Eye movement deficits after ibotenic acid lesions of the nucleus prepositus hypoglossi in monkeys. I. Saccades and fixation

Correlated discharge among cell pairs within the oculomotor horizontal velocity-to-position integrator

In the oculomotor system, temporal integration of velocity commands into position signals may depend on synaptic feedback among neurons of a bilateral brainstem cell assembly known as the "neural integrator." Both ipsilateral excitatory and contralateral inhibitory projections between eye position-related integrator cells are hypothesized as a substrate for positive feedback supporting integration. Presence of feedback interactions should be evident in cross-correlation functions of neuron pairs. Here, unilateral and bilateral paired recordings were obtained during fixation behavior from neurons in goldfish brainstem area I, a key element of the integrator. During fixations, discharge of most unilateral pairs, composed of cells with eye position sensitivities of the same sign, was positively correlated with lag of 0-10 msec (n = 11 of 14 significant). Typically, a very narrow peak (mean half-width <4 msec) near zero lag was observed. Discharge of bilateral pairs, composed of cells with position sensitivities of the opposite sign, was either negatively correlated with lag of 0-10 msec (n = 5 of 13 significant) or not correlated. Troughs in negative correlations always had minima between 3 and 5 msec lag. These results are consistent with the feedback hypothesis of temporal integration, highlighting excitation unilaterally and inhibition bilaterally. Absence of visual input did not weaken correlations, but other sources of correlated input extrinsic to area I were not ruled out. Triplet recordings revealed that unilateral pairwise correlations were primarily independent. Correlation between unilateral pairs systematically decreased with increasing eye position, demonstrating that synchrony is not necessary for persistent activity at high firing rates.

Discharge dynamics of oculomotor neural integrator neurons during conjugate and disjunctive saccades and fixation

Burst-tonic (BT) neurons in the prepositus hypoglossi and adjacent medial vestibular nuclei are important elements of the neural integrator for horizontal eye movements. While the metrics of their discharges have been studied during conjugate saccades (where the eyes rotate with similar dynamics), their role during disjunctive saccades (where the eyes rotate with markedly different dynamics to account for differences in depths between saccadic targets) remains completely unexplored. In this report, we provide the first detailed quantification of the discharge dynamics of BT neurons during conjugate saccades, disjunctive saccades, and disjunctive fixation. We show that these neurons carry both significant eye position and eye velocity-related signals during conjugate saccades as well as smaller, yet important, "slide" and eye acceleration terms. Further, we demonstrate that a majority of BT neurons, during disjunctive fixation and disjunctive saccades, preferentially encode the position and the velocity of a single eye; only few BT neurons equally encode the movements of both eyes (i.e., have conjugate sensitivities). We argue that BT neurons in the nucleus prepositus hypoglossi/medial vestibular nucleus play an important role in the generation of unequal eye movements during disjunctive saccades, and carry appropriate information to shape the saccadic discharges of the abducens nucleus neurons to which they project.

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

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

Effects of partial lidocaine inactivation of the paramedian pontine reticular formation on saccades of macaques

To investigate the brain stem control of saccadic eye movements, the paramedian pontine reticular formation (PPRF) in rhesus monkeys was temporarily and partially inactivated with the local anesthetic lidocaine. The influence on ipsilesional, contralesional, and upward saccades was examined. While the effects of the inactivation on contralesional and upward saccades were inconsistent and small, consistent and marked modifications were observed for ipsilesional movements. For ipsilesional, horizontal saccades, all lidocaine injections caused a decrease in peak velocity and a proportional increase in duration, which substantially altered the shape of the velocity profile. The rise in duration usually fell short of preventing hypometric saccades at the peak of the effect. However, as the lidocaine effect dissipated, the amplitude often returned to control, even though the velocity and duration remained compromised. For ipsilesional, oblique saccades, the effect of lidocaine on the horizontal component was similar to that for horizontal saccades. The vertical component of oblique saccades was also influenced, albeit to a much lesser extent: the duration of the vertical component typically increased, while the vertical peak velocity either decreased or exhibited no significant change. These results were compared with simulations of three prominent models for cross-coupling oblique saccades. In general, these results of the temporary inactivation of PPRF are consistent with the predictions of local feedback models for saccadic control.

