Analysis of primate IBN spike trains using system identification techniques. II. Relationship to gaze, eye, and head movement dynamics during head-free gaze shifts

Pathways for Naturalistic Looking Behavior in Primate I: Behavioral Characteristics and Brainstem Circuits

Organisms control their visual worlds by moving their eyes, heads, and bodies. This control of "gaze" or "looking" is key to survival and intelligence, but our investigation of the underlying neural mechanisms in natural conditions is hindered by technical limitations. Recent advances have enabled measurement of both brain and behavior in freely moving animals in complex environments, expanding on historical head-fixed laboratory investigations. We juxtapose looking behavior as traditionally measured in the laboratory against looking behavior in naturalistic conditions, finding that behavior changes when animals are free to move or when stimuli have depth or sound. We specifically focus on the brainstem circuits driving gaze shifts and gaze stabilization. The overarching goal of this review is to reconcile historical understanding of the differential neural circuits for different "classes" of gaze shift with two inconvenient truths. (1) "classes" of gaze behavior are artificial. (2) The neural circuits historically identified to control each "class" of behavior do not operate in isolation during natural behavior. Instead, multiple pathways combine adaptively and non-linearly depending on individual experience. While the neural circuits for reflexive and voluntary gaze behaviors traverse somewhat independent brainstem and spinal cord circuits, both can be modulated by feedback, meaning that most gaze behaviors are learned rather than hardcoded. Despite this flexibility, there are broadly enumerable neural pathways commonly adopted among primate gaze systems. Parallel pathways which carry simultaneous evolutionary and homeostatic drives converge in superior colliculus, a layered midbrain structure which integrates and relays these volitional signals to brainstem gaze-control circuits.

Nucleus incertus provides eye velocity and position signals to the vestibulo-ocular cerebellum: a new perspective of the brainstem-cerebellum-hippocampus network

The network formed by the brainstem, cerebellum, and hippocampus occupies a central position to achieve navigation. Multiple physiological functions are implicated in this complex behavior. Among these, control of the eye-head and body movements is crucial. The gaze-holding system realized by the brainstem oculomotor neural integrator (ONI) situated in the nucleus prepositus hypoglossi and fine-tuned by the contribution of different regions of the cerebellum assumes the stability of the image on the fovea. This function helps in the recognition of environmental targets and defining appropriate navigational pathways further elaborated by the entorhinal cortex and hippocampus. In this context, an enigmatic brainstem area situated in front of the ONI, the nucleus incertus (NIC), is implicated in the dynamics of brainstem-hippocampus theta oscillation and contains a group of neurons projecting to the cerebellum. These neurons are characterized by burst tonic behavior similar to the burst tonic neurons in the ONI that convey eye velocity-position signals to the cerebellar flocculus. Faced with these forgotten cerebellar projections of the NIC, the present perspective discusses the possibility that, in addition to the already described pathways linking the cerebellum and the hippocampus via the medial septum, these NIC signals related to the vestibulo-ocular reflex and gaze holding could participate in the hippocampal control of navigation.

Dysfunctional mode switching between fixation and saccades: collaborative insights into two unusual clinical disorders

Voluntary rapid eye movements (saccades) redirect the fovea toward objects of visual interest. The saccadic system can be considered as a dual-mode system: in one mode the eye is fixating, in the other it is making a saccade. In this review, we consider two examples of dysfunctional saccades, interrupted saccades in late-onset Tay-Sachs disease and gaze-position dependent opsoclonus after concussion, which fail to properly shift between fixation and saccade modes. Insights and benefits gained from bi-directional collaborative exchange between clinical and basic scientists are emphasized. In the case of interrupted saccades, existing mathematical models were sufficiently detailed to provide support for the cause of interrupted saccades. In the case of gaze-position dependent opsoclonus, existing models could not explain the behavior, but further development provided a reasonable hypothesis for the mechanism underlying the behavior. Collaboration between clinical and basic science is a rich source of progress for developing biologically plausible models and understanding neurological disease. Approaching a clinical problem with a specific hypothesis (model) in mind often prompts new experimental tests and provides insights into basic mechanisms.

Eye position-dependent opsoclonus in mild traumatic brain injury

Opsoclonus consists of bursts of involuntary, multidirectional, back-to-back saccades without an intersaccadic interval. We report a 60-year-old man with post-concussive headaches and disequilibrium who had small amplitude opsoclonus in left gaze, along with larger amplitude flutter during convergence. Examination was otherwise normal and brain MRI was unremarkable. Video-oculography demonstrated opsoclonus predominantly in left gaze and during pursuit in the left hemifield, which improved as post-concussive symptoms improved. Existing theories of opsoclonus mechanisms do not account for this eye position-dependence. We discuss theoretical mechanisms of this behavior, including possible dysfunction of frontal eye field and/or cerebellar vermis neurons; review ocular oscillations in traumatic brain injury; and consider the potential relationship between the larger amplitude flutter upon convergence and post-traumatic ocular oscillations.

Modeling eye-head gaze shifts in multiple contexts without motor planning

During gaze shifts, the eyes and head collaborate to rapidly capture a target (saccade) and fixate it. Accordingly, models of gaze shift control should embed both saccadic and fixation modes and a mechanism for switching between them. We demonstrate a model in which the eye and head platforms are driven by a shared gaze error signal. To limit the number of free parameters, we implement a model reduction approach in which steady-state cerebellar effects at each of their projection sites are lumped with the parameter of that site. The model topology is consistent with anatomy and neurophysiology, and can replicate eye-head responses observed in multiple experimental contexts: 1) observed gaze characteristics across species and subjects can emerge from this structure with minor parametric changes; 2) gaze can move to a goal while in the fixation mode; 3) ocular compensation for head perturbations during saccades could rely on vestibular-only cells in the vestibular nuclei with postulated projections to burst neurons; 4) two nonlinearities suffice, i.e., the experimentally-determined mapping of tectoreticular cells onto brain stem targets and the increased recruitment of the head for larger target eccentricities; 5) the effects of initial conditions on eye/head trajectories are due to neural circuit dynamics, not planning; and 6) "compensatory" ocular slow phases exist even after semicircular canal plugging, because of interconnections linking eye-head circuits. Our model structure also simulates classical vestibulo-ocular reflex and pursuit nystagmus, and provides novel neural circuit and behavioral predictions, notably that both eye-head coordination and segmental limb coordination are possible without trajectory planning.

Activity of long-lead burst neurons in pontine reticular formation during head-unrestrained gaze shifts

Primates explore a visual scene through a succession of saccades. Much of what is known about the neural circuitry that generates these movements has come from neurophysiological studies using subjects with their heads restrained. Horizontal saccades and the horizontal components of oblique saccades are associated with high-frequency bursts of spikes in medium-lead burst neurons (MLBs) and long-lead burst neurons (LLBNs) in the paramedian pontine reticular formation. For LLBNs, the high-frequency burst is preceded by a low-frequency prelude that begins 12-150 ms before saccade onset. In terms of the lead time between the onset of prelude activity and saccade onset, the anatomical projections, and the movement field characteristics, LLBNs are a heterogeneous group of neurons. Whether this heterogeneity is endemic of multiple functional subclasses is an open question. One possibility is that some may carry signals related to head movement. We recorded from LLBNs while monkeys performed head-unrestrained gaze shifts, during which the kinematics of the eye and head components were dissociable. Many cells had peak firing rates that never exceeded 200 spikes/s for gaze shifts of any vector. The activity of these low-frequency cells often persisted beyond the end of the gaze shift and was usually related to head-movement kinematics. A subset was tested during head-unrestrained pursuit and showed clear modulation in the absence of saccades. These "low-frequency" cells were intermingled with MLBs and traditional LLBNs and may represent a separate functional class carrying signals related to head movement.

