Direction of heading and vestibular control of binocular eye movements

Oculomotor stimulation without visual input has no impact on postural control

It has been well established that eye movements have an impact on balance, and it has been hypothesized that extraocular oculomotor signals could play a significant role in this effect. Unfortunately, this hypothesis could not be confirmed as the previous methodology did not allow for the independent assessment of the differential effects of visual and oculomotor stimulation. The objective of the present study is to examine the impact of motor movements of the eyes without visual stimulation on balance. Static postural control, a prerequisite for balance, was assessed using a force platform in 20 participants. They were asked either to remain still without moving or to make movements of the tongue or eyes at a rate of two cycles per second. Movements were monitored using electrophysiological recordings. Each of the conditions was performed with eyes open and with eyes closed. Significant changes in postural control were observed due to eye movements when the eyes were open, but no significant differences were found between the conditions when the eyes were closed. The results confirm that the visual system provides important spatial cues for balance, allowing the body to be better positioned in space, and reject the possibility that extraocular signals are directly involved in postural stability.

Vestibular facilitation of optic flow parsing

Simultaneous object motion and self-motion give rise to complex patterns of retinal image motion. In order to estimate object motion accurately, the brain must parse this complex retinal motion into self-motion and object motion components. Although this computational problem can be solved, in principle, through purely visual mechanisms, extra-retinal information that arises from the vestibular system during self-motion may also play an important role. Here we investigate whether combining vestibular and visual self-motion information improves the precision of object motion estimates. Subjects were asked to discriminate the direction of object motion in the presence of simultaneous self-motion, depicted either by visual cues alone (i.e. optic flow) or by combined visual/vestibular stimuli. We report a small but significant improvement in object motion discrimination thresholds with the addition of vestibular cues. This improvement was greatest for eccentric heading directions and negligible for forward movement, a finding that could reflect increased relative reliability of vestibular versus visual cues for eccentric heading directions. Overall, these results are consistent with the hypothesis that vestibular inputs can help parse retinal image motion into self-motion and object motion components.

Does orbital proprioception contribute to gaze stability during translation?

Translational motion induces retinal image slip which varies with object distance. The brain must know binocular eye position in real time in order to scale eye movements so as to minimize retinal slip. Two potential sources of eye position information are orbital proprioception and an internal representation of eye position derived from central ocular motor signals. To examine the role of orbital proprioceptive information, the position of the left eye was perturbed by microstimulation of the left abducens nerve during translational motion to the right or left along the interaural axis in two rhesus macaques. Microstimulation rotated the eye laterally, activating eye muscle proprioceptors, while keeping central motor commands undisturbed. We found that microstimulation-induced eye position changes did not affect the translational VOR in the abductive (lateral rectus) direction, but it did influence the responses in the adductive (medial rectus) direction. Our findings demonstrate that proprioceptive inputs appear to be involved in the TVOR responses at least during ipsilateral head movements and proprioceptive influences on the TVOR may involve vergence-related signals to the oculomotor nucleus. However, internal representation of eye position, derived from central ocular motor signals, likely plays the dominant role in providing eye position information for scaling eye movements during translational motion, particularly in the abducent direction.

Does the middle temporal area carry vestibular signals related to self-motion?

Recent studies have described vestibular responses in the dorsal medial superior temporal area (MSTd), a region of extrastriate visual cortex thought to be involved in self-motion perception. The pathways by which vestibular signals are conveyed to area MSTd are currently unclear, and one possibility is that vestibular signals are already present in areas that are known to provide visual inputs to MSTd. Thus, we examined whether selective vestibular responses are exhibited by single neurons in the middle temporal area (MT), a visual motion-sensitive region that projects heavily to area MSTd. We compared responses in MT and MSTd to three-dimensional rotational and translational stimuli that were either presented using a motion platform (vestibular condition) or simulated using optic flow (visual condition). When monkeys fixated a visual target generated by a projector, half of MT cells (and most MSTd neurons) showed significant tuning during the vestibular rotation condition. However, when the fixation target was generated by a laser in a dark room, most MT neurons lost their vestibular tuning whereas most MSTd neurons retained their selectivity. Similar results were obtained for free viewing in darkness. Our findings indicate that MT neurons do not show genuine vestibular responses to self-motion; rather, their tuning in the vestibular rotation condition can be explained by retinal slip due to a residual vestibulo-ocular reflex. Thus, the robust vestibular signals observed in area MSTd do not arise through inputs from area MT.