Linear regression of eye velocity on eye position and head velocity suggests a common oculomotor neural integrator

The oculomotor system produces eye-position signals during fixations and head movements by integrating velocity-coded saccadic and vestibular inputs. A previous analysis of nucleus prepositus hypoglossi (nph) lesions in monkeys found that the integration time constant for maintaining fixations decreased, while that for the vestibulo-ocular reflex (VOR) did not. On this basis, it was concluded that saccadic inputs are integrated by the nph, but that the vestibular inputs are integrated elsewhere. We re-analyze the data from which this conclusion was drawn by performing a linear regression of eye velocity on eye position and head velocity to derive the time constant and velocity bias of an imperfect oculomotor neural integrator. The velocity-position regression procedure reveals that the integration time constants for both VOR and saccades decrease in tandem with consecutive nph lesions, consistent with the hypothesis of a single common integrator. The previous evaluation of the integrator time constant relied upon fitting methods that are prone to error in the presence of velocity bias and saccades. The algorithm used to evaluate imperfect fixations in the dark did not account for the nonzero null position of the eyes associated with velocity bias. The phase-shift analysis used in evaluating the response to sinusoidal vestibular input neglects the effect of saccadic resets of eye position on intersaccadic eye velocity, resulting in gross underestimates of the imperfections in integration during VOR. The linear regression method presented here is valid for both fixation and low head velocity VOR data and is easy to implement.

Nitric oxide facilitates GABAergic neurotransmission in the cat oculomotor system: a physiological mechanism in eye movement control

Nitric oxide (NO) synthesis by prepositus hypoglossi (PH) neurons is necessary for the normal performance of horizontal eye movements. We have previously shown that unilateral injections of NO synthase (NOS) inhibitors into the PH nucleus of alert cats produce velocity imbalance without alteration of the eye position control, both during spontaneous eye movements and the vestibulo-ocular reflex (VOR). This NO effect is exerted on the dorsal PH neuropil, whose fibres increase their cGMP content when stimulated by NO. In an attempt to determine whether NO acts by modulation of a specific neurotransmission system, we have now compared the oculomotor effects of NOS inhibition with those produced by local blockade of glutamatergic, GABAergic or glycinergic receptors in the PH nucleus of alert cats. Both glutamatergic antagonists used, 2-amino-5-phosphonovaleric acid (APV) and 2,3-dihydro-6-nitro-7-sulphamoyl-benzo quinoxaline (NBQX), induced a nystagmus contralateral to that observed upon NOS inhibition, and caused exponential eye position drift. In contrast, bicuculline and strychnine induced eye velocity alterations similar to those produced by NOS inhibitors, suggesting that NO oculomotor effects were due to facilitation of some inhibitory input to the PH nucleus. To investigate the anatomical location of the putative NO target neurons, the retrograde tracer Fast Blue was injected in one PH nucleus, and the brainstem sections containing Fast Blue-positive neurons were stained with double immunohistochemistry for NO-sensitive cGMP and glutamic acid decarboxylase. GABAergic neurons projecting to the PH nucleus and containing NO-sensitive cGMP were found almost exclusively in the ipsilateral medial vestibular nucleus and marginal zone. The results suggest that the nitrergic PH neurons control their own firing rate by a NO-mediated facilitation of GABAergic afferents from the ipsilateral medial vestibular nucleus. This self-control mechanism could play an important role in the maintenance of the vestibular balance necessary to generate a stable and adequate eye position signal.

Evidence that the superior colliculus participates in the feedback control of saccadic eye movements

There is general agreement that saccades are guided to their targets by means of a motor error signal, which is produced by a local feedback circuit that calculates the difference between desired saccadic amplitude and an internal copy of actual saccadic amplitude. Although the superior colliculus (SC) is thought to provide the desired saccadic amplitude signal, it is unclear whether the SC resides in the feedback loop. To test this possibility, we injected muscimol into the brain stem region containing omnipause neurons (OPNs) to slow saccades and then determined whether the firing of neurons at different sites in the SC was altered. In 14 experiments, we produced saccadic slowing while simultaneously recording the activity of a single SC neuron. Eleven of the 14 neurons were saccade-related burst neurons (SRBNs), which discharged their most vigorous burst for saccades with an optimal amplitude and direction (optimal vector). The optimal directions for the 11 SRBNs ranged from nearly horizontal to nearly vertical, with optimal amplitudes between 4 and 17 degrees. Although muscimol injections into the OPN region produced little change in the optimal vector, they did increase mean saccade duration by 25 to 192.8% and decrease mean saccade peak velocity by 20.5 to 69.8%. For optimal vector saccades, both the acceleration and deceleration phases increased in duration. However, during 10 of 14 experiments, the duration of deceleration increased as fast as or faster than that of acceleration as saccade duration increased, indicating that most of the increase in duration occurred during the deceleration phase. SRBNs in the SC changed their burst duration and firing rate concomitantly with changes in saccadic duration and velocity, respectively. All SRBNs showed a robust increase in burst duration as saccadic duration increased. Five of 11 SRBNs also exhibited a decrease in burst peak firing rate as saccadic velocity decreased. On average across the neurons, the number of spikes in the burst was constant. There was no consistent change in the discharge of the three SC neurons that did not exhibit bursts with saccades. Our data show that the SC receives feedback from downstream saccade-related neurons about the ongoing saccades. However, the changes in SC firing produced in our study do not suggest that the feedback is involved with producing motor error. Instead, the feedback seems to be involved with regulating the duration of the discharge of SRBNs so that the desired saccadic amplitude signal remains present throughout the saccade.