Contribution of the frontal eye field to gaze shifts in the head-unrestrained rhesus monkey: neuronal activity

The frontal eye field (FEF) has a strong influence on saccadic eye movements with the head restrained. With the head unrestrained, eye saccades combine with head movements to produce large gaze shifts, and microstimulation of the FEF evokes both eye and head movements. To test whether the dorsomedial FEF provides commands for the entire gaze shift or its separate eye and head components, we recorded extracellular single-unit activity in monkeys trained to make large head-unrestrained gaze shifts. We recorded 80 units active during gaze shifts, and closely examined 26 of these that discharged a burst of action potentials that preceded horizontal gaze movements. These units were movement or visuomovement related and most exhibited open movement fields with respect to amplitude. To reveal the relations of burst parameters to gaze, eye, and/or head movement metrics, we used behavioral dissociations of gaze, eye, and head movements and linear regression analyses. The burst number of spikes (NOS) was strongly correlated with movement amplitude and burst temporal parameters were strongly correlated with movement temporal metrics for eight gaze-related burst neurons and five saccade-related burst neurons. For the remaining 13 neurons, the NOS was strongly correlated with the head movement amplitude, but burst temporal parameters were most strongly correlated with eye movement temporal metrics (head-eye-related burst neurons, HEBNs). These results suggest that FEF units do not encode a command for the unified gaze shift only; instead, different units may carry signals related to the overall gaze shift or its eye and/or head components. Moreover, the HEBNs exhibit bursts whose magnitude and timing may encode a head displacement signal and a signal that influences the timing of the eye saccade, thereby serving as a mechanism for coordinating the eye and head movements of a gaze shift.

Human eye-head gaze shifts preserve their accuracy and spatiotemporal trajectory profiles despite long-duration torque perturbations that assist or oppose head motion

Humans routinely use coordinated eye-head gaze saccades to rapidly and accurately redirect the line of sight (Land MF. Vis Neurosci 26: 51–62, 2009). With a fixed body, the gaze control system combines visual, vestibular, and neck proprioceptive sensory information and coordinates two moving platforms, the eyes and head. Classic engineering tools have investigated the structure of motor systems by testing their ability to compensate for perturbations. When a reaching movement of the hand is subjected to an unexpected force field of random direction and strength, the trajectory is deviated and its final position is inaccurate. Here, we found that the gaze control system behaves differently. We perturbed horizontal gaze shifts with long-duration torques applied to the head that unpredictably either assisted or opposed head motion and very significantly altered the intended head trajectory. We found, as others have with brief head perturbations, that gaze accuracy was preserved. Unexpectedly, we found also that the eye compensated well—with saccadic and rollback movements—for long-duration head perturbations such that resulting gaze trajectories remained close to that when the head was not perturbed. However, the ocular compensation was best when torques assisted, compared with opposed, head motion. If the vestibuloocular reflex (VOR) is suppressed during gaze shifts, as currently thought, what caused invariant gaze trajectories and accuracy, early eye-direction reversals, and asymmetric compensations? We propose three mechanisms: a gaze feedback loop that generates a gaze-position error signal; a vestibular-to-oculomotor signal that dissociates self-generated from passively imposed head motion; and a saturation element that limits orbital eye excursion.

Optimal control of saccades by spatial-temporal activity patterns in the monkey superior colliculus

A major challenge in computational neurobiology is to understand how populations of noisy, broadly-tuned neurons produce accurate goal-directed actions such as saccades. Saccades are high-velocity eye movements that have stereotyped, nonlinear kinematics; their duration increases with amplitude, while peak eye-velocity saturates for large saccades. Recent theories suggest that these characteristics reflect a deliberate strategy that optimizes a speed-accuracy tradeoff in the presence of signal-dependent noise in the neural control signals. Here we argue that the midbrain superior colliculus (SC), a key sensorimotor interface that contains a topographically-organized map of saccade vectors, is in an ideal position to implement such an optimization principle. Most models attribute the nonlinear saccade kinematics to saturation in the brainstem pulse generator downstream from the SC. However, there is little data to support this assumption. We now present new neurophysiological evidence for an alternative scheme, which proposes that these properties reside in the spatial-temporal dynamics of SC activity. As predicted by this scheme, we found a remarkably systematic organization in the burst properties of saccade-related neurons along the rostral-to-caudal (i.e., amplitude-coding) dimension of the SC motor map: peak firing-rates systematically decrease for cells encoding larger saccades, while burst durations and skewness increase, suggesting that this spatial gradient underlies the increase in duration and skewness of the eye velocity profiles with amplitude. We also show that all neurons in the recruited population synchronize their burst profiles, indicating that the burst-timing of each cell is determined by the planned saccade vector in which it participates, rather than by its anatomical location. Together with the observation that saccade-related SC cells indeed show signal-dependent noise, this precisely tuned organization of SC burst activity strongly supports the notion of an optimal motor-control principle embedded in the SC motor map as it fully accounts for the straight trajectories and kinematic nonlinearity of saccades.

Mechanisms for generating and compensating for the smallest possible saccades

Microsaccades are small eye movements that occur during gaze fixation. Although taking place only when we attempt to stabilize gaze position, microsaccades can be understood by relating them to the larger voluntary saccades, which abruptly shift gaze position. Starting from this approach to microsaccade analysis, I show how it can lead to significant insight about the generation and functional role of these eye movements. Like larger saccades, microsaccades are now known to be generated by brainstem structures involved not only in compiling motor commands for eye movements, but also in identifying and selecting salient target locations in the visual environment. In addition, these small eye movements both influence and are influenced by sensory and cognitive processes in various areas of the brain, and in a manner that is similar to the interactions between larger saccades and sensory or cognitive processes. By approaching the study of microsaccades from the perspective of what has been learned about their larger counterparts, we are now in a position to make greater strides in our understanding of the function of the smallest possible saccadic eye movements.

Gaze shift duration, independent of amplitude, influences the number of spikes in the burst for medium-lead burst neurons in pontine reticular formation

Changes in the direction of the line of sight (gaze) allow successive sampling of the visual environment. Saccadic eye movements accomplish this goal when the head does not move. Medium-lead burst neurons (MLBs) in the paramedian pontine reticular formation (PPRF) discharge a high frequency burst of action potentials starting ~12 ms before the saccade begins. A subgroup of MLBs rostral of abducens nucleus monosynaptically excites oculomotor neurons. The number of spikes in the presaccadic burst is correlated with the amplitude of the horizontal component of the saccade, and the peak discharge rate is correlated with peak eye velocity. During head-unrestrained gaze shifts, a linear relationship between the number of action potentials in MLB bursts and gaze (but not eye) amplitude has been reported. The anatomical connection of MLBs to motor neurons and the similarity between the phasic motor neuron burst and MLB discharge have raised questions about the usefulness of counting spikes in MLBs to determine their role in eye-head coordination. We investigated this issue using a behavioral technique that permits a dissociation of eye movement amplitude and duration during constant vector gaze shifts. Surprisingly, during gaze shifts of constant amplitude and direction, we observe a nearly linear, positive correlation between saccade duration and spike number associated with a negative correlation between spike number and saccade amplitude. These data constrain models of the oculomotor controller and may further define the time-dependence of hypothesized neural integration in this system.

Target modality determines eye-head coordination in nonhuman primates: implications for gaze control

We have studied eye-head coordination in nonhuman primates with acoustic targets after finding that they are unable to make accurate saccadic eye movements to targets of this type with the head restrained. Three male macaque monkeys with experience in localizing sounds for rewards by pointing their gaze to the perceived location of sources served as subjects. Visual targets were used as controls. The experimental sessions were configured to minimize the chances that the subject would be able to predict the modality of the target as well as its location and time of presentation. The data show that eye and head movements are coordinated differently to generate gaze shifts to acoustic targets. Chiefly, the head invariably started to move before the eye and contributed more to the gaze shift. These differences were more striking for gaze shifts of <20-25° in amplitude, to which the head contributes very little or not at all when the target is visual. Thus acoustic and visual targets trigger gaze shifts with different eye-head coordination. This, coupled to the fact that anatomic evidence involves the superior colliculus as the link between auditory spatial processing and the motor system, suggests that separate signals are likely generated within this midbrain structure.