Primate disconjugate eye movements during the horizontal AVOR in darkness and a plausible mechanism

Disconjugate eye movements during the horizontal angular vestibulo-ocular reflex (AVOR) evoked in response to steps or pulses of head velocity have been previously reported in lateral eyed animals. In this study, we measured binocular responses to sustained sinusoidal and pseudo-random vestibular stimuli in yaw, delivered in darkness, in both human and monkey. The vestibular stimuli used in our experiments had peak velocities in the range of 120-200 degrees /s, frequencies in the range of 0.17-0.5 Hz, and durations between 60 and 75 s. Our results show a large vergence component to the AVOR response that systematically modulated with head velocity. We also examined our results for temporal-nasal preponderance in slow eye velocity. Although each subject showed some degree of directional preference, we did not find a systematically greater eye velocity for temporal-nasal direction across all subjects. Here, we present these findings and discuss that at least two possible sources could result in disconjugate eye movements during the horizontal rotational VOR in darkness: peripheral and central mechanisms.

Self-motion-induced eye movements: effects on visual acuity and navigation

Self-motion disturbs the stability of retinal images by inducing optic flow. Objects of interest need to be fixated or tracked, yet these eye movements can infringe on the experienced retinal flow that is important for visual navigation. Separating the components of optic flow caused by an eye movement from those due to self-motion, as well as using optic flow for visual navigation while simultaneously maintaining visual acuity on near targets, represent key challenges for the visual system. Here we summarize recent advances in our understanding of how the visuomotor and vestibulomotor systems function and interact, given the complex task of compensating for instabilities of retinal images, which typically vary as a function of retinal location and differ for each eye.

Vestibulo-ocular reflex to transient surge translation: complex geometric response ablated by normal aging

The linear vestibulo-ocular reflex (LVOR) to surge (fore-aft) translation has complex kinematics varying with target eccentricity and distance. To determine normal responses and aging changes, 9 younger [age, 28 +/- 2 (SE) yr] and 11 older subjects (age, 69 +/- 2 yr) underwent 0.5 g whole body surge transients while wearing binocular scleral search coils. Linear chair position and head acceleration were measured with a potentiometer and accelerometer. Subjects viewed centered and 10 degrees horizontally and vertically eccentric targets 50, 25, or 15 cm distant before unpredictable onset of randomly directed surge in darkness (LVOR) and light (V-LVOR). Response directions were kinematically appropriate to eccentricity in all subjects, but there were significantly more measurable LVOR and V-LVOR responses (63-79%) in younger than older subjects (38-44%, P < 0.01). Minimal LVOR latency averaged 48 +/- 4 ms for younger and significantly longer at 70 +/- 6 ms for older subjects. In the interval 200-300 ms after surge onset, horizontal LVOR gain (relative to ideal velocity) of younger subjects averaged over all target distances was 0.55 +/- 0.04 and was significantly reduced in older subjects to 0.33 +/- 0.04. Horizontal V-LVOR gain was 0.58 +/- 0.04 in younger and significantly lower at 0.35 +/- 0.06 in older subjects. Vertical gains did not differ significantly between groups. Target visibility had no effect in either group during the initial 200 ms. The LVOR and V-LVOR were augmented by saccades in younger more than older subjects. Aging thus decreases LVOR velocity gain, response rate, and saccade augmentation, but prolongs latency.

Intrinsic and synaptic plasticity in the vestibular system

The vestibular system provides an attractive model for understanding how changes in cellular and synaptic activity influence learning and memory in a quantifiable behavior, the vestibulo-ocular reflex. The vestibulo-ocular reflex produces eye movements that compensate for head motion; simple yet powerful forms of motor learning calibrate the circuit throughout life. Learning in the vestibulo-ocular reflex depends initially on the activity of Purkinje cells in the cerebellar flocculus, but consolidated memories appear to be stored downstream of Purkinje cells, probably in the vestibular nuclei. Recent studies have demonstrated that the neurons of the vestibular nucleus possess the capacity for both synaptic and intrinsic plasticity. Mechanistic analyses of a novel form of firing rate potentiation in neurons of the vestibular nucleus have revealed new rules of plasticity that could apply to spontaneously firing neurons in other parts of the brain.