Analysis of the frequency response of the saccadic circuit: numerical simulations

Fundamental mechanisms of a brain circuit's operation can be revealed by quantitative analysis of the system's dynamic behavior. This approach is particularly useful for investigation of motor circuits, which generate machine-like outputs and where systems control techniques can be applied to reveal the circuit behavior outside the dynamic range of volitional activation. As an extension of our previous study of the step response of the saccadic motor system, this paper presents analytical and numerical considerations for the frequency response of the saccadic circuit-the system response to a steady-state sinusoidal input. Consideration of these responses provides mechanistic explanation for several aspects of the biological circuit and formalizes constraints for viable models of the saccadic circuit. Most importantly, these studies provide quantitative predictions for comparison with experimental data in vivo and make explicit hypotheses about biological mechanisms for experimental verification.

Morphological identification of nitric oxide sources and targets in the cat oculomotor system

Nitric oxide (NO) production by specific neurons in the prepositus hypoglossi (PH) nucleus is necessary for the correct performance of eye movements in alert cats. In an attempt to characterize the morphological substrate of this NO function, the distribution of nitrergic neurons and NO-responding neurons has been investigated in different brainstem structures related to eye movements. Nitrergic neurons were stained by either immunohistochemistry for NO synthase I or histochemistry for reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase. The NO targets were identified by cyclic guanosine monophosphate (cGMP) immunohistochemistry in animals treated with a NO donor immediately before fixation of the brain. Connectivity between cells of the NO-cGMP pathway was analyzed by injections of the retrograde tracers horseradish peroxidase or fast blue in different structures. The motor nuclei commanding extraocular muscles did not contain elements of the NO-cGMP pathway, except for some scattered nitrergic neurons in the most caudal part of the abducens nucleus. The PH nucleus contained the largest number of nitrergic cell bodies and a rich neuropil, distributed in two groups in medial and lateral positions in the caudal part, and one central group in the rostral part of the nucleus. An abundant cGMP positive neuropil was the only NO-sensitive element in the PH nucleus, where no cGMP-producing neuronal cell bodies were observed. The opposite disposition was found in the marginal zone between the PH and the medial vestibular nuclei, with a large number of NO-sensitive cGMP-producing neurons and almost no nitrergic cells. Both nitrergic and NO-sensitive cell bodies were found in the medial and inferior vestibular nuclei and in the superior colliculus, whereas the lateral geniculate nucleus contained nitrergic neuropil and a large number of NO-sensitive cell bodies. Some of the cGMP-positive neurons in the marginal zone and medial vestibular nucleus projected to the PH nucleus, predominantly to the ipsilateral side. These morphological findings may help to explain the mechanism of action of NO in the oculomotor system.

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.

Effects of nucleus prepositus hypoglossi lesions on visual climbing fiber activity in the rabbit flocculus