Matching the oculomotor drive during head-restrained and head-unrestrained gaze shifts in monkey

High-frequency burst neurons in the pons provide the eye velocity command (equivalently, the primary oculomotor drive) to the abducens nucleus for generation of the horizontal component of both head-restrained (HR) and head-unrestrained (HU) gaze shifts. We sought to characterize how gaze and its eye-in-head component differ when an "identical" oculomotor drive is used to produce HR and HU movements. To address this objective, the activities of pontine burst neurons were recorded during horizontal HR and HU gaze shifts. The burst profile recorded on each HU trial was compared with the burst waveform of every HR trial obtained for the same neuron. The oculomotor drive was assumed to be comparable for the pair yielding the lowest root-mean-squared error. For matched pairs of HR and HU trials, the peak eye-in-head velocity was substantially smaller in the HU condition, and the reduction was usually greater than the peak head velocity of the HU trial. A time-varying attenuation index, defined as the difference in HR and HU eye velocity waveforms divided by head velocity [alpha = (H(hr) - E(hu))/H] was computed. The index was variable at the onset of the gaze shift, but it settled at values several times greater than 1. The index then decreased gradually during the movement and stabilized at 1 around the end of gaze shift. These results imply that substantial attenuation in eye velocity occurs, at least partially, downstream of the burst neurons. We speculate on the potential roles of burst-tonic neurons in the neural integrator and various cell types in the vestibular nuclei in mediating the attenuation in eye velocity in the presence of head movements.

Neuronal evidence for individual eye control in the primate cMRF

Previous single unit recordings and electrical stimulation have suggested that separate regions of the MRF participate in the control of vergence and conjugate eye movements. Neurons in the supraoculomotor area (SOA) have been found to encode symmetric vergence [Zhang, Y. et al. (1992). J. Neurophysiol., 67: 944-960] while neurons in the central MRF, the cMRF, located ventral to the SOA and lateral to the oculomotor nucleus are associated with conjugate eye movements [Waitzman, D.M. et al. (1996). J. Neurophysiol., 75(4): 1546-1572]. However, it remains unknown if cMRF neurons are strictly associated with conjugate movements since eye movements were recorded with a single eye coil in monkeys viewing visual stimuli at a distance of at least 50 cm. In the current study we addressed whether neurons in the cMRF might also encode vergence-related information. Interestingly, electrical stimulation elicited disconjugate saccades (contralateral eye moved more than the ipsilateral eye) from locations previously thought to elicit only conjugate saccades. Single unit recordings in this same area made in two rhesus monkeys trained to follow visual stimuli moved rapidly in depth along the axis of sight of an individual eye demonstrate that cMRF neurons do not simply encode conjugate information during disconjugate saccades; in fact our findings provide evidence that cMRF neurons are most closely associated with the movement of an individual eye. These results support the hypothesis that the midbrain shapes the activity of the pre-motor saccadic neurons by encoding integrated conjugate and vergence commands.

Coordination of the eyes and head during visual orienting

Changing the direction of the line of sight is essential for the visual exploration of our environment. When the head does not move, re-orientation of the visual axis is accomplished with high velocity, conjugate movements of the eyes known as saccades. Our understanding of the neural mechanisms that control saccadic eye movements has advanced rapidly as specific hypotheses have been developed, evaluated and sometimes rejected on the basis of new observations. Constraints on new hypotheses and new tests of existing models have often arisen from the careful assessment of behavioral observations. The definition of the set of features (or rules) of saccadic eye movements was critical in the development of hypotheses of their neural control. When the head is free to move, changes in the direction of the line of sight can involve simultaneous saccadic eye movements and movements of the head. When the head moves in conjunction with the eyes to accomplish these shifts in gaze direction, the rules that helped define head-restrained saccadic eye movements are altered. For example, the slope relationship between duration and amplitude for saccadic eye movements is reversed (the slope is negative) during gaze shifts of similar amplitude initiated with the eyes in different orbital positions. Modifications to the hypotheses developed in head-restrained subjects may be needed to account for these new observations. This review briefly recounts features of head-restrained saccadic eye movements, and then describes some of the characteristics of coordinated eye-head movements that have led to development of new hypotheses describing the mechanisms of gaze shift control.

Coordination of eye and head components of movements evoked by stimulation of the paramedian pontine reticular formation

Constant frequency microstimulation of the paramedian pontine reticular formation (PPRF) in head-restrained monkeys evokes a constant velocity eye movement. Since the PPRF receives significant projections from structures that control coordinated eye-head movements, we asked whether stimulation of the pontine reticular formation in the head-unrestrained animal generates a combined eye-head movement or only an eye movement. Microstimulation of most sites yielded a constant-velocity gaze shift executed as a coordinated eye-head movement, although eye-only movements were evoked from some sites. The eye and head contributions to the stimulation-evoked movements varied across stimulation sites and were drastically different from the lawful relationship observed for visually-guided gaze shifts. These results indicate that the microstimulation activated elements that issued movement commands to the extraocular and, for most sites, neck motoneurons. In addition, the stimulation-evoked changes in gaze were similar in the head-restrained and head-unrestrained conditions despite the assortment of eye and head contributions, suggesting that the vestibulo-ocular reflex (VOR) gain must be near unity during the coordinated eye-head movements evoked by stimulation of the PPRF. These findings contrast the attenuation of VOR gain associated with visually-guided gaze shifts and suggest that the vestibulo-ocular pathway processes volitional and PPRF stimulation-evoked gaze shifts differently.

The brain stem saccadic burst generator encodes gaze in three-dimensional space

When we look between objects located at different depths the horizontal movement of each eye is different from that of the other, yet temporally synchronized. Traditionally, a vergence-specific neuronal subsystem, independent from other oculomotor subsystems, has been thought to generate all eye movements in depth. However, recent studies have challenged this view by unmasking interactions between vergence and saccadic eye movements during disconjugate saccades. Here, we combined experimental and modeling approaches to address whether the premotor command to generate disconjugate saccades originates exclusively in "vergence centers." We found that the brain stem burst generator, which is commonly assumed to drive only the conjugate component of eye movements, carries substantial vergence-related information during disconjugate saccades. Notably, facilitated vergence velocities during disconjugate saccades were synchronized with the burst onset of excitatory and inhibitory brain stem saccadic burst neurons (SBNs). Furthermore, the time-varying discharge properties of the majority of SBNs (>70%) preferentially encoded the dynamics of an individual eye during disconjugate saccades. When these experimental results were implemented into a computer-based simulation, to further evaluate the contribution of the saccadic burst generator in generating disconjugate saccades, we found that it carries all the vergence drive that is necessary to shape the activity of the abducens motoneurons to which it projects. Taken together, our results provide evidence that the premotor commands from the brain stem saccadic circuitry, to the target motoneurons, are sufficient to ensure the accurate control shifts of gaze in three dimensions.

Visual-Vestibular Interaction Hypothesis for the Control of Orienting Gaze Shifts by Brain Stem Omnipause Neurons

Models of combined eye-head gaze shifts all aim to realistically simulate behaviorally observed movement dynamics. One of the most problematic features of such models is their inability to determine when a saccadic gaze shift should be initiated and when it should be ended. This is commonly referred to as the switching mechanism mediated by omni-directional pause neurons (OPNs) in the brain stem. Proposed switching strategies implemented in existing gaze control models all rely on a sensory error between instantaneous gaze position and the spatial target. Accordingly, gaze saccades are initiated after presentation of an eccentric visual target and subsequently terminated when an internal estimate of gaze position becomes nearly equal to that of the target. Based on behavioral observations, we demonstrate that such a switching mechanism is insufficient and is unable to explain certain types of movements. We propose an improved hypothesis for how the OPNs control gaze shifts based on a visual-vestibular interaction of signals known to be carried on anatomical projections to the OPN area. The approach is justified by the analysis of recorded gaze shifts interrupted by a head brake in animal subjects and is demonstrated by implementing the switching mechanism in an anatomically based gaze control model. Simulated performance reveals that a weighted sum of three signals: gaze motor error, head velocity, and eye velocity, hypothesized as inputs to OPNs, successfully reproduces diverse behaviorally observed eye-head movements that no other existing model can account for.

