOT (SCWFNOT) neurons. We corroborated the morphological characterization of this neuronal type as a distinct neuronal class with the help of unsupervised hierarchical cluster analysis. Our ultrastructural data demonstrate that SCWFNOT neurons establish excitatory connections with their targets in the NOT. Although, in rodents, the literature about the WF neurons has focused on their extensive projection to the lateral posterior nucleus of the thalamus, as a conduit for information to reach the visual association areas of the cortex, our data suggest that this subclass of WF neurons may participate in the optokinetic nystagmus.

Primacy of vision shapes behavioral strategies and neural substrates of spatial navigation in marmoset hippocampus

The role of the hippocampus in spatial navigation has been primarily studied in nocturnal mammals, such as rats, that lack many adaptations for daylight vision. Here we demonstrate that during 3D navigation, the common marmoset, a new world primate adapted to daylight, predominantly uses rapid head-gaze shifts for visual exploration while remaining stationary. During active locomotion marmosets stabilize the head, in contrast to rats that use low-velocity head movements to scan the environment as they locomote. Pyramidal neurons in the marmoset hippocampus CA3/CA1 regions predominantly show mixed selectivity for 3D spatial view, head direction, and place. Exclusive place selectivity is scarce. Inhibitory interneurons are predominantly mixed selective for angular head velocity and translation speed. Finally, we found theta phase resetting of local field potential oscillations triggered by head-gaze shifts. Our findings indicate that marmosets adapted to their daylight ecological niche by modifying exploration/navigation strategies and their corresponding hippocampal specializations.

Strategies of locomotion composition

This review aims to highlight the importance of saccades during locomotion as a strategy to reduce sensory information loss while the subject is moving. Acquiring sensory data from the environment during movement results in a temporal flow of information, as the sensory precept changes with the position of the observer. Accordingly, the movement pattern shapes the sensory flow. Therefore, the requirements of locomotion and sensation have to be balanced in the behaviour of the organism. Insect vision provides deep insight into the interplay between action and perception. Insects can shape their optic flow by reducing their rotational movements to fast and short saccades. This generates prolonged phases of translations which provide depth information. Extensive behavioural and physiological studies on insects show how shaping the optic flow facilitates the coding of motion vision. Indeed the saccadic strategy provides an elegant solution to optimise sensory flow. Complementary studies in other taxa reported similar locomotion strategies emphasising the crucial influence of sensory flow on locomotion.

Differences in gaze anticipation for locomotion with and without vision

Previous experimental studies have shown a spontaneous anticipation of locomotor trajectory by the head and gaze direction during human locomotion. This anticipatory behavior could serve several functions: an optimal selection of visual information, for instance through landmarks and optic flow, as well as trajectory planning and motor control. This would imply that anticipation remains in darkness but with different characteristics. We asked 10 participants to walk along two predefined complex trajectories (limaçon and figure eight) without any cue on the trajectory to follow. Two visual conditions were used: (i) in light and (ii) in complete darkness with eyes open. The whole body kinematics were recorded by motion capture, along with the participant's right eye movements. We showed that in darkness and in light, horizontal gaze anticipates the orientation of the head which itself anticipates the trajectory direction. However, the horizontal angular anticipation decreases by a half in darkness for both gaze and head. In both visual conditions we observed an eye nystagmus with similar properties (frequency and amplitude). The main difference comes from the fact that in light, there is a shift of the orientations of the eye nystagmus and the head in the direction of the trajectory. These results suggest that a fundamental function of gaze is to represent self motion, stabilize the perception of space during locomotion, and to simulate the future trajectory, regardless of the vision condition.

Eye and head movements shape gaze shifts in Indian peafowl

Animals selectively direct their visual attention toward relevant aspects of their environments. They can shift their attention using a combination of eye, head, and body movements. While we have a growing understanding of eye and head movements in mammals, we know little about these processes in birds. We therefore measured the eye and head movements of freely-behaving Indian peafowl (Pavo cristatus) using a telemetric eye-tracker. Both eye and head movements contributed to gaze changes in peafowl. When gaze shifts were smaller, eye movements played a larger role than when gaze shifts were larger. The duration and velocity of eye and head movements were positively related to the size of the eye and head movements, respectively. In addition, the coordination of eye and head movements in peafowl differed from mammals; peafowl exhibited a near absence of the vestibulo-ocular reflex, which may partly result from the peafowl's ability to move their heads as quickly as their eyes.