The caudal dorsal cap (dc) of the inferior olive is involved in the control of horizontal compensatory eye movements. It provides those climbing fibers to the vestibulocerebellum that modulate optimally to optokinetic stimulation about the vertical axis. This modulation is mediated at least in part via an excitatory input to the caudal dc from the pretectal nucleus of the optic tract and the dorsal terminal nucleus of the accessory optic system. In addition, the caudal dc receives a substantial GABAergic input from the nucleus prepositus hypoglossi (NPH). To investigate the possible contribution of this bilateral inhibitory projection to the visual responsiveness of caudal dc neurons, we recorded the climbing fiber activity (i.e., complex spikes) of vertical axis Purkinje cells in the flocculus of anesthetized rabbits before and after ablative lesions of the NPH. When the NPH ipsilateral to the recorded flocculus was lesioned, the spontaneous complex spike firing frequency did not change significantly; but when both NPHs were lesioned, the spontaneous complex spike firing frequency increased significantly. When only the contralateral NPH was lesioned, the spontaneous complex spike firing frequency decreased significantly. Neither unilateral nor bilateral lesions had a significant influence on the depth of complex spike modulation during constant velocity optokinetic stimulation or on the transient continuation of complex spike modulation that occurred when the constant velocity optokinetic stimulation stopped. The effects of the lesions on the spontaneous complex spike firing frequency could not be explained when only the projections from the NPH to the inferior olive were considered. Therefore we investigated at the electron microscopic level the nature of the commissural connection between the two NPHs. The terminals of this projection were found to be predominantly GABAergic and to terminate in part on GABAergic neurons. When this inhibitory commissural connection is taken into consideration, then the effects of NPH lesions on the spontaneous firing frequency of floccular complex spikes are qualitatively explicable in terms of relative weighting of the commissural and caudal dc projections of the NPH. In summary, we conclude that in the anesthetized rabbit the inhibitory projection of the NPH to the caudal dc influences the spontaneous firing frequency of floccular complex spikes but not their modulation by optokinetic stimulation.

Why move the eyes if we can move the head?

To see while moving is a very basic and integrative sensorimotor function in vertebrates. To maintain visual acuity, the oculomotor system provides efficient compensatory eye movements for head and visual field displacements. Other types of eye movement allow the selection of new visual targets and binocular vision and stereopsis. Motor and premotor neuronal circuits involved in the genesis and control of eye movements are briefly described. The peculiar properties and robust biomechanics of the oculomotor system have allowed it to survive almost unchanged through vertebrate evolution.

Anatomy and discharge properties of pre-motor neurons in the goldfish medulla that have eye-position signals during fixations

Previous work in goldfish has suggested that the oculomotor velocity-to-position neural integrator for horizontal eye movements may be confined bilaterally to a distinct group of medullary neurons that show an eye-position signal. To establish this localization, the anatomy and discharge properties of these position neurons were characterized with single-cell Neurobiotin labeling and extracellular recording in awake goldfish while monitoring eye movements with the scleral search-coil method. All labeled somata (n = 9) were identified within a region of a medially located column of the inferior reticular formation that was approximately 350 microm in length, approximately 250 microm in depth, and approximately 125 microm in width. The dendrites of position neurons arborized over a wide extent of the ventral half of the medulla with especially heavy ramification in the initial 500 microm rostral of cell somata (n = 9). The axons either followed a well-defined ventral pathway toward the ipsilateral abducens (n = 4) or crossed the midline (n = 2) and projected toward the contralateral group of position neurons and the contralateral abducens. A mapping of the somatic region using extracellular single unit recording revealed that position neurons (n > 120) were the dominant eye-movement-related cell type in this area. Position neurons did not discharge below a threshold value of horizontal fixation position of the ipsilateral eye. Above this threshold, firing rates increased linearly with increasing temporal position [mean position sensitivity = 2.8 (spikes/s)/ degrees, n = 44]. For a given fixation position, average rates of firing were higher after a temporal saccade than a nasal one (n = 19/19); the magnitude of this hysteresis increased with increasing position sensitivity. Transitions in firing rate accompanying temporal saccades were overshooting (n = 43/44), beginning, on average, 17.2 ms before saccade onset (n = 17). Peak firing rate change accompanying temporal saccades was correlated with eye velocity (n = 36/41). The anatomical findings demonstrate that goldfish medullary position neurons have somata that are isolated from other parts of the oculomotor system, have dendritic fields overlapping with axonal terminations of neurons with velocity signals, and have axons that are capable of relaying commands to the abducens. The physiological findings demonstrate that the signals carried by position neurons could be used by motoneurons to set the fixation position of the eye. These results are consistent with a role for position neurons as elements of the velocity-to-position neural integrator for horizontal eye movements.