Premotor correlates of integrated feedback control for eye-head gaze shifts

Simple activities like picking up the morning newspaper or catching a ball require finely coordinated movements of multiple body segments. How our brain readily achieves such kinematically complex yet remarkably precise multijoint movements remains a fundamental and unresolved question in neuroscience. Many prevailing theoretical frameworks ensure multijoint coordination by means of integrative feedback control. However, to date, it has proven both technically and conceptually difficult to determine whether the activity of motor circuits is consistent with integrated feedback coding. Here, we tested this proposal using coordinated eye-head gaze shifts as an example behavior. Individual neurons in the premotor network that command saccadic eye movements were recorded in monkeys trained to make voluntary eye-head gaze shifts. Head-movement feedback was experimentally controlled by unexpectedly and transiently altering the head trajectory midflight during a subset of movements. We found that the duration and dynamics of neuronal responses were appropriately updated following head perturbations to preserve global movement accuracy. Perturbation-induced increases in gaze shift durations were accompanied by equivalent changes in response durations so that neuronal activity remained tightly synchronized to gaze shift offset. In addition, the saccadic command signal was updated on-line in response to head perturbations applied during gaze shifts. Nearly instantaneous updating of responses, coupled with longer latency changes in overall discharge durations, indicated the convergence of at least two levels of feedback. We propose that this strategy is likely to have analogs in other motor systems and provides the flexibility required for fine-tuning goal-directed movements.

Temporal characteristics of neurons in the central mesencephalic reticular formation of head unrestrained monkeys

The accompanying paper demonstrated two distinct types of central mesencephalic reticular formation (cMRF) neuron that discharged before or after the gaze movement: pre-saccadic or post-saccadic. The movement fields of pre-saccadic neurons were most closely associated with gaze displacement. The movement fields of post-saccadic neurons were most closely associated with head displacement. Here we examine the relationships of the discharge patterns of these cMRF neurons with the temporal aspects of gaze or head movement. For pre-saccadic cMRF neurons with monotonically open movement fields, we demonstrate that burst duration correlated closely with gaze duration. In addition, the peak discharge of the majority of pre-saccadic neurons was closely correlated with peak gaze velocity. In contrast, discharge parameters of post-saccadic neurons were best correlated with the time of peak head velocity. However, the duration and peak discharge of post-saccadic discharge was only weakly related to the duration and peak velocity of head movement. As a result, for the majority of post-saccadic neurons the discharge waveform poorly correlated with the dynamics of head movement. We suggest that the discharge characteristics of pre-saccadic cMRF neurons with monotonically open movement fields are similar to that of direction long-lead burst neurons found previously in the paramedian portion of the pontine reticular formation (PPRF; Hepp and Henn 1983). In light of their anatomic connections with the PPRF, these pre-saccadic neurons could form a parallel pathway that participates in the transformation from the spatial coding of gaze in the superior colliculus (SC) to the temporal coding displayed by excitatory burst neurons of the PPRF. In contrast, closed and non-monotonically open movement field pre-saccadic neurons could play a critical role in feedback to the SC. The current data do not support a role for post-saccadic cMRF neurons in the direct control of head movements, but suggest that they may serve a feedback or reafference function, providing a signal of current head amplitude to upstream regions involved in head control.

Behavior of the position vestibular pause (PVP) interneurons of the vestibuloocular reflex during head-free gaze shifts in the monkey

Most behavioral studies indicate that the efficacy (gain) of the vestibuloocular reflex (VOR) in primates is modulated during the voluntary head movements that accompany large shifts in the direction of gaze. However, the timing and degree of this modulation is the subject of some debate. The neurophysiological substrate for this apparent gain reduction has been sought in the behavior of the type I position vestibular pause (PVP) neuron, a well-known type of interneuron in the direct VOR pathway. With the head fixed, PVPs increase their firing rates with contraversive eye position and with ipsiversive passive head rotation and also cease firing (pause) for the duration of ipsiversive saccades. During head-free ipsiversive gaze shifts, the eyes and head move in the same direction. If the vestibular signal carried by PVPs provides the primary drive for the VOR, the vestibular signal should be present during ipsiversive gaze shifts to the degree that the VOR is present. Of 25 type I PVPs recorded, 21 ceased their discharge for the entire duration of the rapid, eye-saccade component of an ipsiversive gaze shift. The resumption of activity occurred, on average, 13 ms after the end of the saccade. These results suggest that the activity of the vast majority of PVP neurons do not reflect the state of the VOR, but rather PVPs are completely eliminated from participation in the reflex during head-free gaze movements. We conclude that if any modulation of the VOR does exist, it must occur through other, probably longer-latency, pathways.

Time course of vestibuloocular reflex suppression during gaze shifts

Although numerous investigations have probed the status of the vestibuloocular (VOR) during gaze shifts, its exact status remains strangely elusive. The goal of the present study was to precisely evaluate the dynamics of VOR suppression immediately before, throughout, and just after gaze shifts. A torque motor was used to apply rapid (100 degrees/s), short-duration (20-30 ms) horizontal head perturbations in three Rhesus monkeys. The status of the VOR elicited by this transient head perturbation was first compared during 15, 40, and 60 degrees gaze shifts. The level of VOR suppression just after gaze-shift onset (40 ms) increased with gaze-shift amplitude in two monkeys, approaching values of 80 and 35%. In contrast, in the third monkey, the VOR was not significantly attenuated for all gaze-shift amplitudes. The time course of VOR attenuation was then studied in greater detail for all three monkeys by imposing the same short-duration head perturbations 40, 100, and 150 ms after the onset of 60 degrees gaze shifts. Overall we found a consistent trend, in which VOR suppression was maximal early in the gaze shift and progressively recovered to reach normal values near gaze-shift end. However, the high variability across subjects prevented establishing a unifying description of the absolute level and time course of VOR suppression during gaze shifts. We propose that differences in behavioral strategies may account, at least in part, for these differences between subjects.

Dissociating self-generated from passively applied head motion: neural mechanisms in the vestibular nuclei

The ability to distinguish sensory inputs that are a consequence of our own actions from those that result from changes in the external world is essential for perceptual stability and accurate motor control. To accomplish this, it has been proposed that an internal prediction of the consequences of our actions is compared with the actual sensory input to cancel the resultant self-generated activation. Here, we provide evidence for this hypothesis at an early stage of processing in the vestibular system. Previous studies have shown that neurons in the vestibular nucleus, which receive direct inputs from vestibular afferent fibers, are responsive to passively applied head movements. However, these same neurons do not reliably encode head velocity resulting from self-generated movements of the head on the body. In this study, we examined the mechanism that underlies the selective elimination of vestibular sensitivity to active head-on-body rotations. Individual neurons were recorded in monkeys making active head movements. The correspondence between intended and actual head movement was experimentally controlled. We found that a cancellation signal was gated into the vestibular nuclei only in conditions in which the activation of neck proprioceptors matched that expected on the basis of the neck motor command. This finding suggests that vestibular signals that arise from self-generated head movements are inhibited by a mechanism that compares the internal prediction of the sensory consequences by the brain to the actual resultant sensory feedback. Because self-generated vestibular inputs are selectively cancelled early in processing, we propose that this gating is important for the computation of spatial orientation and control of posture by higher-order structures.