Scaling of the fore-aft vestibulo-ocular reflex by eye position during smooth pursuit

An eye position signal scales the amplitude of compensatory eye velocity in the translational vestibulo-ocular reflex (TVOR). To investigate the origin of such a modulatory signal, we studied the kinematics of the fore-aft TVOR as rhesus monkeys pursued a horizontally moving target at velocities between 0.5 and 30 degrees /s. We found that the "V-shaped" curve of the fore-aft TVOR amplitude as a function of eye position was shifted opposite to the direction of pursuit eye movement. As a result, the tip of the V-shaped curve that occurred close to zero eye position during steady-state fixation was shifted to the right during leftward pursuit and to the left during rightward pursuit eye movements. The faster the pursuit velocity the larger the observed shift. These results suggest that the scaling of the TVOR can precede actual eye position changes by several tens of milliseconds, which averaged 169 +/- 87 ms in three rhesus monkeys. Thus, central motor commands, rather than low-level efference copy or proprioceptive information, may be the signals scaling TVOR amplitude.

Skew Deviation Revisited

Skew deviation is a vertical misalignment of the eyes caused by damage to prenuclear vestibular input to ocular motor nuclei. The resultant vertical ocular deviation is relatively comitant in nature, and is usually seen in the context of brainstem or cerebellar injury from stroke, multiple sclerosis, or trauma. Skew deviation is usually accompanied by binocular torsion, torticollis, and a tilt in the subjective visual vertical. This constellation of findings has been termed the ocular tilt reaction. In the past two decades, a clinical localizing value for skew deviation has been assigned, and a cogent vestibular mechanism for comitant and incomitant variants of skew deviation has been proposed. Our understanding of skew deviation as a manifestation of central otolithic dysfunction in different planes of three-dimensional space is evolving. The similar spectrum of vertical ocular deviations arising in patients with congenital strabismus may further expand the nosology of skew deviation to include vergence abnormalities caused by the effects of early binocular visual imbalance on the developing visual system.

Neural correlates of the dependence of compensatory eye movements during translation on target distance and eccentricity

To stabilize objects of interest on the fovea during translation, vestibular-driven compensatory eye movements [translational vestibulo-ocular reflex (TVOR)] must scale with both target distance and eccentricity. To identify the neural correlates of these properties, we recorded from different groups of eye movement-sensitive neurons in the prepositus hypoglossi and vestibular nuclei of macaque monkeys during lateral and fore-aft displacements. All neuron types exhibited some increase in modulation amplitude as a function of target distance during high-frequency (4 Hz) lateral motion in darkness, with slopes that were correlated with the cell's pursuit gain, but not eye position sensitivity. Vergence angle dependence was largest for burst-tonic (BT) and contralateral eye-head (EH) neurons and smallest for ipsilateral EH and position-vestibular-pause (PVP) cells. On the other hand, the EH and PVP neurons with ipsilateral eye movement preferences exhibited the largest vergence-independent responses, which would be inappropriate to drive the TVOR. In addition to target distance, the TVOR also scales with target eccentricity, as evidenced during fore-aft motion, where eye velocity amplitude exhibits a "V-shaped " dependence and phase shifts 180 degrees for right versus left eye positions. Both the modulation amplitude and phase of BT and contralateral EH cells scaled with eye position, similar to the evoked eye movements during fore-aft motion. In contrast, the response modulation of ipsilateral EH and PVP cells during fore-aft motion was characterized by neither the V-shaped scaling nor the phase reversal. These results show that distinct premotor cell types carry neural signals that are appropriately scaled by vergence angle and eye position to generate the geometrically appropriate compensatory eye movements in the translational vestibulo-ocular reflex.

Foveal visual strategy during self-motion is independent of spatial attention

Translational self-motion disturbs the stability of retinal images by inducing a pattern of retinal optic flow that cannot be compensated globally by a single eye movement. The eyes must rotate by different amounts, depending on which spatial location needs to be stabilized on the retina. However, compensatory eye movements during steady fixation are always such as to maintain visual acuity on the fovea at the expense of significant image slip on the peripheral retina. We investigated whether such a foveal visual strategy during translation is hard-wired or whether it embeds enough flexibility to also allow for behaviorally relevant objects outside the foveae to be stabilized preferentially on the retinas. Monkeys were moved forward or backward and leftward or rightward passively in darkness while planning a saccade or bar release to peripheral dimmed targets. By comparing the eye movements made during these tasks with those under conditions of steady fixation, we found that the motion-induced eye movements depended only on current fixation. This was true even during the last milliseconds just before a saccade to the peripheral target. We conclude that the foveal stabilization strategy is invariant and solely dependent on current eye position, a strategy that is optimal for both processing speed and efficiency in the extraction of heading information from retinal flow during self-motion.