Stability of the memory of eye position in a recurrent network of conductance-based model neurons

Studies of the neural correlates of short-term memory in a wide variety of brain areas have found that transient inputs can cause persistent changes in rates of action potential firing, through a mechanism that remains unknown. In a premotor area that is responsible for holding the eyes still during fixation, persistent neural firing encodes the angular position of the eyes in a characteristic manner: below a threshold position the neuron is silent, and above it the firing rate is linearly related to position. Both the threshold and linear slope vary from neuron to neuron. We have reproduced this behavior in a biophysically plausible network model. Persistence depends on precise tuning of the strength of synaptic feedback, and a relatively long synaptic time constant improves the robustness to mistuning.

Frequency-dependent effects of glutamate antagonists on the vestibulo-ocular reflex of the cat

In the central nervous system, sensory and motor signals at different frequencies are transmitted most effectively by neural elements that have different dynamic characteristics. Dynamic differences may be due, in part, to the dynamics of neurotransmitter receptors. For example, N-methyl-D-aspartate (NMDA) receptors are thought to be a component of the "neural integrator" of the vestibulo-ocular reflex (VOR), which generates a signal proportional to eye position. We measured the effects of blockade of NMDA and AMPA/kainate receptors on the gain and phase of the VOR at frequencies between 0.1 and 8 Hz in alert cats. The competitive NMDA antagonist, APV, and the non-competitive antagonists, MK-801 and ketamine, all caused a pronounced reduction in VOR gain. Gain was more strongly attenuated at low frequencies (0.1-1 Hz) than at higher frequencies (2-8 Hz). The phase lead of the eye with respect to the head was increased up to 30 degrees. In contrast, the reduction in gain associated with drowsiness or surgical anesthesia was not frequency-dependent. Blockade of AMPA/kainate receptors by the competitive antagonists, CNQX and NBQX, reduced the gain of the VOR at all frequencies tested. We evaluated our results using a control systems model. Our data are consistent with participation of NMDA receptors in neural integration, but suggest that NMDA receptors also participate in transmission by other components of the VOR pathway, and that neural integration also employs other receptors. One possibility is that between 0.1 and 10 Hz, higher-frequency signals are transmitted primarily by AMPA/kainate receptors, and lower frequencies by NMDA receptors. This arrangement would provide a biological substrate for selective motor learning within a small frequency range.

A distributed model of the saccade system: simulations of temporally perturbed saccades using position and velocity feedback

Interrupted saccades, movements that are perturbed in mid-flight by pulsatile electrical stimulation in the omnipause neuron region, are known to achieve final eye displacements with accuracies that are similar to normal saccades even in the absence of visual input following the perturbation. In an attempt to explain the neurophysiological basis for this phenomenon, the present paper describes a model of the saccadic system that represents the superior colliculus as a dynamic two-dimensional, topographically arranged array of laterally interconnected units. A distributed feedback pathway to the colliculus from downstream elements, providing both eye position and velocity signals is incorporated in the model. With the help of a training procedure based on a genetic algorithm and gradient descent, the model is optimized to produce both the normal as well as slow saccades with similar accuracy. The slow movements are included in the training set to mimic the accurate saccades that occur despite alterations in alertness, as well as following various degenerative oculomotor diseases. Although interrupted saccades were not included in the training set, the model is able to produce accurate movement of this type as an emergent property for a wide range of perturbed eye velocity trajectories. Our model demonstrates for the first time, that by means of an appropriate feedback mechanism, a single-layered dynamic network can be made to retain a distributed memory of the remaining ocular displacement error even for interrupted and slow saccades. These results support the hypothesis that saccades are controlled by error feedback of signals that code efference copies of eye motion, and further, suggest a possible answer to a long standing question about the kind of the feedback signal, if any, that is received by the superior colliculus during saccadic eye movements.

Mechanisms of action and targets of nitric oxide in the oculomotor system

Nitric oxide (NO) production by neurons in the prepositus hypoglossi (PH) nucleus is necessary for the normal performance of eye movements in alert animals. In this study, the mechanism(s) of action of NO in the oculomotor system has been investigated. Spontaneous and vestibularly induced eye movements were recorded in alert cats before and after microinjections in the PH nucleus of drugs affecting the NO-cGMP pathway. The cellular sources and targets of NO were also studied by immunohistochemical detection of neuronal NO synthase (NOS) and NO-sensitive guanylyl cyclase, respectively. Injections of NOS inhibitors produced alterations of eye velocity, but not of eye position, for both spontaneous and vestibularly induced eye movements, suggesting that NO produced by PH neurons is involved in the processing of velocity signals but not in the eye position generation. The effect of neuronal NO is probably exerted on a rich cGMP-producing neuropil dorsal to the nitrergic somas in the PH nucleus. On the other hand, local injections of NO donors or 8-Br-cGMP produced alterations of eye velocity during both spontaneous eye movements and vestibulo-ocular reflex (VOR), as well as changes in eye position generation exclusively during spontaneous eye movements. The target of this additional effect of exogenous NO is probably a well defined group of NO-sensitive cGMP-producing neurons located between the PH and the medial vestibular nuclei. These cells could be involved in the generation of eye position signals during spontaneous eye movements but not during the VOR.