Electrical stimulation of rhesus monkey nucleus reticularis gigantocellularis. II. Effects on metrics and kinematics of ongoing gaze shifts to visual targets

Saccade kinematics are altered by ongoing head movements. The hypothesis that a head movement command signal, proportional to head velocity, transiently reduces the gain of the saccadic burst generator (Freedman 2001, Biol Cybern 84:453-462) can account for this observation. Using electrical stimulation of the rhesus monkey nucleus reticularis gigantocellularis (NRG) to alter the head contribution to ongoing gaze shifts, two critical predictions of this gaze control hypothesis were tested. First, this hypothesis predicts that activation of the head command pathway will cause a transient reduction in the gain of the saccadic burst generator. This should alter saccade kinematics by initially reducing velocity without altering saccade amplitude. Second, because this hypothesis does not assume that gaze amplitude is controlled via feedback, the added head contribution (produced by NRG stimulation on the side ipsilateral to the direction of an ongoing gaze shift) should lead to hypermetric gaze shifts. At every stimulation site tested, saccade kinematics were systematically altered in a way that was consistent with transient reduction of the gain of the saccadic burst generator. In addition, gaze shifts produced during NRG stimulation were hypermetric compared with control movements. For example, when targets were briefly flashed 30 degrees from an initial fixation location, gaze shifts during NRG stimulation were on average 140% larger than control movements. These data are consistent with the predictions of the tested hypothesis, and may be problematic for gaze control models that rely on feedback control of gaze amplitude, as well as for models that do not posit an interaction between head commands and the saccade burst generator.

Single cell signals: an oculomotor perspective

We examine the activity of individual neurons in three different brain areas where firing rate, number of spikes (the integral of discharge rate), and the location of the active cell within a motor map are used as coding schemes. The correlations between single cell activity and the parameters of a movement range from extremely tight (motoneurons) to non-existent (superior colliculus). We argue that the relationship between the activity of single cell activity and global aspects of behavior are best described as coarse coding for all three types of neuron. We also present evidence, in some cases in a preliminary and suggestive form, that the distribution of spikes in time, rather than average firing rate, may be important for all three neuron types, including those using a place code. Finally, we describe difficulties encountered in obtaining an estimate of the motor command when more than one oculomotor system is active.

Neural control of 3-D gaze shifts in the primate

The neural mechanisms that specify target locations for gaze shifts and then convert these into desired patterns of coordinated eye and head movements are complex. Much of this complexity is only revealed when one takes a realistic three-dimensional (3-D) view of these processes, where fundamental computational problems such as kinematic redundancy, reference-frame transformations, and non-commutativity emerge. Here we review the underlying mechanisms and solutions for these problems, starting with a consideration of the kinematics of 3-D gaze shifts in human and non-human primates. We then consider the neural mechanisms, including cortical representation of gaze targets, the nature of the gaze motor command used by the superior colliculus, and how these gaze commands are decomposed into brainstem motor commands for the eyes and head. A general conclusion is that fairly simple coding mechanisms may be used to represent gaze at the cortical and collicular level, but this then necessitates complexity for the spatial updating of these representations and in the brainstem sensorimotor transformations that convert these signals into eye and head movements.

Firing behaviour of squirrel monkey eye movement-related vestibular nucleus neurons during gaze saccades

The firing behaviour of vestibular nucleus neurons putatively involved in producing the vestibulo-ocular reflex (VOR) was studied during active and passive head movements in squirrel monkeys. Single unit recordings were obtained from 14 position-vestibular (PV) neurons, 30 position-vestibular-pause (PVP) neurons and 9 eye-head-vestibular (EHV) neurons. Neurons were sub-classified as type I or II based on whether they were excited or inhibited during ipsilateral head rotation. Different classes of cell exhibited distinctive responses during active head movements produced during and after gaze saccades. Type I PV cells were nearly as sensitive to active head movements as they were to passive head movements during saccades. Type II PV neurons were insensitive to active head movements both during and after gaze saccades. PVP and EHV neurons were insensitive to active head movements during saccadic gaze shifts, and exhibited asymmetric sensitivity to active head movements following the gaze shift. PVP neurons were less sensitive to on-direction head movements during the VOR after gaze saccades, while EHV neurons exhibited an enhanced sensitivity to head movements in their on direction. Vestibular signals related to the passive head movement were faithfully encoded by vestibular nucleus neurons. We conclude that central VOR pathway neurons are differentially sensitive to active and passive head movements both during and after gaze saccades due primarily to an input related to head movement motor commands. The convergence of motor and sensory reafferent inputs on VOR pathways provides a mechanism for separate control of eye and head movements during and after saccadic gaze shifts.

Dynamics of abducens nucleus neuron discharges during disjunctive saccades

In this report, we provide the first characterization of abducens nucleus neuron (ABN) discharge dynamics during horizontal disjunctive saccades. These movements function to rapidly transfer the visual axes between targets located at different eccentricities and depths. Our primary objective was to determine whether the signals carried by ABNs during these movements are appropriate to drive the motion of the eye to which they project. We also asked whether ABNs encode eye movements similarly during disjunctive saccades and disjunctive fixation. To address the first objective we 1) assessed whether we could predict the discharge dynamics of individual neurons during disjunctive saccades based on their discharge properties during conjugate saccades and 2) directly estimated the sensitivity of individual neurons to either the ipsilateral/contralateral eye or the conjugate/vergence position and velocity using bootstrap statistics. Our main finding was that during disjunctive saccades in the direction ipsilateral to the recording site (ON-direction), the majority of ABNs preferentially encoded the velocity and the position of the ipsilateral eye. The remaining neurons predominantly encoded the conjugate motion of the eyes (i.e., were equally sensitive to the motion of both eyes). Generally, ipsilateral/contralateral eye based models better described neuronal discharges than conjugate/vergence based models, yet both model structures yielded similar conclusions. Moreover, the preferred eye of individual neurons based on their position and velocity sensitivities were generally well matched. We also found that for saccades in the OFF-direction, the pausing behavior of ABNs was similar during conjugate and disjunctive saccades, with the exception that for movements of small amplitudes, more ABNs paused during conjugate saccades. Finally, we found that putative motoneurons and internuclear neurons encoded ON- and OFF-direction disjunctive saccades in a similar manner. To address our second objective, we compared the discharge properties of individual ABNs during disjunctive saccades and disjunctive fixation. Good coherence was observed between the preferred eye of individual ABNs during the two behaviors. Taken together, our results indicate that although individual ABNs can encode the motion of both eyes to various degrees, the population drive of ABNs accounts for most of the movement of the ipsilateral eye during disjunctive saccades and disjunctive fixation.

In multiple-step gaze shifts: omnipause (OPNs) and collicular fixation neurons encode gaze position error; OPNs gate saccades

The superior colliculus (SC), via its projections to the pons, is a critical structure for driving rapid orienting movements of the visual axis, called gaze saccades, composed of coordinated eye-head movements. The SC contains a motor map that encodes small saccade vectors rostrally and large ones caudally. A zone in the rostral pole may have a different function. It contains superior colliculus fixation neurons (SCFNs) with probable projections to omnipause neurons (OPNs) of the pons. SCFNs and OPNs discharge tonically during visual fixation and pause during single-step gaze saccades. The OPN tonic discharge inhibits saccades and its cessation (pause) permits saccade generation. We have proposed that SCFNs control the OPN discharge. We compared the discharges of SCFNs and OPNs recorded while cats oriented horizontally, to the left and right, in the dark to a remembered target. Cats used multiple-step gaze shifts composed of a series of small gaze saccades, of variable amplitude and number, separated by periods of variable duration (plateaus) in which gaze was immobile or moving at low velocity (<25 degrees /s). Just after contralaterally (ipsilaterally) presented targets, the firing frequency of SCFNs decreased to almost zero (remained constant at background). As multiple-step gaze shifts progressed in either direction in the dark, these activity levels prevailed until the distance between gaze and target [gaze position error (GPE)] reached approximately 16 degrees. At this point, firing frequency gradually increased, without saccade-related pauses, until a maximum was reached when gaze arrived on target location (GPE = 0 degrees). SCFN firing frequency encoded GPE; activity was not correlated to characteristics or occurrence of gaze saccades. By comparison, after target presentation to left or right, OPN activity remained steady at pretarget background until first gaze saccade onset, during which activity paused. During the first plateau, activity resumed at a level lower than background and continued at this level during subsequent plateaus until GPE approximately 8 degrees was reached. As GPE decreased further, tonic activity during plateaus gradually increased until a maximum (greater than background) was reached when gaze was on goal (GPE = 0 degrees). OPNs, like SCFNs, encoded GPE, but they paused during every gaze saccade, thereby revealing, unlike for SCFNs, strong coupling to motor events. The firing frequency increase in SCFNs as GPE decreased, irrespective of trajectory characteristics, implies these cells get feedback on GPE, which they may communicate to OPNs. We hypothesize that at the end of a gaze-step sequence, impulses from SCFNs onto OPNs may suppress further movements away from the target.