Binocular coordination in fore/aft motion

Stabilization of images on the fovea during either fore/aft translation of a subject or fore/aft movement of a visual target in front of a stationary observer imposes complex geometrical requirements that depend upon the eccentricity of the object of interest with respect to the eyes. Each eye needs to be rotated independently with varying proportions of conjugate (version) and disconjugate (vergence) eye movements to maintain fixation of the target. Here, we describe binocular coordination in the early response to translational movements of normal subjects along their naso-occipital axis. We recorded the responses evoked by small (about 4 cm), abrupt (about 0.7 g), fore/aft translations in four normal subjects while they viewed a near target. In the forward and backward starting positions the target was 15 or 10.5 cm away, respectively. Each subject was tested with the target centered between the eyes, aligned on the right eye, and placed to the right of the right eye by approximately 3 cm. The three conditions differed only in the lateral eccentricity of the target, yet the geometrical requirements for image stabilization are very different: pure vergence, one eye still, or mostly version. We found that the eye-movement responses closely matched what was needed for visual stabilization of the target, though responses to stimuli calling for divergence were less accurate than those for convergence. The latency of these responses ranged from 40 to 65 ms and achieved about 80% of the ideal response by 90 to 100 ms after the onset of the stimulus. Next, we asked whether these eye movements were generated by the vestibular system or by high-level strategies for image stabilization, such as pursuit. Thus, in a second set of experiments we used the mean profile of fore\aft body motion computed for each subject to drive a small visual target across the same distances and in the same eccentricities used during body translations. We found that visually driven responses had longer latencies (by at least 80 ms, ranging from 144 to 155 ms) and slower dynamics (with significantly lower peak eye velocities), highlighting the different subsystems producing the two types of responses. Saccades were also an important component of the response to both visual and vestibular stimuli, less frequent during the centered-target configuration and more frequent during viewing of eccentric targets. Visual stimuli evoked saccadic corrections more often and at shorter latencies than did vestibular stimuli. Both smooth and saccadic eye movements were appropriately disconjugate and their pattern depended on whether the eyes were converging or diverging.

Similar kinematic properties for ocular following and smooth pursuit eye movements

Ocular following (OFR) is a short-latency visual stabilization response to the optic flow experienced during self-motion. It has been proposed that it represents the early component of optokinetic nystagmus (OKN) and that it is functionally linked to the vestibularly driven stabilization reflex during translation (translational vestibuloocular reflex, TVOR). Because no single eye movement can eliminate slip from the whole retina during translation, the OFR and the TVOR appear to be functionally related to maintaining visual acuity on the fovea. Other foveal-specific eye movements, like smooth pursuit and saccades, exhibit an eye-position-dependent torsional component, as dictated by what is known as the "half-angle rule" of Listing's law. In contrast, eye movements that stabilize images on the whole retina, such as the rotational vestibuloocular reflex (RVOR) and steady-state OKN do not. Consistent with the foveal stabilization hypothesis, it was recently shown that the TVOR is indeed characterized by an eye-position-dependent torsion, similar to pursuit eye movements. Here we have investigated whether the OFR exhibits three-dimensional kinematic properties consistent with a foveal response (i.e., similar to the TVOR and smooth pursuit eye movements) or with a whole-field stabilization function (similar to steady-state OKN). The OFR was elicited using 100-ms ramp motion of a full-field random dot pattern that moved horizontally at 20, 62, or 83 degrees/s. To study if an eye-position-dependent torsion is generated during the OFR, we varied the initial fixation position vertically within a range of +/-20 degrees . As a control, horizontal smooth pursuit eye movements were also elicited using step-ramp target motion (10, 20, or 30 degrees/s) at similar eccentric positions. We found that the OFR followed kinematic properties similar to those seen in pursuit and the TVOR with the eye-position-dependent torsional tilt of eye velocity having slopes that averaged 0.73 +/- 0.16 for OFR and 0.57 +/- 0.12 (means +/- SD) for pursuit. These findings support the notion that the OFR, like the TVOR and pursuit, are foveal image stabilization systems.