Resolving the active versus passive conundrum for head direction cells

Head direction (HD) cells have been identified in a number of limbic system structures. These cells encode the animal's perceived directional heading in the horizontal plane and are dependent on an intact vestibular system. Previous studies have reported that the responses of vestibular neurons within the vestibular nuclei are markedly attenuated when an animal makes a volitional head turn compared to passive rotation. This finding presents a conundrum in that if vestibular responses are suppressed during an active head turn how is a vestibular signal propagated forward to drive and update the HD signal? This review identifies and discusses four possible mechanisms that could resolve this problem. These mechanisms are: (1) the ascending vestibular signal is generated by more than just vestibular-only neurons, (2) not all vestibular-only neurons contributing to the HD pathway have firing rates that are attenuated by active head turns, (3) the ascending pathway may be spared from the affects of the attenuation in that the HD system receives information from other vestibular brainstem sites that do not include vestibular-only cells, and (4) the ascending signal is affected by the inhibited vestibular signal during an active head turn, but the HD circuit compensates and uses the altered signal to accurately update the current HD. Future studies will be needed to decipher which of these possibilities is correct.

Interaction of egocentric and world-centered reference frames in the rat posterior parietal cortex

Navigation requires coordination of egocentric and allocentric spatial reference frames and may involve vectorial computations relative to landmarks. Creation of a representation of target heading relative to landmarks could be accomplished from neurons that encode the conjunction of egocentric landmark bearings with allocentric head direction. Landmark vector representations could then be created by combining these cells with distance encoding cells. Landmark vector cells have been identified in rodent hippocampus. Given remembered vectors at goal locations, it would be possible to use such cells to compute trajectories to hidden goals. To look for the first stage in this process, we assessed parietal cortical neural activity as a function of egocentric cue light location and allocentric head direction in rats running a random sequence to light locations around a circular platform. We identified cells that exhibit the predicted egocentric-by-allocentric conjunctive characteristics and anticipate orienting toward the goal.

Spatial and temporal characteristics of vestibular convergence

In all species studied, afferents from semicircular canals and otolith organs converge on central neurons in the brainstem. However, the spatial and temporal relationships between converging inputs and how these contribute to vestibular behaviors is not well understood. In the current study, we used discrete rotational and translational motion stimuli to characterize canal- and otolith-driven response components of convergent non-eye movement (NEM) neurons in the vestibular nuclear complex of alert pigeons. When compared to afferent responses, convergent canal signals had similar gain and phase ranges but exhibited greater spatial variability in their axes of preferred rotation. Convergent otolith signals also had similar mean gain and phase values to the afferent population but were spatially well-matched with the corresponding canal signals, cell-by-cell. However, neither response component alone nor a simple linear combination of these components was sufficient to predict actual net responses during combined canal-otolith stimulation. We discuss these findings in the context of previous studies of pigeon vestibular behaviors, and we compare our findings to similar studies in other species.

Effects of common anesthetics on eye movement and electroretinogram

High-resolution magnetic resonance imaging (MRI) provides non-invasive images of retinal anatomy, physiology, and function with depth-resolved laminar resolution. Eye movement and drift, however, could limit high spatial resolution imaging, and anesthetics that minimize eye movement could significantly attenuate retinal function. The aim of this study was to determine the optimal anesthetic preparations to minimize eye movement and maximize visual-evoked retinal response in rats. Eye movements were examined by imaging of the cornea with a charge-coupled device (CCD) camera under isoflurane, urethane, ketamine/xylazine, and propofol anesthesia at typical dosages in rats. Combination of the paralytic pancuronium bromide with isoflurane or ketamine/xylazine anesthesia was also examined for the eye movement studies. Visual-evoked retinal responses were evaluated using full-field electroretinography (ERG) under isoflurane, ketamine/xylazine, urethane, and ketamine/xylazine + pancuronium anesthesia in rats. The degree of eye movement, measured as displacement per unit time, was the smallest under 1% isoflurane + pancuronium anesthesia. The ketamine/xylazine groups showed larger dark-adapted ERG a- and b-waves than other anesthetics tested. The isoflurane group showed the shortest b-wave implicit times. Photopic ERGs in the ketamine/xylazine groups showed the largest b-waves with the isoflurane group showing slightly shorter implicit times at the higher flash intensities. Oscillatory potentials revealed an early peak in the isoflurane group compared with ketamine/xylazine and urethane groups. Pancuronium did not affect the a- and b-wave, but did increase oscillatory potential amplitudes. Compared with the other anesthetics tested here, ketamine/xylazine + pancuronium was the best combination to minimize eye movement and maximize retinal function. These findings should set the stage for further development and application of high-resolution functional imaging techniques, such as MRI, to study retinal anatomy, physiology, and function in anesthetized rats.