Neck muscle responses to stimulation of monkey superior colliculus. II. Gaze shift initiation and volitional head movements

We report neck muscle activity and head movements evoked by electrical stimulation of the superior colliculus (SC) in head-unrestrained monkeys. Recording neck electromyography (EMG) circumvents complications arising from the head's inertia and the kinetics of muscle force generation and allows precise assessment of the neuromuscular drive to the head plant. This study served two main purposes. First, we sought to test the predictions made in the companion paper of a parallel drive from the SC onto neck muscles. Low-current, long-duration stimulation evoked both neck EMG responses and head movements either without or prior to gaze shifts, testifying to a SC drive to neck muscles that is independent of gaze-shift initiation. However, gaze-shift initiation was linked to a transient additional EMG response and head acceleration, confirming the presence of a SC drive to neck muscles that is dependent on gaze-shift initiation. We forward a conceptual neural architecture and suggest that this parallel drive provides the oculomotor system with the flexibility to orient the eyes and head independently or together, depending on the behavioral context. Second, we compared the EMG responses evoked by SC stimulation to those that accompanied volitional head movements. We found characteristic features in the underlying pattern of evoked neck EMG that were not observed during volitional head movements in spite of the seemingly natural kinematics of evoked head movements. These features included reciprocal patterning of EMG activity on the agonist and antagonist muscles during stimulation, a poststimulation increase in the activity of antagonist muscles, and synchronously evoked responses on agonist and antagonist muscles regardless of initial horizontal head position. These results demonstrate that the electrically evoked SC drive to the head cannot be considered as a neural replicate of the SC drive during volitional head movements and place important new constraints on the interpretation of electrically evoked head movements.

Neural network models for the gaze shift system in the superior colliculus and cerebellum

We investigate the role that the superior colliculus (SC) and the cerebellum might play in generating gaze shifts. The discharge of cells in the intermediate layers of the SC is tightly linked to the occurrence of saccades. Many studies have demonstrated that the cerebellum is involved in both eye and head movements. When the head is unrestrained, large amplitude gaze shifts are composed of coordinated eye and head movements. In this study, we propose that the gaze saccades system is controlled by a feedback loop between the SC and the cerebellum. The SC only encodes retinal coordinates and controls the eye displacement (to move the fovea to the target), while the cerebellum deals with the gaze programming and controls the head displacement. When a target appears in space, the buildup cells within the SC decode the target signal in the retina before the saccade onset, and input the signal of the gaze displacement to the cerebellum. The cells in the cerebellum vermis encode the initial position of the eye in the orbit. The gaze displacement is decomposed into the head amplitude and the eye amplitude within the cerebellum. There are two output signals from the cerebellum. One signal controls the head movement. The other is projected back to the SC, and forms a component of the saccade vector to control the eye movement. The sum of the vectors provided by the cerebellum and the vector provided by the burst cells in the SC indicates the direction and the amplitude of the desired movement of the eye during the saccade. We propose a cerebellum model to predict the displacements of the eye and head under the condition that the position of the target signal in the retina and the initial position of the eye in the orbit are known. The results from the model are close to that observed physiologically. We conclude that before gaze shift onset, the cerebellum may play an important role in decomposing the gaze displacement into an eye amplitude and head amplitude signal.

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.

Contribution of the superior colliculus and the mesencephalic reticular formation to gaze control

Converging lines of evidence support a role for the intermediate and deep layers of the superior colliculus (SC) and the mesencephalic reticular formation (MRF) in the control of combined head and eye movements (i.e., gaze). Recent microstimulation, single-cell recording, and lesion experiments are reviewed in which monkeys are free to move their heads. Cells in the SC discharge in advance of combined head and eye movements and most likely provide a gaze error signal to downstream structures. In contrast, the neurons in the MRF are of at least two types. Eye cells have features that are similar to neurons in the rostral portion of the SC, but fire before the onset of horizontal eye movments. A second group of MRF neurons begin to fire after the onset of the gaze shift and are most closely associated with movements of the head. The peak discharge of these late-onset MRF neurons occurs near the peak head velocity. Stimulation in the rostral SC generates eye movements with fixed amplitude and direction. A similar response is noted after stimulation of the more dorsal portion of the caudal MRF. Stimulation in the caudal portion of the SC produces combined head and eye movements of fixed amplitude. Electrical activation of the more ventral portions of the caudal MRF generates goal-directed and centering eye movements. Temporary inactivation of the SC with the GABA agonist muscimol generated hypometria and curved trajectories of contralateral eye movements. Inactivation of the caudal MRF produced contralateral hypermetria and ipsilateral hypometria of saccades. Release of the monkey's head demonstrated a profound contralateral head tilt. Taken together, these data suggest that the gaze signal generated in the SC is filtered by neurons in the MRF to generate a feedback signal of eye motor error. The head signal found in the MRF could cancel a portion of the gaze signal coming from the SC in the form of head velocity feedback.

Cortical and subcortical contributions to coordinated eye and head movements

This paper summarizes recent experiments conducted by the authors - experiments that studied the behavioral characteristics of large gaze shifts and the neural bases of coordinated movements of the eyes and head.

Gaze-, eye-, and head-movement dynamics during closed- and open-loop gaze pursuit

Horizontal step-ramp stimuli were used to examine gaze-, eye-, and head-movement dynamics during head-unrestrained pursuit in two rhesus monkeys. In a first series of experiments, we characterized and compared head-restrained (HR) and -unrestrained (HU) pursuit responses to unpredictable, nonperiodic, constant velocity (20-80 degrees/s) stimuli. When the head was free to move, both monkeys used a combination of eye and head motion to initially fixate and then pursue the target. The pursuit responses (i.e., gaze responses) were highly stereotyped and nearly identical among the HR and HU conditions for a given step-ramp stimulus. In the HU condition, initial eye and initial head acceleration tended to increase as a function of target velocity but did not vary systematically with initial target eccentricity. In a second series of experiments, step-ramp stimuli (40 degrees/s) were presented, and, approximately 125 ms after pursuit onset, a constant retinal velocity error (RVE) was imposed for a duration of 300 ms. In each monkey, HR and HU gaze velocity was similarly affected by stabilizing the target with respect to the monkey's fovea (i.e., RVE = 0 degrees/s) and by moving the target with constant retinal velocity errors (i.e., RVE = +/- 10 degrees/s). In the HU condition, changes in both eye and head velocity trajectories contributed to the observed gaze velocity responses to imposed RVEs. We conclude that eye and head movements are not independently controlled during HU pursuit but rather are controlled, at least in part, by a shared upstream controller within the pursuit pathways.