Eyes on Target: What Neurons Must do for the Vestibuloocular Reflex During Linear Motion

A gaze-stabilization reflex that has been conserved throughout evolution is the rotational vestibuloocular reflex (RVOR), which keeps images stable on the entire retina during head rotation. An ethological newer reflex, the translational or linear VOR (TVOR), provides fast foveal image stabilization during linear motion. Whereas the sensorimotor processing has been extensively studied in the RVOR, much less is currently known about the neural organization of the TVOR. Here we summarize the computational problems faced by the system and the potential solutions that might be used by brain stem and cerebellar neurons participating in the VORs. First and foremost, recent experimental and theoretical evidence has shown that, contrary to popular beliefs, the sensory signals driving the TVOR arise from both the otolith organs and the semicircular canals. Additional unresolved issues include a scaling by both eye position and vergence angle as well as the temporal transformation of linear acceleration signals into eye-position commands. Behavioral differences between the RVOR and TVOR, as well as distinct differences in neuroanatomical and neurophysiological properties, raise multiple functional questions and computational issues, only some of which are readily understood. In this review, we provide a summary of what is known about the functional properties and neural substrates for this oculomotor system and outline some specific hypotheses about how sensory information is centrally processed to create motor commands for the VORs.

Viewing distance dependence of the vestibulo-ocular reflex during translation: extra-otolith influences

Despite nearly perfect gaze stability during natural head movements, the amplitude of the vestibulo-ocular reflex during passive head and body translation (TVOR) has been consistently reported to be undercompensatory during near target viewing. Here we have compared the rhesus monkey TVOR during pure head and body translation with the eye movements generated during eccentric yaw rotations, where both semicircular canal and otolith signals are activated. We found a significant increase in both the near target TVOR amplitude and its viewing distance dependence during eccentric rotations, as compared to pure translations. We conclude that the simultaneous activation of the horizontal semicircular canals result in an improvement of the viewing distance-dependence of the rhesus monkey TVOR.

Does Head Rotation Contribute to Gaze Stability During Passive Translations?

Active translations of human subjects are nearly perfectly compensated by a combined rotation of both the eyes and the head. Because vestibuloocular reflex (VOR) gain is less than perfect during passive translations with near targets in head-fixed subjects, there is a possibility that the compensatory head rotation observed during natural behavior represents a vestibularly driven head reflex [translational vestibulocollic reflex (TVCR)]. The TVCR could elicit a horizontal rotation of the head during lateral linear acceleration that contributes to gaze stabilization. To investigate this hypothesis, we examined whether a horizontal rotation of the head contributes to gaze stability during passive lateral translation in rhesus monkeys whose head was free to rotate in the horizontal plane. Motion frequency was varied between 0.5 and 5 Hz while animals fixated targets at distances of 12–102 cm. We did not find evidence supporting the existence of a TVCR. Specifically, during motion at frequencies between 0.5 and 2 Hz, horizontal head rotation was negligible. During 4- and 5-Hz oscillations, there was a clear and consistent horizontal rotation of the head, but responses were always anticompensatory to gaze stabilization; that is, the head rotated in the same direction as head translation and oppositely to the direction of gaze rotation. Furthermore, there was no difference in gaze stability between the head-free and head-fixed conditions. Thus we conclude that the compensatory head rotation observed in human studies of active gaze movements could represent a strategy and/or a motor command contribution to gaze stabilization, rather than a simple vestibularly driven reflex.

Do visual cues contribute to the neural estimate of viewing distance used by the oculomotor system?

Perceived shape and depth judgments that require knowledge of viewing distance are strongly influenced by both vergence angle and the pattern of vertical disparities across large visual fields. On the basis of this established contribution of visual cues to the neural estimate of viewing distance, we hypothesized that the oculomotor system would also make use of high-level visual cues to distance. To address this hypothesis, we investigated how compensatory eye movements during whole-body translation scale with viewing distance. Monkeys viewed large-field (85 x 68 degrees ) random-dot stereograms that were rear projected onto a fixed screen and simulated either a textured wall or pyramid at different viewing distances. In these stereograms, we independently manipulated vergence angle, horizontal and vertical disparity gradients, relative horizontal disparities, and textural cues to viewing distance. For comparison, random-dot patterns were also projected onto a moveable screen placed at different physical distances from the animal. Several cycles of left-right sinusoidal motion of the monkey at 5 Hz were interleaved with several cycles of motion in darkness, and the relationship between eye movement responses and viewing distance was quantified. As expected from previous work, the amplitude of compensatory eye movements depended strongly on vergence angle. Although visual cues to distance had a statistically significant effect on eye movements, these effects were approximately 20-fold weaker than the effect of vergence angle. We conclude that sensory and motor systems do not share a common neural estimate of viewing distance and that the oculomotor system relies far less on visual cues than the perceptual system.