Dynamics of Active Sensing and perceptual selection

Sensory processing is often regarded as a passive process in which biological receptors like photoreceptors and mechanoreceptors transduce physical energy into a neural code. Recent findings, however, suggest that: first, most sensory processing is active, and largely determined by motor/attentional sampling routines; second, owing to rhythmicity in the motor routine, as well as to its entrainment of ambient rhythms in sensory regions, sensory inflow tends to be rhythmic; third, attentional manipulation of rhythms in sensory pathways is instrumental to perceptual selection. These observations outline the essentials of an Active Sensing paradigm, and argue for increased emphasis on the study of sensory processes as specific to the dynamic motor/attentional context in which inputs are acquired.

Eye velocity asymmetry, ocular orientation, and convergence induced by angular rotation in the rabbit

We studied ocular asymmetries and orienting responses induced by angular rotation in rabbits with binocular video recordings. Slow phase velocities were significantly larger in the eye moving temporonasally than nasotemporally. The eyes also converged and pitched down during rotation, which increased and refocused binocular overlap in the visual fields. Eye position also shifted into the slow phase direction. Vergence and pitch outlasted the induced nystagmus, suggesting that they were generated by a separate vestibulo-oculomotor subsystem(s). Thus, mechanisms in the rabbit increase compensatory eye velocity in the eye that leads into the direction of rotation and enhance binocular vision.

Vestibular gaze stabilization: different behavioral strategies for arboreal and terrestrial avians

In birds, it is thought that head movements play a major role in the reflexive stabilization of gaze and vision. In this study, we investigated the contributions of the eye and head to gaze stabilization during rotations under both head-fixed [vestibuloocular (VOR)] and head-free conditions in two avian species: pigeons and quails. These two species differ both in ocular anatomy (the pigeon has 2 distinct foveal regions), as well as in behavioral repertoires. Pigeons are arboreal, fly extended distances, and can navigate. Quails are primarily engrossed in terrestrial niches and fly only short distances. Unlike the head-fixed VOR gains that were under-compensatory for both species, gaze gains under head-free conditions were completely compensatory at high frequencies. This compensation was achieved primarily with head movements in pigeons, but with combined head and eye-in-head contributions in the quail. In contrast, eye-in-head motion, which was significantly reduced for head-free compared with head-fixed conditions, contributed very little to overall gaze stability in pigeons. These results suggest that disparity between the stabilization strategies employed by these two birds may be attributed to differences in species-specific behavior and anatomy.

Passive movements of the head do not abolish anticipatory firing properties of head direction cells

Neurons in the anterior dorsal thalamic nucleus (ADN) of the rat selectively discharge in relation to the animal's head direction (HD) in the horizontal plane. Temporal analyses of cell firing properties reveal that their discharge is optimally correlated with the animal's future directional heading by approximately 24 ms. Among the hypotheses proposed to explain this property is that ADN HD cells are informed of future head movement via motor efference copy signals. One prediction of this hypothesis is that when the rat's head is moved passively, the anticipatory time interval (ATI) will be attenuated because the motor efference signal reflects only the active contribution to the movement. The present study tested this hypothesis by loosely restraining the animal and passively rotating it through the cell's preferred direction. Contrary to our prediction, we found that ATI values did not decrease during passive movement but in fact increased significantly. HD cells in the postsubiculum did not show the same effect, suggesting independence between the two sites with respect to anticipatory firing. We conclude that it is unlikely that a motor efference copy signal alone is responsible for generating anticipatory firing in ADN HD cells.