Neurology of saccades and smooth pursuit

During the period covered by this review a number of papers have been published on saccade and smooth pursuit research, conducted experimentally in monkeys and clinically in humans. In monkeys, using mainly electrophysiological methods, the roles of the frontal eye field, parietal eye field and supplementary eye field at the cortical level, and those of the paramedian pontine reticular formation, nucleus prepositus hypoglossi, interstitial nucleus of Cajal and superior colliculus at the brainstem level have been studied in great detail. In humans the same cortical areas have also been examined, mainly using functional imaging resulting in new information on the location of these areas and new hypotheses on the role of the superior parietal lobule in visual attention and that of the posterior part of the anterior cingulate cortex in motivation. Saccades, smooth pursuit and clinical applications of eye movement research are dealt with separately.

Eye movement deficits following ibotenic acid lesions of the nucleus prepositus hypoglossi in monkeys II. Pursuit, vestibular, and optokinetic responses

The eyes are moved by a combination of neural commands that code eye velocity and eye position. The eye position signal is supposed to be derived from velocity-coded command signals by mathematical integration via a single oculomotor neural integrator. For horizontal eye movements, the neural integrator is thought to reside in the rostral nucleus prepositus hypoglossi (nph) and project directly to the abducens nuclei. In a previous study, permanent, serial ibotenic acid lesions of the nph in three rhesus macaques compromised the neural integrator for fixation but saccades were not affected. In the present study, to determine further whether the nph is the neural substrate for a single oculomotor neural integrator, the effects of those lesions on smooth pursuit, the vestibulo-ocular reflex (VOR), vestibular nystagmus (VN), and optokinetic nystagmus (OKN) are documented. The lesions were correlated with long-lasting deficits in eye movements, indicated most clearly by the animals' inability to maintain steady gaze in the dark. However, smooth pursuit and sinusoidal VOR in the dark, like the saccades in the previous study, were affected minimally. The gain of horizontal smooth pursuit (eye movement/target movement) decreased slightly (<25%) and phase lead increased slightly for all frequencies (0.3-1.0 Hz, +/-10 degrees target tracking), most noticeably for higher frequencies (0.8-0.7 and approximately 20 degrees for 1.0-Hz tracking). Vertical smooth pursuit was not affected significantly. Surprisingly, horizontal sinusoidal VOR gain and phase also were not affected significantly. Lesions had complex effects on both VN and OKN. The plateau of per- and postrotatory VN was shortened substantially ( approximately 50%), whereas the initial response and the time constant of decay decreased slightly. The initial OKN response also decreased slightly, and the charging phase was prolonged transiently then recovered to below normal levels like the VN time constant. Maximum steady-state, slow eye velocity of OKN decreased progressively by approximately 30% over the course of the lesions. These results support the previous conclusion that the oculomotor neural integrator is not a single neural entity and that the mathematical integrative function for different oculomotor subsystems is most likely distributed among a number of nuclei. They also show that the nph apparently is not involved in integrating smooth pursuit signals and that lesions of the nph can fractionate the VOR and nystagmic responses to adequate stimuli.

Neuropharmacologic aspects of the ocular motor system and the treatment of abnormal eye movements

Neuropharmacology is aiding our understanding of the control of eye movements in at least three ways. First, neurotransmitters have been identified in the pathways that coordinate gaze. Second, the technique of pharmacologic inactivation has provided a powerful method to determine the contributions of populations of neurons to specific behaviors, such as steady gaze holding. Finally, the results of basic neuropharmacologic studies have been used to identify candidate drugs for therapeutic trials of abnormal eye movements.