Vestibular contributions to gaze stability during transient forward and backward motion

The accuracy with which the vestibular system anticipates and compensates for the visual consequences of translation during forward and backward movements was investigated with transient motion profiles in rhesus monkeys trained to fixate targets on an isovergence screen. Early during motion when visuomotor reflexes remain relatively ineffective and vestibular-driven mechanisms have an important role for controlling the movement of the eyes, a large asymmetry was observed for forward and backward heading directions. During forward motion, ocular velocity gains increased steeply and reached near unity gains as early as 40-50 ms after motion onset. In addition, instantaneous directional errors also remained <10 degrees for forward headings. In contrast, backward motion was characterized by smaller vestibular gains and larger directional errors during the first 70 ms of the movement. To evaluate the accumulated retinal slip and vergence errors during the early epochs of motion when vestibular-driven mechanisms dominate gaze stability, the movement of a virtual fixation point defined by the intersection of the two gaze lines was quantitatively compared with the respective movement of the extinguished target in head coordinates. Both conjugate retinal slip and vergence errors were <0.2 degrees during the first 70 ms of the movement, with forward motion conjugate errors typically being smaller as compared with backward motion directions. Thus vestibularly driven gaze stabilization mechanisms can effectively minimize conjugate retinal slip errors as well as keep binocular disparity errors low during the open loop interval of head movement.

Three-dimensional ocular kinematics during eccentric rotations: evidence for functional rather than mechanical constraints

Previous studies have reported that the translational vestibuloocular reflex (TVOR) follows a three-dimensional (3D) kinematic behavior that is more similar to visually guided eye movements, like pursuit, rather than the rotational VOR (RVOR). Accordingly, TVOR rotation axes tilted with eye position toward an eye-fixed reference frame rather than staying relatively fixed in the head like in the RVOR. This difference arises because, contrary to the RVOR where peripheral image stability is functionally important, the TVOR like pursuit and saccades cares to stabilize images on the fovea. During most natural head and body movements, both VORs are simultaneously activated. In the present study, we have investigated in rhesus monkeys the 3D kinematics of the combined VOR during yaw rotation about eccentric axes. The experiments were motivated by and quantitatively compared with the predictions of two distinct hypotheses. According to the first (fixed-rule) hypothesis, an eye-position-dependent torsion is computed downstream of a site for RVOR/TVOR convergence, and the combined VOR axis would tilt through an angle that is proportional to gaze angle and independent of the relative RVOR/TVOR contributions to the total eye movement. This hypothesis would be consistent with the recently postulated mechanical constraints imposed by extraocular muscle pulleys. According to the second (image-stabilization) hypothesis, an eye-position-dependent torsion is computed separately for the RVOR and the TVOR components, implying a processing that takes place upstream of a site for RVOR/TVOR convergence. The latter hypothesis is based on the functional requirement that the 3D kinematics of the combined VOR should be governed by the need to keep images stable on the fovea with slip on the peripheral retina being dependent on the different functional goals of the two VORs. In contrast to the fixed-rule hypothesis, the data demonstrated a variable eye-position-dependent torsion for the combined VOR that was different for synergistic versus antagonistic RVOR/TVOR interactions. Furthermore, not only were the eye-velocity tilt slopes of the combined VOR as much as 10 times larger than what would be expected based on extraocular muscle pulley location, but also eye velocity during antagonistic RVOR/TVOR combinations often tilted opposite to gaze. These results are qualitatively and quantitatively consistent with the image-stabilization hypothesis, suggesting that the eye-position-dependent torsion is computed separately for the RVOR and the TVOR and that the 3D kinematics of the combined VOR are dependent on functional rather than mechanical constraints.