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.

Gravity modulates Listing's plane orientation during both pursuit and saccades

Previous studies have shown that the spatial organization of all eye orientations during visually guided saccadic eye movements (Listing's plane) varies systematically as a function of static and dynamic head orientation in space. Here we tested if a similar organization also applies to the spatial orientation of eye positions during smooth pursuit eye movements. Specifically, we characterized the three-dimensional distribution of eye positions during horizontal and vertical pursuit (0.1 Hz, +/-15 degrees and 0.5 Hz, +/-8 degrees) at different eccentricities and elevations while rhesus monkeys were sitting upright or being statically tilted in different roll and pitch positions. We found that the spatial organization of eye positions during smooth pursuit depends on static orientation in space, similarly as during visually guided saccades and fixations. In support of recent modeling studies, these results are consistent with a role of gravity on defining the parameters of Listing's law.

Influence of dynamic tilts on the perception of earth-vertical

The aim of this study was to test the hypothesis that optimal activation of both the semicircular canals and the otoliths provides reliable vestibular cues about self-orientation in space. For this, we measured the ability of subjects to estimate the subjective vertical immediately, 20 s and 90 s after a rapid tilt (180 degrees /s(2)) from upright into different roll positions between 90 degrees left and right side down. Subjects had to estimate the earth-vertical and earth-horizontal direction in the dark by (a) setting a luminous line, (b) performing saccades, and (c) verbally declaring body position relative to gravity. The mean error curves from the three paradigms showed consistent E (Müller)- and A (Aubert)-effects, which did not significantly change over time. Horizontal and vertical saccade tasks exhibited different response characteristics, as previously reported by others, which likely reflect different computation mechanisms. The verbal estimation paradigm yielded complementary results to those of the luminous line paradigm and vertical saccade task. The E-effect of the luminous line and the vertical saccade paradigm might be explained by a bias towards earth-vertical due to interactions of vestibular and neck afferent signals. The invariably small A-effect of the luminous line and the vertical saccade paradigm probably reflects somatosensory signals that had relatively weak influence in our experiments. We conclude that phasic activation of the vestibular system reduces the influence of non-vestibular cues observed in low tilt velocity or static experiments. Although this activation generates an E-effect, the total error in the range of +/-90 degrees is reduced.

Vestibulo-ocular and optokinetic impairments in left unilateral neglect

Right brain damaged patients affected by left unilateral neglect (N+) typically fail to explore the contralesional space. For the first time, this study investigates the dynamic and spatial features of the horizontal vestibular-ocular response (VOR), the optokinetic response (OKR) and the VOR-OKR interaction in six N+ and in five right brain damaged patients without neglect (N-). No lateral asymmetry of the gain (i.e. eye velocity to head velocity ratio) of VOR slow phases was found in either group. In the VOR, N+ had higher frequency of slow-rightward/fast-leftward phases and higher contralesional shift of the beating field (i.e. orbital position of fast phases). In the VOR-OKR, there was an increase of gain in both lateral directions and in both groups even though in N-, there was a lower phase shift between eye and head velocity. In contrast to the VOR, in the VOR-OKR, N+ had higher frequency of slow-leftward/fast-rightward phases. The VOR-OKR interaction also introduced an ipsilesional shift of the beating field in both N+ and N-. In the OKR, N+ showed a drop in the velocity, amplitude and frequency of slow-rightward/fast-leftward phases. These findings potentially suggest that each hemisphere modulates VOR with contralaterally directed slow phases and OKR with ipsilaterally directed slow phases. This organisation could facilitate maintenance or fast recovery of combined VOR + OKR after unilateral brain damage. The same findings suggest that by inducing slow-leftward phases, vestibular and optokinetic stimulation improve left side neglect through the activation of different hemispheric pathways. No ipsilesional deviation of the subjective "straight ahead" was found in N+. These results show that chronic unilateral neglect can be dissociated both from deficits of ipsilesionally directed VOR and from ipsilesional deviation of the subjective midsagittal plane of the body.