Effects of lesions of the oculomotor vermis on eye movements in primate: saccades

We studied the effects on saccades of ablation of the dorsal cerebellar vermis (lesions centered on lobules VI and VII) in three monkeys in which the deep cerebellar nuclei were spared. One animal, with a symmetrical lesion, showed bilateral hypometric horizontal saccades. Two animals, with asymmetrical lesions, showed hypometric ipsilateral saccades, and saccades to vertically positioned targets were misdirected, usually deviating away from the side to which horizontal saccades were hypometric. Postlesion, all animals showed an increase (2- to 5-fold) in trial-to-trial variability of saccade amplitude. They also showed a change in the ratio of the amplitudes of centripetal to centrifugal saccades (orbital-position effect); usually centrifugal saccades became smaller. In the two animals with asymmetrical lesions, for saccades in the hypometric direction, latencies were markedly increased (up to approximately 500 ms). There was also an absence of express and anticipatory saccades in the hypometric direction. When overall saccade latency was increased, centrifugal saccades became relatively more delayed than centripetal saccades. The dynamic characteristics of saccades were affected to some extent in all monkeys with changes in peak velocity, eye acceleration, and especially eye deceleration. There was relatively little effect of orbital position on saccade dynamics, however, with the exception of one animal that showed an orbital position effect for eye acceleration. In a double-step adaptation paradigm, animals showed an impaired ability to adaptively adjust saccade amplitude, though increased amplitude variability postlesion may have played a role in this deficit. During a single training session, however, the latency to corrective saccades-which had been increased postlesion-gradually decreased and so enabled the animal to reach the final position of the target more quickly. Overall, both in the early postlesion period and during recovery, changes in saccade amplitude and latency tended to vary together but not with changes in saccade dynamics or adaptive capability, both of which behaved relatively independently. These findings suggest that the cerebellum can adjust saccade amplitude and saccade dynamics independently. Our results implicate the cerebellar vermis directly in every aspect of the on-line control of saccades: initiation (latency), accuracy (amplitude and direction), and dynamics (velocity and acceleration) and also in the acquisition of adaptive ocular motor behavior.

Discharge characteristics of vestibular saccade neurons in alert monkeys

We previously described a class of neurons, located in and around the interstitial nucleus of Cajal of the cat, that discharged during vestibular stimulation and before saccades. We called these neurons vestibular saccade neurons (VSNs). In the present study, we characterized similar neurons in the monkey. These neurons discharged before vertical saccades and during vertical vestibular stimulation as well as vertical smooth pursuit. Like cat VSNs, the discharge metrics of these VSNs were poorly related to saccade metrics and showed only occasional, weak sensitivity to eye position. They discharged most intensely (on-direction) for movements that were either upward or downward, and their on-directions were consistent during pitch and pursuit but not for eye position. For saccades, the correlation coefficient of number of spikes and vertical saccade size varied from 0.08 to 0.90 with a mean of approximately 0.6. The average sensitivity (i.e., slope) of the number of spikes and vertical saccade size linear regression was 0.3 +/- 0.2 spike/deg. Average correlations between peak discharge rate and peak saccade velocity and between burst duration and saccade duration were 0.5 and 0.4; sensitivities were 0.2 +/- 0.2 spike per s per deg/s and 0.6 +/- 0.5 ms/ms, respectively. Average vestibular sensitivities during 0.5 Hz, +/-10 degrees sinusoidal pitch while the animals suppressed their vestibular ocular reflex were 0.97 spike/s per deg/s for up VSNs and 0.66 spike/s per deg/s for down VSNs. The average static position sensitivity for the population of 39 VSNs tested was 0.55 spike/s per deg. The average gain for VSNs tested during 0.5 Hz, +/-10 degrees sinusoidal smooth pursuit tracking was 1.4 spike/s per deg/s. As we could not identify analogous neurons in the region of the monkey ponto-medullary junction, we conclude that horizontal on-direction VSNs do not exist in the monkey. We discuss a possible functional role for VSNs and similar neurons described in previous studies and conclude that these neurons are most likely involved with the process of neural integration (in a mathematical sense) of velocity-coded inputs from a variety of oculomotor subsystems and are not a pivotal element in saccade generation.

Anatomical substrates of oculomotor control

It is convenient to describe oculomotor neuroanatomy in terms of five to six different eye movement types, each with relatively independent neural circuitry: saccades, vestibulo-ocular reflex, optokinetic response, smooth pursuit, vergence and, most recently added to the list, gaze-holding. Current research indicates that many structures participate in several eye movement types, such as the nucleus reticularis tegmenti pontis, frontal eye fields and pretectum. However, the circuits appear to run in parallel rather than being integrated.