Conjugate and vergence oscillations during saccades and gaze shifts: implications for integrated control of binocular movement

Saccades made between targets at optical infinity require both eyes to rotate by the same angle. Nevertheless, these saccades are consistently accompanied by transient vergence eye movements. Here we have investigated whether the dynamics of these vergence movements depend on the trajectory of the coincident conjugate movement, and whether moving the head during eye-head gaze shifts modifies vergence dynamics. In agreement with previous reports, saccades with more symmetric (i.e., "bell-shaped") conjugate velocity profiles were accompanied by stereotyped biphasic vergence transients (i.e., a divergence phase immediately followed by a convergence phase). However, we found that saccades with more asymmetric, oscillatory-like dynamics (characterized by a typical conjugate reacceleration of the eyes following the initial peak velocity) were systematically accompanied by more complex vergence movements that also exhibited oscillatory-like dynamics. These findings could be extended to conditions where the head was free to move: comparable conjugate and vergence oscillations were observed during head-restrained saccades and combined eye-head gaze shifts. The duration of the vergence oscillation increased with gaze shift amplitude, such that as many as four vergence phases (divergence-convergence-divergence-convergence) were recorded during 55 degrees gaze shifts (approximately 240 ms). To quantify these observations, we first determined whether conjugate and vergence peak velocities were systematically correlated. Conjugate peak velocity was linearly related to the peak velocity of the initial divergence phase for saccades and gaze shifts of all amplitudes, regardless of their dynamics. However, for more asymmetric saccades and gaze shifts, the subsequent convergence and divergence peak velocities were not correlated with either the initial peak conjugate velocity or the peak velocity of the conjugate reacceleration. Next, we determined that the duration of the different conjugate and vergence oscillation phases remained relatively constant across all saccades and gaze shifts, and that the conjugate and vergence profiles oscillated together at approximately 7.5-10 Hz. Using computer simulations, we show that a classic feed-forward model is unable to reproduce vergence oscillations based solely on peripheral mechanisms. Furthermore, we demonstrate that small modifications to the gain and delay of a simple feedback model for saccade generation can generate conjugate oscillations, and propose that such changes reflect the influence of lowered alertness on the tecto-reticular pathways. We conclude that peripheral mechanisms can only account for the initial divergence that accompanies all saccades, and that the conjugate and vergence oscillations observed during asymmetric movements arise centrally from an integrative binocular controller.

Neck muscles in the rhesus monkey. I. Muscle morphometry and histochemistry

Morphometric methods were used to describe the musculotendinous lengths, fascicle lengths, pennation angles, and cross-sectional areas of neck muscles in adult Macaca mulatta monkeys. Additionally, muscles were frozen, sectioned, and stained for ATPase activity to determine fiber-type composition. Individual rhesus muscles were found to vary widely in their degree of similarity to feline and human muscles studied previously. Suboccipital muscles and muscles supplied by the spinal accessory nerve were most similar to human homologs, whereas most other muscles exhibited architectural specializations. Many neck muscles were architecturally complex, with multiple attachments and internal aponeuroses or tendinous inscriptions that affected the determination of their cross-sectional areas. All muscles were composed of a mixture of type I, IIa, and IIb fiber types the relative proportions of which varied. Typically, head-turning muscles had lower proportions of type II (fast) fibers than homologous feline muscles, whereas extensor muscles contained higher proportions of type II fibers. The physical and histochemical specializations described here are known to have a direct bearing on functional properties, such as force-developing capacity and fatigue-resistance. These specializations must be recognized if muscles are to be modeled accurately or studied electrophysiologically.

Selective processing of vestibular reafference during self-generated head motion

The vestibular sensory apparatus and associated vestibular nuclei are generally thought to encode head-in-space motion. Angular head-in-space velocity is detected by vestibular hair cells that are located within the semicircular canals of the inner ear. In turn, the afferent fibers of the vestibular nerve project to neurons in the vestibular nuclei, which, in head-restrained animals, similarly encode head-in-space velocity during passive whole-body rotation. However, during the active head-on-body movements made to generate orienting gaze shifts, neurons in the vestibular nuclei do not reliably encode head-in-space motion. The mechanism that underlies this differential processing of vestibular information is not known. To address this issue, we studied vestibular nuclei neural responses during passive head rotations and during a variety of tasks in which alert rhesus monkeys voluntarily moved their heads relative to space. Neurons similarly encoded head-in-space velocity during passive rotations of the head relative to the body and during passive rotations of the head and body together in space. During all movements that were generated by activation of the neck musculature (voluntary head-on-body movements), neurons were poorly modulated. In contrast, during a task in which each monkey actively "drove" its head and body together in space by rotating a steering wheel with its arm, neurons reliably encoded head-in-space motion. Our results suggest that, during active head-on-body motion, an efferent copy of the neck motor command, rather than the monkey's knowledge of its self-generated head-in-space motion or neck proprioceptive information, gates the differential processing of vestibular information at the level of the vestibular nuclei.

Experimental control of eye and head positions prior to head-unrestrained gaze shifts in monkey

A coordinated movement of the eyes and head in the head-unrestrained condition is often used to change orientation between targets. Under natural conditions, these gaze shifts are typically generated with the eyes roughly centered in the orbits. To achieve experimental control of eye and head positions, a miniature laser was mounted on the head implants of monkeys that were trained to point the head to one target and direct gaze to another before generating a head-unrestrained gaze shift to a third target (dissociation paradigm). For comparison, monkeys were also required to make gaze shifts between stimuli, without any constraints on eye and head positions (standard paradigm). Analyses indicated that movement parameters, limited to horizontal gaze shifts, were similar for both behavioral conditions. Thus, the proposed technique and behavioral paradigm, when used in conjunction with electrophysiological and pharmacological experiments, may facilitate the study of neural control of gaze.

Comparing extraocular motoneuron discharges during head-restrained saccades and head-unrestrained gaze shifts

Burst neurons (BNs) in the paramedian pontine reticular formation provide the primary input to the extraocular motoneurons (MNs) during head-restrained saccades and combined eye-head gaze shifts. Prior studies have shown that BNs carry eye movement-related signals during saccades and carry head as well as eye movement-related signals during gaze shifts. Therefore MNs receive signals related to head motion during gaze shifts, yet they solely drive eye motion. Here we addressed whether the relationship between MN firing rates and eye movements is influenced by the additional premotor signals present during gaze shifts. Neurons in the abducens nucleus of monkeys were first studied during saccades made with the head stationary. We then recorded from the same neurons during voluntary combined eye-head gaze shifts. We conclude that the activity of MNs, in contrast to that of BNs, is related to eye motion by the same dynamic relationship during head-restrained saccades and head-unrestrained gaze shifts. In addition, we show that a standard metric-based analysis [i.e., counting the number of spikes (NOS) in a burst] yields misleading results when applied to the same data set. We argue that this latter approach fails because it does not properly consider the system's dynamics or the strong interactions between eye and head motion.

Conceptual issues related to the role of the superior colliculus in the control of gaze

Various conceptual issues have been brought into focus by recent experiments studying the role of the superior colliculus in the control of coordinated movements of the eyes and head, the interaction of saccadic and vergence movements, and cognitive processes influencing the initiation and execution of saccades.

Apparent dissociation between saccadic eye movements and the firing patterns of premotor neurons and motoneurons

Saccadic eye movements result from high-frequency bursts of activity in ocular motoneurons. This phasic activity originates in premotor burst neurons. When the head is restrained, the number of action potentials in the bursts of burst neurons and motoneurons increases linearly with eye movement amplitude. However, when the head is unrestrained, the number of action potentials now increase as a function of the change in the direction of the line of sight during eye movements of relatively similar amplitudes. These data suggest an apparent uncoupling of premotor neuron and motoneuron activity from the resultant eye movement.