Modulation of spatial attention with unidirectional field motion: an implication for the shift of the OKN beating field

During optokinetic nystagmus (OKN) the mean eye position of gaze (the beating field) shifts in the direction of the fast phases. The function of this shift may be to re-orient the eyes in the direction of self-motion which optic flow implies (in-coming field). This idea leads to the hypothesis that visual attention may be directed toward the In-coming field. In Experiment 1, subjects detected a visual flash presented against unidirectional field motion. The OKN beating field was shifted toward the In-coming field, and manual reaction times were shorter when the target appeared in the In-coming field. Experiment 2 revealed that this In-coming field advantage occurred even when OKN (and thus the mean eye-position shift) was suppressed. Subsequent experiments showed that the In-coming field advantage is not due to a local motion interaction (Experiment 3), survives subject's voluntary allocation of attention (Experiment 4), and develops over less than 320 ms after the onset of the motion field (Experiment 5). These results suggest that unidirectional field motion tends to automatically shift visual attention toward the In-coming field.

Eye movements induced by linear acceleration in humans

The otolith-function study is remarkably behind the semicanal-function study. In the present paper, we introduced briefly our on-going studies on eye movements including nystagmic elicitation during lateral (Gy) linear acceleration with step and sinusoidal modes using a sled-type accelerator. The eye movements were recorded by EOGs (DC) from subjects who looked at an imaginary target of their straight ahead in darkness during G-loading up to 0.5 G. Corresponding to the +Gy and -Gy segments, nystagmus and/or deviation in eye position were frequently induced in some subjects, but none or slightly in the other. The nystagmus changed the beating direction dependently on the Gy direction, while the eye-deviation could be either direction of compensatory or anticompensatory. In half of subjects, nystagmus elicitation was absent or low at 0.3 G, while it tended to increase above 0.3 G. The nystagmic elicitation was similar to each other between the both modes of acceleration, and directional preponderance (DP) was observed in some subjects. There was no correlation between the DP and the nystagmic slow-phase velocity. Functional meanings of these findings were discussed.

Shift of the beating field of vestibular nystagmus: an orientation strategy?

We investigated in humans whether the shift of the beating field, which is often observed during vestibular nystagmus, could be related to some strategy of orientation. Eye movements were measured with an infrared system during an experiment on self-motion perception in the dark. Subjects were asked to rotate, by means of a joystick, a mobile robot on which they were seated in order to reproduce a previously imposed passive rotation. We suggest that the shift of the ocular beating field is the manifestation of two different orientation strategies based on allocentric and egocentric reference frames, respectively. It is also proposed that subjects who preferably used the first strategy exhibited large shifts of the beating field, while the others who probably used egocentric memory did not exhibit any shift.

Kinematic principles of primate rotational vestibulo-ocular reflex. I. Spatial organization of fast phase velocity axes

The spatial organization of fast phase velocity vectors of the vestibulo-ocular reflex (VOR) was studied in rhesus monkeys during yaw rotations about an earth-horizontal axis that changed continuously the orientation of the head relative to gravity ("barbecue spit" rotation). In addition to a velocity component parallel to the rotation axis, fast phases also exhibited a velocity component that invariably was oriented along the momentary direction of gravity. As the head rotated through supine and prone positions, torsional components of fast phase velocity axes became prominent. Similarly, as the head rotated through left and right ear-down positions, fast phase velocity axes exhibited prominent vertical components. The larger the speed of head rotation the greater the magnitude of this fast phase component, which was collinear with gravity. The main sequence properties of VOR fast phases were independent of head position. However, peak amplitude as well as peak velocity of fast phases were both modulated as a function of head orientation, exhibiting a minimum in prone position. The results suggest that the fast phases of vestibulo-ocular reflexes not only redirect gaze and reposition the eye in the direction of head motion but also reorient the eye with respect to earth-vertical when the head moves relative to gravity. As further elaborated in the companion paper, the underlying mechanism could be described as a dynamic, gravity-dependent modulation of the coordinates of ocular rotations relative to the head.