Foveal versus full-field visual stabilization strategies for translational and rotational head movements

Because we view the world from a constantly shifting platform when our head and body move in space, vestibular and visuomotor reflexes are critical to maintain visual acuity. In contrast to the phylogenetically old rotational vestibulo-ocular reflex (RVOR), it has been proposed that the translational vestibulo-ocular reflex (TVOR) represents a newly developed vestibular-driven mechanism that is important for foveal vision and stereopsis. To investigate the hypothesis that the function of the TVOR is indeed related to foveal (as opposed to full-field) image stabilization, we compared the three-dimensional ocular kinematics during lateral translation and rotational movements with those during pursuit of a small moving target in four rhesus monkeys. Specifically, we tested whether TVOR rotation axes tilt with eye position as in visually driven systems such as pursuit, or whether they stay relatively fixed in the head as in the RVOR. We found a significant dependence of three-dimensional eye velocity on eye position that was independent of viewing distance and viewing conditions (full-field, single target, or complete darkness). The slopes for this eye-position dependence averaged 0.7 +/- 0.07 for the TVOR, compared with 0.6 +/- 0.07 for visually guided pursuit eye movements and 0.18 +/- 0.09 for the RVOR. Because the torsional tilt versus vertical gaze slopes during translation were slightly higher than those during pursuit, three-dimensional eye movements during translation could partly reflect a compromise between the two different solutions for foveal gaze control, that of Listing's law and minimum velocity strategies. These results with respect to three-dimensional kinematics provide additional support for a functional difference in the two vestibular-driven mechanisms for visual stability during rotations and translations and establish clearly the functional goal of the TVOR as that for foveal visual acuity.

Motor scaling by viewing distance of early visual motion signals during smooth pursuit

The geometry of gaze stabilization during head translation requires eye movements to scale proportionally to the inverse of target distance. Such a scaling has indeed been demonstrated to exist for the translational vestibuloocular reflex (TVOR), as well as optic flow-selective translational visuomotor reflexes (e.g., ocular following, OFR). The similarities in this scaling by a neural estimate of target distance for both the TVOR and the OFR have been interpreted to suggest that the two reflexes share common premotor processing. Because the neural substrates of OFR are partly shared by those for the generation of pursuit eye movements, we wanted to know if the site of gain modulation for TVOR and OFR is also part of a major pathway for pursuit. Thus, in the present studies, we investigated in rhesus monkeys whether initial eye velocity and acceleration during the open-loop portion of step ramp pursuit scales with target distance. Specifically, with visual motion identical on the retina during tracking at different distances (12, 24, and 60 cm), we compared the first 80 ms of horizontal pursuit. We report that initial eye velocity and acceleration exhibits either no or a very small dependence on vergence angle that is at least an order of magnitude less than the corresponding dependence of the TVOR and OFR. The results suggest that the neural substrates for motor scaling by target distance remain largely distinct from the main pathway for pursuit.

Differential sensorimotor processing of vestibulo-ocular signals during rotation and translation

Rotational and translational vestibulo-ocular reflexes (RVOR and TrVOR) function to maintain stable binocular fixation during head movements. Despite similar functional roles, differences in behavioral, neuroanatomical, and sensory afferent properties suggest that the sensorimotor processing may be partially distinct for the RVOR and TrVOR. To investigate the currently poorly understood neural correlates for the TrVOR, the activities of eye movement-sensitive neurons in the rostral vestibular nuclei were examined during pure translation and rotation under both stable gaze and suppression conditions. Two main conclusions were made. First, the 0.5 Hz firing rates of cells that carry both sensory head movement and motor-like signals during rotation were more strongly related to the oculomotor output than to the vestibular sensory signal during translation. Second, neurons the firing rates of which increased for ipsilaterally versus contralaterally directed eye movements (eye-ipsi and eye-contra cells, respectively) exhibited distinct dynamic properties during TrVOR suppression. Eye-ipsi neurons demonstrated relatively flat dynamics that was similar to that of the majority of vestibular-only neurons. In contrast, eye-contra cells were characterized by low-pass filter dynamics relative to linear acceleration and lower sensitivities than eye-ipsi cells. In fact, the main secondary eye-contra neuron in the disynaptic RVOR pathways (position-vestibular-pause cell) that exhibits a robust modulation during RVOR suppression did not modulate during TrVOR suppression. To explain these results, a simple model is proposed that is consistent with the known neuroanatomy and postulates differential projections of sensory canal and otolith signals onto eye-contra and eye-ipsi cells, respectively, within a shared premotor circuitry that generates the VORs.