Three-dimensional eye-head coordination during gaze saccades in the primate

The purpose of this investigation was to describe the neural constraints on three-dimensional (3-D) orientations of the eye in space (Es), head in space (Hs), and eye in head (Eh) during visual fixations in the monkey and the control strategies used to implement these constraints during head-free gaze saccades. Dual scleral search coil signals were used to compute 3-D orientation quaternions, two-dimensional (2-D) direction vectors, and 3-D angular velocity vectors for both the eye and head in three monkeys during the following visual tasks: radial to/from center, repetitive horizontal, nonrepetitive oblique, random (wide 2-D range), and random with pin-hole goggles. Although 2-D gaze direction (of Es) was controlled more tightly than the contributing 2-D Hs and Eh components, the torsional standard deviation of Es was greater (mean 3.55 degrees ) than Hs (3.10 degrees ), which in turn was greater than Eh (1.87 degrees ) during random fixations. Thus the 3-D Es range appeared to be the byproduct of Hs and Eh constraints, resulting in a pseudoplanar Es range that was twisted (in orthogonal coordinates) like the zero torsion range of Fick coordinates. The Hs fixation range was similarly Fick-like, whereas the Eh fixation range was quasiplanar. The latter Eh range was maintained through exquisite saccade/slow phase coordination, i.e., during each head movement, multiple anticipatory saccades drove the eye torsionally out of the planar range such that subsequent slow phases drove the eye back toward the fixation range. The Fick-like Hs constraint was maintained by the following strategies: first, during purely vertical/horizontal movements, the head rotated about constantly oriented axes that closely resembled physical Fick gimbals, i.e., about head-fixed horizontal axes and space-fixed vertical axes, respectively (although in 1 animal, the latter constraint was relaxed during repetitive horizontal movements, allowing for trajectory optimization). However, during large oblique movements, head orientation made transient but dramatic departures from the zero-torsion Fick surface, taking the shortest path between two torsionally eccentric fixation points on the surface. Moreover, in the pin-hole goggle task, the head-orientation range flattened significantly, suggesting a task-dependent default strategy similar to Listing's law. These and previous observations suggest two quasi-independent brain stem circuits: an oculomotor 2-D to 3-D transformation that coordinates anticipatory saccades with slow phases to uphold Listing's law, and a flexible "Fick operator" that selects head motor error; both nested within a dynamic gaze feedback loop.

A neural correlate for vestibulo-ocular reflex suppression during voluntary eye-head gaze shifts

The vestibulo-ocular reflex (VOR) is classically associated with stabilizing the visual world on the retina by producing an eye movement of equal and opposite amplitude to the motion of the head. Here we have directly measured the efficacy of VOR pathways during voluntary combined eye-head gaze shifts by recording from individual vestibular neurons in monkeys whose heads were unrestrained. We found that the head-velocity signal carried by VOR pathways is reduced during gaze shifts in an amplitude-dependent manner, consistent with results from behavioral studies in humans and monkeys. Our data support the hypothesis that the VOR is not a hard-wired reflex, but rather a pathway that is modulated in a manner that depends on the current gaze strategy.

Analysis of primate IBN spike trains using system identification techniques. I. Relationship To eye movement dynamics during head-fixed saccades

The dynamic behavior of primate (Macaca fascicularis) inhibitory burst neurons (IBNs) during head-fixed saccades was analyzed by using system identification techniques. Neurons were categorized as IBNs on the basis of their anatomic location as well as by their activity during horizontal head-fixed saccadic and smooth pursuit eye movements and vestibular nystagmus. Each IBN's latency or "dynamic lead time" (td) was determined by shifting the unit discharge in time until an optimal fit to the firing rate frequency B(t) profile was obtained by using the simple model based on eye movement dynamics,B(t) = r + b1(t); where is eye velocity. For the population of IBNs, the dynamic estimate of lead time provided a significantly lower value than a method that used the onset of the first spike. We then compared the relative abilities of different eye movement-based models to predict B(t) by using objective optimization algorithms. The most important terms for predicting B(t) were eye velocity gain (b1) and bias terms (r) mentioned above. The contributions of higher-order velocity, acceleration, and/or eye position terms to model fits were found to be negligible. The addition of a pole term [the time derivative of B(t)] in conjunction with an acceleration term significantly improved model fits to IBN spike trains, particularly when the firing rates at the beginning of each saccade [initial conditions (ICs)] were estimated as parameters. Such a model fit the data well (a fit comparable to a linear regression analysis with a R2 value of 0.5, or equivalently, a correlation coefficient of 0.74). A simplified version of this model [B(t) = rk + b1(t)], which did not contain a pole term, but in which the bias term (rk) was estimated separately for each saccade, provided nearly equivalent fits of the data. However, models in which ICs or rks were estimated separately for each saccade contained too many parameters to be considered as useful models of IBN discharges. We discovered that estimated ICs and rks were correlated with saccade amplitude for the majority of short-lead IBNs (SLIBNs; 56%) and many long-lead IBNs (LLIBNs; 42%). This observation led us to construct a more simple model that included a term that was inversely related to the amplitude of the saccade, in addition to eye velocity and constant bias terms. Such a model better described neuron discharges than more complex models based on a third-order nonlinear function of eye velocity. Given the small number of parameters required by this model (only 3) and its ability to fit the data, we suggest that it provides the most valuable description of IBN discharges. This model emphasizes that the IBN discharges are dependent on saccade amplitude and implies further that a mechanism must exist, at the motoneuron (MN) level, to offset the effect of the bias and amplitude-dependent terms. In addition, we did not find a significant difference in the variance accounted for by any of the downstream models tested for SLIBNs versus LLIBNs. Therefore we conclude that the eye movement signals encoded dynamically by SLIBNs and LLIBNs are similar in nature. Put another way, SLIBNs are not closer, dynamically, to MNs than LLIBNs.

Analysis of primate IBN spike trains using system identification techniques. III. Relationship To motor error during head-fixed saccades and head-free gaze shifts

The classic model of saccade generation assumes that the burst generator is driven by a motor-error signal, the difference between the actual eye position and the final "desired" eye position in the orbit. Here we evaluate objectively, using system identification techniques, the dynamic relationship between motor-error signals and primate inhibitory burst neuron (IBN) discharges (upstream analysis). The IBNs presented here are the same neurons whose downstream relationships were characterized during head-fixed saccades and head-free gaze shifts in our companion papers. In our analysis of head-fixed saccades we determined how well IBN discharges encode eye motor error (epsilone) compared with downstream saccadic eye movement dynamics and whether long-lead IBN (LLIBN) discharges encode epsilone better than short-lead IBNs (SLIBNs), given that it is commonly assumed that short-lead burst neurons (BNs) are closer than long-lead BNs to the motor output and thus further from the epsilone signal. In the epsilone-based models tested, IBN firing frequency B(t) was represented by one of the following: 1) model 1u, a nonlinear function of epsilone; 2) model 2u, a linear function of epsilone [B(t) = rk + a1epsilone(t)] where the bias term rk was estimated separately for each saccade; 3) model 3u, a version of model 2u wherein the bias term was a function of saccade amplitude; or 4) model 4u, a linear function of epsilone with an added pole term (the derivative of firing rate). Models based on epsilone consistently produced worse predictions of IBN activity than models of comparable complexity based on eye movement dynamics (e.g., eye velocity). Hence, the simple two parameter downstream model 2d [B(t) = r + b1(t)] was much better than any upstream (epsilone-based) model with a comparable number of parameters. The link between B(t) and epsilone is due primarily to the correlation between the declining phases of B(t) and epsilone; motor-error models did not predict well the rising phase of the discharge. We could improve substantially the performance of upstream models by adding an e term. Because e = -, this process was equivalent to incorporating terms into upstream models thereby erasing the distinction between upstream and downstream analyses. Adding an e term to the upstream models made them as good as downstream ones in predicting B(t). However, the epsilone term now became redundant because its removal did not affect model accuracy. Thus, when is available as a parameter, epsilone becomes irrelevant. In the head-free monkey the ability of upstream models to predict IBN firing during head-free gaze shifts when gaze, eye, or head motor-error signals were model inputs was poor and similar to the upstream analysis of the head-fixed condition. We conclude that during saccades (head-fixed) or gaze shifts (head-free) the activity of both SLIBNs and LLIBNs is more closely linked to downstream events (i.e., the dynamics of ongoing movements) than to the coincident upstream motor-error signal. Furthermore, SLIBNs and LLIBNs do not differ in their characteristics; the latter are not, as is usually hypothesized, closer to a motor-error signal than the former.