The early phase of horizontal optokinetic responses in the pigmented rat and the effects of lesions of the visual cortex

Horizontal optokinetic responses of pigmented rats were studied both in intact animals and in animals that had received lesions of the visual area of the cerebral cortex. In response to uniform velocity stimulation, there was an initial phase of rapid acceleration, larger than that reported in earlier studies, followed by a period of fairly uniform acceleration until the eye velocity approached that of the stimulus. As reported previously, responses to monocular stimulation were highly asymmetric, with the responses to nasotemporal stimulation being much weaker than those to temporonasal stimulation. Responses to sinusoidal stimulation were also studied. No significant effect of cortical lesions on the responses was seen.

Gaze shift during optokinetic stimulation in head free cats

During horizontal optokinetic stimulation (moving random dot pattern) in freely standing cats, gaze, head, and eye movements were measured with the search coil technique. The position values ('beating field') were evaluated. Gaze position depended on stimulus direction and stimulus velocity; it shifted to where the stimulus came from, the more the faster the stimulus velocity (mean values reached 15 degrees during high velocities). The head contributed about two-thirds of the gaze shift and eye-in-head about one-third, thus remaining near to the center of the orbit. During monocular optokinetic stimulation, the cats showed a directional asymmetry: a prominent position shift of gaze and head during temporal-to-nasal stimulation and almost no shift during nasal-to-temporal stimulation. It is discussed that the shift of gaze position represents an orienting movement towards a 'center of interest'.

Orienting-related eye-neck neurons of the medial ponto-bulbar reticular formation do not participate in horizontal canal-dependent vestibular reflexes of alert cats

Ponto-bulbar reticular formation neurons, including identified reticulospinal neurons, were studied in alert, head-fixed cats. Orienting-related neurons of "eye-neck" type (ENNs) were selected on the basis of qualitative correlations of their discharges with visually triggered eye saccades and electromyographic activity (EMG) of dorsal neck muscles. It was tested whether ENNs participate both in visually triggered gaze shifts requiring eye-head coordination and in gaze-stabilizing movements, such as vestibulo-ocular and vestibulo-collic reflexes (VOR, VCR). Firing patterns were studied during passive sinusoidal rotation (0.2-1.0 Hz; 2.0-21.5 deg peak-to-peak) in the horizontal plane. Responses to electrical stimulation of the superior colliculus and the vestibular nerve were recorded to assess the convergence of tectal and vestibular synaptic inputs. The same methods were applied to a control sample of neurons with discharges apparently "unrelated" to orienting movements. ENNs did not show any modulation of firing rate correlated to compensatory VOR or VCR during passive sinusoidal rotations. Among "unrelated" cells, the fraction of modulated units was close to that reported for reticular neurons projecting in the medial reticulospinal tract. Phasic and sustained components of ENN bursts were associated with anticompensatory movements induced by rotation, such as quick phases, ocular beating field shift, and the increase of EMG activity in neck muscles acting in the direction of passive rotation. Monosynaptic excitation from the contralateral superior colliculus was observed in 92.3% of ENNs, but only 2 out of 17 tested showed an excitatory response to vestibular nerve stimulation. In the control group of "unrelated" neurons the proportions of monosynaptic tectal and excitatory vestibular nerve inputs were, respectively, 75.6 and 71.4%. It is concluded that ENNs are specifically related to active gaze shifts, derived from either visual or from head velocity inputs. Rhombencephalic connections of vestibular nuclei to these neurons appear to be quite weak. Parallel inputs from the mid- or forebrain must be assumed to explain their firing patterns during rotation-induced anticompensatory gaze shifts. Within the studied range of frequencies and amplitudes of passive rotation, ENNs did not participate in the vestibulo-collic reflex. It is therefore unlikely that reticular neurons controlling orienting eye-neck synergies act also as a premotor pathway for gaze-stabilizing movements.

Evidence for separate eye and head position command signals in unrestrained rats

Compensatory horizontal eye-head movements of unrestrained rats were recorded with search coils in a magnetic field in response to combined optokinetic plus vestibular sinusoidal oscillations (0.05-1 Hz). The velocity contribution of compensatory slow head movements for image stabilization was relatively small (about 30%). The beating field of ocular nystagmus shifted during each half cycle in quick phase direction. These changes in eye position were counterbalanced by concomitant changes in head position. As a result, the orientation of gaze position was kept straight ahead with respect to the body length axis. These results imply independent and task-specific recruitment orders for the ocular and neck motor system.